School encyclopedia. Radio wave propagation Features of radio wave propagation
Radio is one of the types of wireless communication, in which the carrier of the signal is a radio wave, which spreads widely over a distance. There is an opinion that it is impossible to transmit radio signals under water. Let's try to figure it out why it is impossible to carry out radio communication between submarines, and is it really so.
How radio communication between submarines works:
The propagation of radio waves is carried out according to the following principle: the one who transmits a signal, with a certain frequency and power, sets the radio wave. After that, the sent signal is modulated to a high-frequency oscillation. The picked up modulated signal is emitted by a special antenna at certain distances. Where a radio wave signal is received, a modulated signal is directed to the antenna, which is first filtered and demodulated. And only then we can receive the signal, with a certain distinction with the signal, the one that was originally transmitted.
Radio waves with the lowest range (VLF, VLF, 3-30 kHz) can easily penetrate sea water, up to 20 meters deep.
For example, a submarine that is not too deep under water could use this range to establish and maintain communication with the crew. And if we take a submarine, but located much deeper under water, and it has a long cable on which a buoy with an antenna is attached, then it will also be able to use this range. Due to the fact that the buoy is installed at a depth of several meters, and even has a small size, it is very problematic to find it with sonars of enemies. "Goliath", is one of the first VLF transmitters, built during the Second World War (1943) in Germany, after the end of the war it was transported to the USSR, and in 1949-1952 it was reanimated in the Nizhny Novgorod region and is used there to this day.
Aerial photograph of ELF transmitter (Clam Lake, Wisconsin, 1982)
The lowest frequency radio waves (ELF, ELF, up to 3 kHz) easily penetrate the Earth's crust and seas. The creation of an ELF transmitter is an extremely difficult task due to the enormous wavelength of , 4 km). Their waves are commensurate with the radius of the Earth. From here we see that the construction of a dipole antenna at half the wavelength (length ≈ 2000 km) is an unattainable goal at the current stage.
Summing up everything that has been said above, we need to find such a part of the earth's surface that will be characterized by relatively low conductivity, and attach 2 giant electrodes to it, which would be located at a distance of 60 kilometers relative to each other.
Since we know the specific conductivity of the Earth in terms of the electrodes is satisfactorily at a low level, thus, the electric current between the electrodes would penetrate fundamentally into the depths of our planet, using them as an element of a giant antenna. It should be noted that the primary source of the unusually high technical difficulties of such an antenna was that only the USSR and the USA had ELF transmitters.
Radio waves, and their propagation, are an undeniable mystery for aspiring airwaves. Here you can get acquainted with the basics of the theory of radio wave propagation. This article is intended to familiarize beginner fans of the airwaves, as well as for those who have some knowledge of it.
The most important introduction, which is often forgotten to say before introducing the theory of radio wave propagation, is that radio waves propagate around our planet due to reflection from the ionosphere and from the earth as a beam of light is reflected from translucent mirrors.
Peculiarities of medium wave propagation and cross modulation
Medium waves include radio waves with a length of 1000 to 100 m (frequencies of 0.3 - 3.0 MHz). Medium waves are mainly used for broadcasting. And they are also the cradle of domestic radio piracy. They can spread by terrestrial and ionospheric paths. Medium waves experience significant absorption in the semiconducting surface of the Earth, the propagation distance of the Earth's wave 1 (see Fig. 1) is limited by a distance of 500-700 km. Over long distances, radio waves 2 and 3 are propagated by an ionospheric (spatial) wave.
At night, mean waves propagate by reflection from the E layer of the ionosphere (see Fig. 2), the electron density of which is sufficient for this. In the daytime, layer D is located in the path of the wave propagation, which absorbs medium waves extremely strongly. Therefore, at normal transmitter powers, the electric field strength is insufficient for reception, and during the daytime, medium waves propagate practically only by an earth wave over relatively short distances, of the order of 1000 km. In the medium wavelength range, longer wavelengths experience less absorption, and the ionospheric wave electric field strength is greater at longer wavelengths. Absorption increases in the summer months and decreases in the winter months. Ionospheric disturbances do not affect the propagation of medium waves, since the E layer is slightly disturbed during ionospheric magnetic storms.
At night see fig. 1, at a certain distance from the transmitter (point B), the arrival of both spatial 3 and surface waves 1 is possible, and the length of the spatial wave path changes with a change in the electron density of the ionosphere. A change in the phase difference of these waves leads to an oscillation of the electric field strength, called near field fading.
Waves 2 and 3 can arrive at a considerable distance from the transmitter (point C) through one or two reflections from the ionosphere. A change in the phase difference between these two waves also leads to an oscillation in the electric field strength, called far-field fading.
To combat fading at the transmitting end of the communication line, antennas are used whose maximum radiation pattern is "pressed" to the earth's surface; these include the simplest "Inverted-V" antenna, which is often used by radio amateurs. With such a radiation pattern, the near fading zone moves away from the transmitter, and at large distances the field of the wave arriving through two reflections is weakened.
Unfortunately, not all novice broadcasters working in the 1600-3000 kHz frequency range know that a weak signal from a low-power transmitter is subject to ionospheric distortion. The signal from more powerful radio transmitters is less susceptible to ionospheric distortion. Due to the nonlinear ionization of the ionosphere, a weak signal is modulated by the modulating voltage of signals from powerful stations. This phenomenon is called cross modulation. The depth of the modulation ratio reaches 5-8%. From the side of the reception, the impression is made of a poorly made transmitter, with all kinds of hums and wheezes, this is especially noticeable in the AM modulation mode.
Due to cross modulation, intense lightning interference often penetrates into the receiver, which cannot be filtered out - the lightning discharge modulates the received signal. It is for this reason that broadcasters began to use single-sideband transmitters for two-way radio communications and began to work more often at higher frequencies. Foreign radio broadcasters of CB stations power them up and compress the modulating signals, and for undistorted work on the air, they use inverse frequencies.
The phenomena of demodulation and cross modulation in the ionosphere are observed only in the medium wave (MW) range. In the range of short waves (SW), the speed of an electron under the action of an electric field is negligible compared to its thermal speed, and the presence of a field does not change the number of collisions of an electron with heavy particles.
The most favorable, in the frequency range from 1500 to 3000 kHz for long-distance communications, are winter nights and periods of minimum solar activity. Particularly long distance communications, over 10,000 km, are usually possible at sunset and sunrise. During the daytime, communication is possible at a distance of up to 300 km. Free FM radio broadcasters can only envy such large radio routes.
During the summer, this range is often interfered with by static discharges in the atmosphere.
Features of the propagation of short waves and their characteristics
Short waves include radio waves with a length of 100 to 10 m (frequencies 3-30 MHz). The advantage of operating at short wavelengths over operating at longer wavelengths is that directional antennas can be easily created in this range. Short waves can propagate both terrestrial, in the low-frequency part of the range, and as ionospheric.
With increasing frequency, the absorption of waves in the semiconducting surface of the Earth increases. Therefore, at conventional transmitter powers, short-wave terrestrial waves propagate over distances not exceeding several tens of kilometers. On the sea surface, this distance increases significantly.
The ionospheric wave can propagate short waves over many thousands of kilometers, and this does not require high-power transmitters. Therefore, at present, short wavelengths are mainly used for communication and broadcasting over long distances.
Short waves travel long distances by reflection from the ionosphere and the Earth's surface. This method of propagation is called jump-like, see fig. 2 and is characterized by hop distance, number of hops, exit and arrival angles, maximum usable frequency (MUF) and least usable frequency (LFR).
If the ionosphere is uniform in the horizontal direction, then the wave trajectory is also symmetric. Typically, radiation occurs in a certain range of angles, since the width of the radiation pattern of short-wave antennas in the vertical plane is 10-15 °. The minimum jump distance for which the reflection condition is satisfied is called the silence zone distance (ZM). For wave reflection, it is necessary that the operating frequency is not higher than the value of the maximum applicable frequency (MUF), which is the upper limit of the operating range for a given distance. Wave 4.
The use of anti-aircraft radiation antennas, as one of the methods for reducing the silence zone, is limited to the concept of the maximum usable frequency (MUF), taking into account its reduction by 15-20% of the MUF. Antennas of zenith radiation are used for broadcasting in the near zone by the method of one-hop reflection from the ionosphere.
The second condition limits the operating range from below: the lower the operating frequency (within the short-wavelength range), the stronger the wave absorption in the ionosphere. The smallest - applicable frequency (APF) is determined from the condition that with a transmitter power of 1 kW, the electric field strength of the signal must exceed the noise level, and therefore, the signal absorption in the layers of the ionosphere must be no more than the allowable one. The electron density of the ionosphere changes during the day, during the year, and during the period of solar activity. This means that the boundaries of the working range also change, which leads to the need to change the working wavelength during the day.
Frequency range 1.5-3 MHz, is nocturnal. It is clear that for a successful radio communication session, you need to choose the right frequency (wavelength) every time, besides, this complicates the design of the station, but for a true connoisseur of long-distance communications this is not a difficulty, it is part of a hobby. Let's assess the HF band by site.
Frequency range 5-8 MHz, in many respects it is similar to the 3 MHz range, and unlike it, here in the daytime you can communicate up to 2000 km, the zone of silence (ZM) is absent and is several tens of kilometers. At night, communication is possible at any distance with the exception of 3M, which increases to several hundred kilometers. During the hours of changing the time of day (sunset / sunrise), they are most convenient for long-distance communications. Atmospheric interference is less pronounced than in the 1.5-3 MHz range.
In the frequency range 10-15 MHz during periods of solar activity, daytime connections are possible with almost any point in the world. In summer, the duration of radio communication in this frequency range is round-the-clock, with the exception of certain days. The zone of silence at night has distances of 1500-2000 km, and therefore only long-distance communications are possible. In the daytime, they decrease to 400-1000 km.
Frequency range 27-30 MHz suitable for communication only during daylight hours. This is the most capricious range. It usually opens for several hours, days or weeks, especially when the seasons change, i.e. autumn and spring. The zone of silence (ZM) reaches 2000-2500 km. This phenomenon belongs to the MUF topic, here the angle of the reflected wave must be small in relation to the ionosphere, otherwise it has a large attenuation in the ionosphere, or a simple escape into space. Small angles of radiation correspond to large jumps and correspondingly large zones of silence. During periods of maximum solar activity, communication is possible at night.
In addition to the above models, cases of anomalous radio wave propagation are possible. Abnormal propagation can occur when a sporadic layer appears on the path of the wave, from which shorter waves, up to meter waves, can be reflected. This phenomenon can be observed in practice by the passage of distant TV stations and FM radio stations. The MUF of the radio signal during these hours reaches 60-100 MHz during the years of solar activity.
VHF FM, except in rare cases of anomalous propagation of radio waves, propagation is strictly due to the so-called "line of sight". The propagation of radio waves within the line of sight speaks for itself, and is due to the height of the transmitting and receiving antennas. It is clear that in the conditions of urban development, one cannot speak of any visual and direct visibility, but radio waves pass through urban buildings with some weakening. The higher the frequency, the higher the attenuation in urban areas. The 88-108 MHz frequency range is also subject to some attenuation in urban environments.
Fading of HF radio signals
The reception of short radio waves is always accompanied by a measurement of the received signal level, and this change is of a random and temporary nature. This phenomenon is called fading (fading) of the radio signal. Fast and slow signal fading is observed on the air. The depth of fading can be up to several tens of decibels.
The main cause of fast signal fading is radio multipath. In this case, the cause of fading is the arrival at the receiving point of two beams propagating through one and two reflections from the ionosphere, wave 1 and wave 3, see Fig. 2.
Since the rays travel different paths along the distance, their arrival phases are not the same. Changes in the electron density, continuously occurring in the ionosphere, lead to a change in the path length of each of the rays, and, consequently, to a change in the phase difference between the rays. To change the phase of the wave by 180 °, it is enough for the path length to change by only ½. It should be recalled that when beams of the same signal arrive at the receiving point with the same strength and with a phase difference of 180 °, they are completely subtracted according to the law of vectors, and the strength of the incoming signal in this case can be zero. Such small changes in path length can occur continuously, therefore, fluctuations in the strength of the electric field in the short wave range are frequent and deep. The observation interval of 3-7 minutes can be at low frequencies in the HF range, and up to 0.5 seconds at frequencies closer to 30 MHz.
In addition, signal fading is caused by scattering of radio waves by ionospheric irregularities and interference of scattered waves.
In addition to interference fading, at short wavelengths, polarization fading takes place. The cause of polarization fading is the rotation of the plane of polarization of the wave relative to the received antenna. This occurs when a wave propagates in the direction of the lines of force of the Earth's magnetic field, and with a change in the electron density of the ionosphere. If the transmitting and receiving antennas are horizontal dipoles, then the radiated horizontally polarized wave, after passing through the ionosphere, will undergo a rotation of the plane of polarization. This leads to fluctuations in e. etc., induced in the antenna, which has an additional attenuation of up to 10 dB.
In practice, all of the indicated causes of signal fading act, as a rule, in a complex manner and obey the described Rayleigh distribution law.
In addition to fast fading, slow fading is observed, which is observed with a period of 40-60 minutes in the low-frequency part of the HF range. The reason for these fading is the change in the absorption of radio waves in the ionosphere. The distribution of the envelope amplitude of the signal at slow fading obeys a normally logarithmic law with a decrease in the signal to 8-12 dB.
To combat fading, diversity reception is used at short wavelengths. The fact is that the increase and decrease in the electric field strength do not occur simultaneously, even on a relatively small area of the earth's surface. In the practice of shortwave communication, usually two antennas are used, separated by a distance of several wavelengths, and the signals are added after detection. Diversity of antennas in polarization is effective, i.e. simultaneous reception on vertical and horizontal antennas with subsequent addition of signals after detection.
It should be noted that these control measures are effective only to eliminate fast fading, slow signal changes are not eliminated, since this is associated with a change in the absorption of radio waves in the ionosphere.
In amateur radio practice, the diversity antenna method is used quite rarely, due to its structural high cost and the lack of the need to receive sufficiently reliable information. This is due to the fact that amateurs often use resonant and band antennas, the number of which in his household is about 2-3 pieces. Using diversity reception requires at least doubling the antenna fleet.
Another thing is when an amateur lives in a rural area, while having a sufficient area to accommodate an anti-fading structure, he can simply use two broadband vibrators for this, covering all or almost all of the necessary ranges. One vibrator should be vertical, the other horizontal. For this it is not at all necessary to have several masts. It is enough to place them so, on one mast, so that they are oriented relative to each other at an angle of 90 °. The two antennas, in this case, will resemble the well-known "Inverted-V" antenna.
Calculation of the radius of coverage with a radio signal in the VHF / FM bands
Frequencies of the meter range are distributed within the line of sight. The radius of the radio wave propagation within the line of sight without taking into account the radiation power of the transmitter and other natural phenomena that reduce the communication efficiency looks like this:
r = 3.57 (√h1 + √h2), km,
Calculate line-of-sight radii when installing the receiving antenna at different heights, where h1 is a parameter, h2 = 1.5 m.Let us summarize them in Table 1.
Table 1
| h1 (m) | 10 | 20 | 25 | 30 | 35 | 40 | 50 | 60 |
| r (km) | 15,6 | 20,3 | 22.2 | 24 | 25.5 | 27,0 | 29,6 | 32 |
This formula does not take into account the attenuation of the signal and the power of the transmitter, it only speaks about the possibility of line of sight, taking into account the perfectly round ground.
Let's make a calculation the required level of radio signal together with reception for a wavelength of 3 m.
Since on the paths between the transmitting station and the moving object there are always such phenomena as reflection, scattering, absorption of radio signals by various objects, etc., corrections should be made to the level of signal attenuation, which was suggested by a Japanese scientist Okumura. The standard deviation for this range with urban buildings will be 3 dB, and with a communication probability of 99%, we will introduce a factor of 2, which will make the total correction P in the radio signal level in
P = 3 × 2 = 6 dB.
The sensitivity of the receivers is determined by the ratio of the useful signal over the noise of 12 dB, i.e. 4 times. Such a ratio is not acceptable for high-quality broadcasting, so we will introduce an additional correction of 12–20 dB, we will accept 14 dB.
In total, the total correction in the level of the received signal, taking into account its attenuation along the path and the specifics of the receiving device, will be: 6 + 16 20 dB (10 times). Then, with a receiver sensitivity of 1.5 μV. at the receiving site, a field with an intensity of 15 μV / m.
Calculate using the Vvedensky formula range at a given field strength of 15 μV / m, taking into account the transmitter power, receiver sensitivity and urban areas:
where r is km; Р - kW; G - dB (= 1); h - m; λ - m; E - mV.
This calculation does not take into account the gain of the receiving antenna, as well as the attenuation in the feeder and bandpass filter.
Answer: With a power of 10 W, a radiation height of h1 = 27 meters and h2 = 1.5 m, a really high-quality radio reception with a radius in urban buildings will be 2.5-2.6 km. If we take into account that the reception of radio signals from your radio transmitter will be carried out on the middle and high floors of residential buildings, then this range will increase by about 2-3 times. If you receive radio signals to a remote antenna, then the range will be tens of kilometers.
73! UA9LBG & Radio-Vector-Tyumen
The laws of propagation of radio waves in free space are relatively simple, but most often radio engineering deals not with free space, but with the propagation of radio waves over the earth's surface. As experience and theory show, the Earth's surface strongly affects the propagation of radio waves, and both the physical properties of the surface, for example, spills between the sea and land), and its geometric shape (the general curvature of the surface, for example, the differences between the sea and land), and its geometric shape (the general curvature of the earth's surface and individual irregularities in the relief - mountains, gorges, etc.). This effect is different for waves of different wavelengths and for waves of different lengths and for different distances between the transmitter and receiver.
The influence exerted on the propagation of radio waves by the shape of the earth's surface is clear from the previous one. After all, we have here, in essence, various manifestations of the diffraction of waves coming from the emitter (§ 41), both on the globe as a whole and on individual features of the relief. We know that diffraction is highly dependent on the relationship between the wavelength and the size of the body in the path of the wave. It is not surprising, therefore, that the curvature of the earth's surface and its relief have different effects on the propagation of waves of different lengths.
So, for example, a mountain range casts "radio shade" in the case of short waves, while rather long (several kilometers) waves go around this obstacle well and on the mountain slope opposite the radio station, they are slightly weakened (Fig. 147).
Rice. 147. The mountain drops "radio shade" in the case of short waves. Long waves go round the mountain
As for the globe as a whole, it is extremely large even in comparison with the longest waves used in radio. Very short waves, for example, meter ones, do not wrap any noticeably beyond the horizon, that is, beyond the line of sight. The longer the waves, the better they go around the surface of the globe, but even the longest of the applied waves could not, due to diffraction, wrap so much as to go around the globe - from us to the antipodes. If, nevertheless, radio communication is carried out between any points of the globe, and at waves of very different lengths, then this is possible not due to diffraction, but for a completely different reason, which we will talk about a little further.
The influence of the physical properties of the earth's surface on the propagation of radio waves is due to the fact that under the influence of these waves in the soil and in sea water, high-frequency electric currents arise, which are strongest near the transmitter antenna. Part of the energy of the radio wave is spent on maintaining these currents, which release the corresponding amount of Joule heat in the soil or water. These energy losses (and hence the attenuation of the wave due to losses) depend, on the one hand, on the conductivity of the soil, and on the other, on the wavelength. Short waves are attenuated much more than long ones. With good conductivity (sea water), high-frequency currents penetrate to a shallower depth from the surface than with poor conductivity (soil), and the energy loss in the first case is significantly less. As a result, the operating range of the same transmitter turns out to be much (several times) greater when waves propagate over the sea than when propagating over land.
We have already noted that the propagation of radio waves over very long distances cannot be explained by diffraction around the globe. Meanwhile, long-distance radio communication (several thousand kilometers) was carried out already in the first years after the invention of radio. Nowadays, every radio amateur knows that longwave (more) and mediumwave 
stations on winter nights are heard at a distance of many thousands of kilometers, while during the day, especially in the summer months, these same stations are heard at a distance of only a few hundred kilometers. In the short wave range 
the situation is different. Here, at any time of the day and at any time of the year, you can find such wavelengths at which any distances are reliably covered. To ensure round-the-clock communication, you have to work at different times of the day on waves of different lengths. The dependence of the range of propagation of radio waves on the time of year and day made it necessary to associate the conditions of propagation of radio waves on Earth with the influence of the Sun. This connection is now well studied and explained.
The sun emits, along with visible light, strong ultraviolet radiation and a large number of fast charged particles, which, falling into the earth's atmosphere, strongly ionize its upper regions. As a result, several layers of ionized gases are formed, located at different heights. 
.
The presence of such traces gave reason to call the upper layers of the earth's atmosphere the ionosphere.
The presence of ions and free electrons gives the ionosphere properties that sharply distinguish it from the rest of the atmosphere. While retaining the ability to transmit visible light, infrared radiation and meter radio waves, the ionosphere strongly reflects longer waves; for such waves (more) the earth is surrounded by a kind of spherical "mirror", and the propagation of these radio waves occurs between two reflecting spherical surfaces - the surface of the Earth and the "surface" of the ionosphere (Fig. 148). That is why radio waves are able to bend around the globe.
Rice. 148. The wave goes between the Earth and the ionosphere
Of course, the words “surface of the spherical mirror of the ionosphere” should not be taken literally. The ionized layers have no sharp boundary; the correct spherical shape is also not observed (at least simultaneously around the entire globe); ionization is different in different layers (in the upper layers it is greater than in the lower ones), and the layers themselves consist of continuously moving and changing "clouds". Such an inhomogeneous "mirror" not only reflects, but also absorbs and scatters radio waves, and again differently depending on the wavelength. In addition, the properties of the "mirror" change over time. During the day, under the action of solar radiation, ionization is significantly greater than at night, when only the reunification of positive ions and negative electrons into neutral molecules (recombination) occurs. The difference in ionization between day and night is especially great in the lower layers of the ionosphere. Here, the air density is higher, collisions between ions and electrons occur more often, and recombination is more intense. During the night, the ionization of the lower layers of the ionosphere may have time to drop to zero. Ionization is also different depending on the season, that is, on the height of the Sun's rise above the horizon.
The study of diurnal and seasonal changes in the state of the ionosphere made it possible not only to explain, but also to predict the conditions for the passage of radio waves of different lengths at different times of the day and year (radio forecasts).
The presence of the ionosphere not only makes possible short-wave communication over long distances, but also allows radio waves to sometimes circle the entire globe, and even several times. Because of this, a peculiar phenomenon occurs during radio reception, the so-called radio echo, in which the signal is perceived by the receiver several times: after the arrival of the signal along the shortest path from the transmitter, repeated signals can be heard that have circled the globe.
It often happens that a wave travels from a transmitter to a receiver along several different paths, having experienced a different number of reflections from the ionosphere and the earth's surface (Fig. 149). Obviously, the waves coming from the same transmitter are coherent and can interfere at the receiving point, weakening or amplifying each other depending on the path difference. Since the ionosphere is not an absolutely stable "mirror", but changes over time, the difference in the paths of waves arriving along different paths from the transmitter to the receiver also changes, resulting in amplification, etc. We can say that the interference fringes "creep" over the surfaces of the Earth, and the receiver is now in the maximum, now in the minimum of oscillations. In the received transmission, a change in good audibility and reception fading is obtained, in which audibility can drop to zero.
Rice. 149. Different wave paths from transmitter to receiver
A similar phenomenon is observed on the TV screen if an airplane flies over the vicinity of the receiving antenna. The radio wave reflected by the airplane interferes with the wave from the transmitting station, and we see how the image "blinks" due to the fact that interference "bands" of alternating gain and attenuation of the signal run (due to the movement of the aircraft) past the receiving antenna.
Note that when receiving a television broadcast in a city, doubling (and even "multiplication") of the image on the kinescope screen is quite often observed: it consists of two or more images, in varying degrees, horizontally shifted relative to each other. This is the result of the reflection of radio waves from houses, towers, etc. The reflected waves travel a longer path than the distance between the transmitting and receiving antennas, and therefore are delayed giving the picture. shifted in the direction of scanning the electron beam in the CRT. In essence, we are witnessing here with our own eyes the result of the propagation of radio waves with a finite speed.
The transparency of the ionosphere for radio waves, the length of which is shorter, made it possible to detect radio emission coming from extraterrestrial sources. It also appeared in the 40s. of our century, radio astronomy is rapidly developing, which has opened up new possibilities for studying the Universe, beyond those available to ordinary (optical) astronomy. More and more radio telescopes are being built, the size of their antennas is increasing, the sensitivity of receivers is increasing, and as a result, the number and variety of discovered extraterrestrial radio sources is constantly increasing.
It turned out that radio waves are emitted by both the Sun and the planets, and outside our solar system - many nebulae and the so-called supernovae. Many sources of radio emission are discovered outside our star system (Galaxy). Basically, these are other galactic systems, and only a small fraction of them are identified with optically observed nebulae. "Radio galaxies" have also been discovered at such great distances from us (many billions of years) that are beyond the reach of the most powerful modern optical telescopes. Intense sources of radio emission with very small angular dimensions (fractions of an arc second) were discovered. Initially, they were considered a special kind of stars belonging to our Galaxy, and therefore were called quasi-stellar sources or quasars. But since 1962, it has become clear that quasars are extragalactic objects with an enormous power of radio emission.
Individual, or, as they say, discrete radio sources in our Galaxy emit a wide range of wavelengths. But there was also discovered "monochromatic" radio emission with a wavelength emitted by interstellar hydrogen. The study of this radiation made it possible to find the total mass of interstellar hydrogen and establish how it is distributed throughout the Galaxy. Most recently, it has been possible to detect monochromatic radio emission at wavelengths characteristic of other chemical elements.
For all the sources of radio emission mentioned above, the intensity is very constant. Only in some cases (in particular, near the Sun) are individual random flashes of radio emission observed against a general constant background. 1968 was marked by a new radio astronomy discovery of great importance: sources were discovered (mostly located within the Galaxy) emitting strictly periodic pulses of radio waves. These sources are called pulsars. Pulse repetition periods for different pulsars are different and range from a few seconds to a few hundredths of a second or even less. The nature of the radio emission from pulsars seems to get the most plausible explanation if we assume that pulsars are rotating stars, consisting mainly of neutrons (neutron stars). The discovery and possibility of observing such stars is the great scientific significance of this radio astronomy discovery.
In addition to receiving their own radio emission from bodies of the solar system, their radar is also used. This is the so-called radar astronomy. By receiving radio signals from powerful radars reflected from any of the planets, one can very accurately measure the distance to this planet, estimate the speed of its rotation around the axis and judge (by the intensity of the reflection of radio waves of various lengths) about the properties of the planet's surface and atmosphere.
In conclusion, we note that the transparency of the ionosphere for sufficiently short radio waves also makes it possible to carry out all types of radio communications with artificial earth satellites and spacecraft (communication proper, radio control, television, as well as telemetry - the transmission of readings of various measuring instruments to the earth). For the same reason, it is now possible to use meter radio waves for communication and television between points of the earth's surface that are very distant from each other (for example, between Moscow and our Far Eastern cities), using a single relay of transmissions by special satellites on which receiving and transmitting radio equipment is installed.
Radio frequency range and its use for radio communication
2.1 Basics of radio propagation
Radio communication provides the transmission of information over a distance using electromagnetic waves (radio waves).
Radio waves- these are electromagnetic oscillations that propagate in space at the speed of light (300,000 km / sec). By the way, light also refers to electromagnetic waves, which determines their very similar properties (reflection, refraction, attenuation, etc.).
Radio waves carry energy emitted by an electromagnetic oscillator through space. And they are born when the electric field changes, for example, when an alternating electric current passes through a conductor or when sparks jump through space, i.e. a series of rapidly following one after another current pulses.
Rice. 2.1 Structure of an electromagnetic wave.
Electromagnetic radiation is characterized by frequency, wavelength and power of the transmitted energy. The frequency of electromagnetic waves shows how many times per second the direction of the electric current changes in the emitter and, therefore, how many times per second the magnitude of the electric and magnetic fields changes at each point in space.
The frequency is measured in hertz (Hz) - units named after the great German scientist Heinrich Rudolf Hertz. 1Hz is one oscillation per second, 1 MegaHertz (MHz) is one million oscillations per second. Knowing that the speed of movement of electromagnetic waves is equal to the speed of light, it is possible to determine the distance between points in space where the electric (or magnetic) field is in the same phase. This distance is called the wavelength.
Wavelength (in meters) is calculated using the formula:
, or roughly
where f is the frequency of electromagnetic radiation in MHz.
It can be seen from the formula that, for example, a frequency of 1 MHz corresponds to a wavelength of about 300 m.With an increase in frequency, the wavelength decreases, with a decrease, it increases.
Electromagnetic waves freely pass through air or outer space (vacuum). But if a metal wire, antenna or any other conducting body meets on the path of the wave, then they give it their energy, thereby causing an alternating electric current in this conductor. But not all of the wave energy is absorbed by the conductor; some of it is reflected from the surface. By the way, the use of electromagnetic waves in radar is based on this.
Another useful property of electromagnetic waves (as well as any other waves) is their ability to bend around bodies on their way. But this is possible only in the case when the size of the body is less than the wavelength, or comparable to it. For example, to detect an airplane, the length of the radar radio wave must be less than its geometric dimensions (less than 10m). If the body is longer than the wavelength, it can reflect it. But it may not reflect - remember "Stealth".
The energy carried by electromagnetic waves depends on the power of the generator (emitter) and the distance to it, i.e. the energy flux per unit area is directly proportional to the radiation power and inversely proportional to the square of the distance to the radiator. This means that the communication range depends on the power of the transmitter, but to a much greater extent on the distance to it.
For example, the energy flow of electromagnetic radiation from the Sun to the Earth's surface reaches 1 kilowatt per square meter, while the energy flow of a medium-wave broadcasting radio station is only thousandths or even millionths of a watt per square meter.
2.2 Allocation of radio spectrum
Radio waves (radio frequencies) used in radio engineering cover a spectrum from 10,000 m (30 kHz) to 0.1 mm (3,000 GHz). This is only part of the vast spectrum of electromagnetic waves. Radio waves (in decreasing length) are followed by heat or infrared rays. After them there is a narrow section of visible light waves, then - the spectrum of ultraviolet, X-ray and gamma rays - all these are electromagnetic oscillations of the same nature, differing only in wavelength and, therefore, in frequency.
Although the entire spectrum is divided into regions, the boundaries between them are roughly outlined. Regions follow continuously one after another, pass one into another, and in some cases overlap.
But these ranges are very extensive and, in turn, are divided into sections, which include the so-called broadcasting and television bands, ranges for terrestrial and aviation, space and maritime communications, for data transmission and medicine, for radar and radio navigation, etc. Each radio service is allocated its own section of the range or fixed frequencies. In reality, for radio communication purposes, oscillations are used in the frequency range from 10 kHz to 100 GHz. The use of a particular frequency interval for communication depends on many factors, in particular, on the propagation conditions of radio waves of different ranges, the required communication range, the feasibility of the transmitter power values in the selected frequency interval, etc.
By international agreements, the entire spectrum of radio waves used in radio communication is divided into ranges (Table 1):
Table 1
| No. of item | Range name | Range boundaries | |||
| Waves | Obsolete terms | Frequencies | Radio waves | Frequencies | |
| 1 | DKMGMVDecaMega Meters | Extremely low frequencies (ELF) | 100.000-10.000km | 3-30 Hz | |
| 2 | MGMV | Ultra-low frequencies (ELF) | 10.000-1.000 km | 30-3.000Hz | |
| 3 | GCMMVHect-kilometer | Infra-low frequencies (LF) | 1.000-100 km | 0.3-3 kHz | |
| 4 | MRMV | ADV | Very Low Frequency (VLF) VLF | 100-10 km | 3-30kHz |
| 5 | KMVKilometer | DV | Low frequencies (LF) LF | 10-1 km | 30-300kHz |
| 6 | GCMVHectameter | SV | Mid frequencies (MF) VF | 1000-100m | 0.3-3 MHz |
| 7 | DKMVDecameter | Kv | Treble (HF) HF | 100-10m | 3-30 MHz |
| 8 | MVMeter | VHF | Very high frequency (VHF) VHF | 10-1m | 30-300 MHz |
| 9 | DCMV | VHF | Ultra High Frequency (UHF) UHF | 10-1 dm | 0.3-3 GHz |
| 10 | SMVS centimeter | VHF | Ultra-high frequency (microwave) SHF | 10-1 cm | 3-30 GHz |
| 11 | MMVMillimeter | VHF | Extreme High Frequency (EHF) EHF | 10-1 mm | 30-300 GHz |
| 12 | DCMMVDetsimilli- meter Submillie- meter |
SUM | Hyper-high frequencies (HHF) | 1-0.1 mm | 0.3-3 THz |
| 13 | Light | < 0,1 мм | > 3 THz | ||
Rice. 2.2 An example of spectrum allocation between different services.
Radio waves are radiated through the antenna into space and propagated as energy in an electromagnetic field. Although the nature of radio waves is the same, their propagation ability is highly dependent on the wavelength.
For radio waves, earth is a conductor of electricity (although not a very good one). Passing over the surface of the earth, radio waves gradually weaken. This is due to the fact that electromagnetic waves excite electric currents in the surface of the earth, for which part of the energy is spent. Those. energy is absorbed by the earth, and the more, the shorter the wavelength (higher frequency).
In addition, the energy of the wave also weakens because the radiation propagates in all directions of space and, therefore, the further the receiver is from the transmitter, the less energy falls on a unit area and the less it gets into the antenna.
Long-wave broadcasts can be received at a distance of up to several thousand kilometers, and the signal level decreases smoothly, without jumps. Medium wave stations can be heard within a thousand kilometers. As for short waves, their energy sharply decreases with distance from the transmitter. This explains the fact that at the dawn of radio development, waves from 1 to 30 km were mainly used for communication. Waves shorter than 100 meters were generally considered unsuitable for long-distance communications.
However, further studies of short and ultrashort waves have shown that they quickly decay when they travel near the Earth's surface. When the radiation is directed upward, short waves come back.
Back in 1902, the English mathematician Oliver Heaviside and the American electrical engineer Arthur Edwin Kennelly predicted almost simultaneously that there is an ionized layer of air above the Earth - a natural mirror that reflects electromagnetic waves. This layer was named ionosphere.
The ionosphere of the Earth was supposed to allow increasing the range of propagation of radio waves to distances exceeding the line of sight. This assumption was experimentally proven in 1923. RF pulses were transmitted vertically upward and returned signals were received. Measurements of the time between sending and receiving pulses made it possible to determine the height and number of reflection layers.
2.3 Influence of the atmosphere on radio wave propagation
The nature of the propagation of radio waves depends on the wavelength, curvature of the Earth, soil, atmospheric composition, time of day and year, the state of the ionosphere, the Earth's magnetic field, and meteorological conditions.
Let us consider the structure of the atmosphere, which has a significant effect on the propagation of radio waves. Moisture content and air density change depending on the time of day and year.
The air surrounding the earth's surface forms an atmosphere that is approximately 1000-2000 km high. The composition of the earth's atmosphere is heterogeneous.
Rice. 2.3 The structure of the atmosphere.
Layers of the atmosphere up to about 100-130 km in height are homogeneous in composition. These layers contain air containing (by volume) 78% nitrogen and 21% oxygen. The lower layer of the atmosphere 10-15 km thick (Fig. 2.3) is called troposphere... This layer contains water vapor, the content of which fluctuates sharply with changes in meteorological conditions.
The troposphere gradually turns into stratosphere... The boundary is the height at which the temperature drop stops.
At altitudes of about 60 km and higher above the Earth, under the influence of solar and cosmic rays in the atmosphere, air ionization occurs: some of the atoms decay into free electrons and ions... In the upper atmosphere, ionization is negligible, since the gas is very rarefied (there are a small number of molecules per unit volume). As the sun's rays penetrate into the denser layers of the atmosphere, the degree of ionization increases. With approaching the Earth, the energy of the sun's rays decreases, and the degree of ionization decreases again. In addition, in the lower layers of the atmosphere, due to the high density, negative charges cannot exist for a long time; there is a process of restoration of neutral molecules.
Ionization in a rarefied atmosphere at altitudes of 60-80 km from the Earth and higher persists for a long time. At these altitudes, the atmosphere is very rarefied, the density of free electrons and ions is so low that collisions, and hence the restoration of neutral atoms, are relatively rare.
The upper atmosphere is called the ionosphere. Ionized air has a significant effect on the propagation of radio waves.
During the day, four regular layers or ionization maxima are formed - layers D, E, F 1 and F 2. The F 2 layer has the highest ionization (the largest number of free electrons per unit volume).
After sunset, ionizing radiation drops sharply. The restoration of neutral molecules and atoms occurs, which leads to a decrease in the degree of ionization. Layers disappear completely at night D and F 2, layer ionization E decreases significantly, and the layer F 2 retains ionization with some attenuation.
Rice. 2.4 Dependence of radio wave propagation on frequency and time of day.
The height of the layers of the ionosphere changes all the time depending on the intensity of the sun's rays. During the day, the height of the ionized layers is lower, at night, it is higher. In summer, at our latitudes, the electron concentration of ionized layers is higher than in winter (with the exception of the layer F 2). The degree of ionization also depends on the level of solar activity, determined by the number of spots on the sun. The period of solar activity is approximately 11 years.
Irregular ionization processes associated with so-called ionospheric disturbances are observed at polar latitudes.
There are several ways in which the radio wave arrives at the receiving antenna. As already noted, radio waves propagating above the earth's surface and enveloping it due to the phenomenon of diffraction are called surface or earth waves (direction 1, Fig. 2.5). Waves propagating in directions 2 and 3 are called spatial... They are divided into ionospheric and tropospheric. The latter are observed only in the VHF range. Ionospheric waves are called, reflected or scattered by the ionosphere, tropospheric- waves reflected or scattered by inhomogeneous layers or "grains" of the troposphere.
Rice. 2.5 Ways of propagation of radio waves.
Surface wave the base of its front touches the Earth, as shown in Fig. 2.6. With a point source, this wave always has vertical polarization, since the horizontal component of the wave is absorbed by the Earth. With sufficient distance from the source, expressed in wavelengths, any segment of the wave front is a plane wave.
The surface of the Earth absorbs part of the energy of surface waves propagating along it, since the Earth has an active resistance.
Rice. 2.6 Propagation of surface waves.
The shorter the wave, i.e. the higher the frequency, the more current is induced in the Earth and the greater the loss. Losses in the Earth decrease with an increase in the conductivity of the soil, since the waves penetrate into the Earth, the less, the higher the conductivity of the soil. Dielectric losses also occur in the Earth, which also increase with the shortening of the wave.
For frequencies above 1 MHz, the surface wave is in fact highly attenuated due to absorption by the Earth and is therefore not used except in the local coverage area. At television frequencies, the attenuation is so great that the surface wave can be used at distances of no more than 1-2 km from the transmitter.
Communication over long distances is carried out mainly by space waves.
To receive refraction, that is, the return of a wave to the Earth, the wave must be emitted at a certain angle with respect to the earth's surface. The largest angle of radiation at which a radio wave of a given frequency returns to the ground is called critical angle for a given ionized layer (Fig. 2.7). 
Rice. 2.7 Influence of the angle of radiation on the passage of the sky wave.
Each ionized layer has its own critical frequency and critical angle.
In fig. 2.7 shows a ray that is easily refracted by a layer E since the ray enters at an angle below the critical angle of this layer. Beam 3 passes the area E but returns to Earth in a layer F 2 because it enters at an angle below the critical angle of the layer F 2. Beam 4 also passes through the layer E... It enters the layer F 2 at its critical angle and returns to Earth. Beam 5 passes through both areas and is lost in space.
All rays shown in Fig. 2.7 refer to one frequency. If a lower frequency is used, larger critical angles are required for both regions; conversely, if the frequency increases, both regions have smaller critical angles. If you continue to increase the frequency, then there will come a moment when the wave propagating from the transmitter parallel to the Earth will exceed the critical angle for any region. This condition occurs at a frequency of about 30 MHz. Above this frequency, skywave communication becomes unreliable.
So, each critical frequency has its own critical angle, and, conversely, each critical angle has its own critical frequency. Consequently, any sky wave, the frequency of which is equal to or lower than the critical one, will return to Earth at a certain distance from the transmitter.
In fig. 2.7 ray 2 falls on layer E at a critical angle. Note where the reflected wave hits the Earth (when the critical angle is exceeded, the signal is lost); The space wave, having reached the ionized layer, is reflected from it and returns to the Earth at a great distance from the transmitter. At some distance from the transmitter, depending on the transmitter power and wavelength, it is possible to receive a surface wave. From the point where the reception of the surface wave ends, silence zone and it ends where the reflected spatial wave appears. The zone of silence does not have a sharp border.
Rice. 2.8 Reception areas of surface and spatial waves.
As the frequency increases, the quantity dead zone increases due to a decrease in the critical angle. To communicate with a correspondent at a certain distance from the transmitter at certain times of the day and seasons of the year, there is maximum permissible frequency that can be used for skywave communication. Each ionospheric region has its own maximum allowable frequency for communication.
Short and, moreover, ultrashort waves in the ionosphere lose an insignificant part of their energy. The higher the frequency, the less path the electrons travel during their oscillations, as a result of which the number of their collisions with molecules decreases, i.e., the energy losses of the wave decrease.
In lower ionized layers, the losses are greater, since an increased pressure indicates a higher gas density, and with a higher gas density, the probability of collision of particles increases.
Long waves are reflected from the lower layers of the ionosphere, which have the lowest electron concentration, at any elevation angles, including those close to 90 °. Medium moisture soil is almost a conductor for long waves, so they reflect well from the Earth. Multiple reflections from the ionosphere and the Earth explain the long-range propagation of long waves.
Long wave propagation does not depend on the season and meteorological conditions, on the period of solar activity and on ionospheric disturbances. When reflected from the ionosphere, long waves undergo large absorption. This is why high power transmitters are necessary for long distance communication.
Medium waves are noticeably absorbed in the ionosphere and soil of poor and medium conductivity. During the day, only a surface wave is observed, since a space wave (longer than 300 m) is almost completely absorbed in the ionosphere. For complete internal reflection, the average waves must travel a certain path in the lower layers of the ionosphere, which, although they have a low concentration of electrons, have a significant air density.
At night, with the disappearance of the D layer, the absorption in the ionosphere decreases, as a result of which it is possible to maintain communication on space waves at distances of 1500-2000 km with a transmitter power of about 1 kW. Communication conditions are somewhat better in winter than in summer.
The virtue of medium waves is that they are not affected by ionospheric disturbances.
According to international agreement, distress signals (SOS signals) are transmitted on waves of about 600 m.
The positive side of skywave communication at short and medium waves is the possibility of long-distance communication with a low transmitter power. But space wave link has and significant disadvantages.
At first, the instability of communication due to changes in the height of the ionized layers of the atmosphere during the day and year. To maintain communication with the same point, you have to change the wavelength 2-3 times per day. Often, due to a change in the state of the atmosphere, communication is completely disrupted for some time.
Secondly, the presence of a zone of silence.
Waves shorter than 25 m are referred to as "daytime waves" as they travel well during the day. "Night waves" include waves longer than 40 m. These waves propagate well at night.
The conditions for the propagation of short radio waves are determined by the state of the ionized layer Fg. The electron concentration of this layer is often disturbed due to uneven solar radiation, which causes ionospheric disturbances and magnetic storms. As a result, the energy of short radio waves is significantly absorbed, which degrades radio communication, even sometimes makes it completely impossible. Ionospheric disturbances are especially often observed at latitudes close to the poles. Therefore, shortwave communication there is unreliable.
Most notable ionospheric disturbances have their own periodicity: they are repeated after 27 days(time of rotation of the Sun around its axis).
In the short wave range, the influence of industrial, atmospheric and mutual interference is strongly affected.
Optimal communication frequencies on short waves are selected on the basis of radio forecasts, which are divided into long-term and short term... Long-term forecasts indicate the expected average state of the ionosphere for a certain period of time (month, season, year or more), while short-term forecasts are made for a day, five days and characterize possible deviations of the ionosphere from its average state. Forecasts are drawn up in the form of graphs as a result of processing systematic observations of the ionosphere, solar activity and the state of terrestrial magnetism.
Ultrashort waves(VHF) are not reflected from the ionosphere, they freely pass through it, that is, these waves do not have a spatial ionospheric wave. The surface ultrashort wave, on which radio communication is possible, has two significant drawbacks: firstly, the surface wave does not go around the earth's surface and large obstacles, and, secondly, it is strongly absorbed in the soil.
Ultrashort waves are widely used where a short range of a radio station is required (communication is usually limited to line-of-sight). In this case, communication is carried out by a spatial tropospheric wave. It usually consists of two components: a direct ray and a ray reflected from the Earth (Fig. 2.9).
Rice. 2.9 Direct and reflected rays of the sky wave.
If the antennas are close enough, both beams usually reach the receiving antenna, but their intensities are different. The beam reflected from the Earth is weaker due to losses that occur during the reflection from the Earth. A direct beam has almost the same attenuation as a free-space wave. In the receiving antenna, the total signal is equal to the vector sum of these two components.
The receiving and transmitting antennas are usually at the same height, so the path length of the reflected beam is slightly different from the direct beam. The reflected wave is 180 ° out of phase. Thus, neglecting the losses in the Earth during reflection, if two beams have traveled the same distance, their vector sum is zero, as a result, there will be no signal in the receiving antenna.
In reality, the reflected beam travels a slightly greater distance, therefore, the phase difference in the receiving antenna will be about 180 °. The phase difference is determined by the path difference in terms of wavelength, not in linear units. In other words, the total signal received under these conditions depends mainly on the frequency used. For example, if the operating wavelength is 360 m and the path difference is 2 m, the phase shift will differ from 180 ° by only 2 °. As a result, there is an almost complete absence of a signal in the receiving antenna. If the wavelength is 4 m, the same 2 m path difference will cause a 180 ° phase difference, fully compensating for the 180 ° phase shift in reflection. In this case, the signal is doubled in voltage.
It follows from this that at low frequencies the use of space waves is not of interest for communication. Only at high frequencies, where the path difference is commensurate with the wavelength used, is the skywave widely used.
The range of VHF transmitters is significantly increased when aircraft communicate in the air and with the Earth.
TO advantages of VHF should include the possibility of using small antennas. In addition, a large number of radio stations can operate simultaneously in the VHF band without mutual interference. More stations can be deployed simultaneously in the 10 to 1 m wavelength range than in the short, medium and long wavelengths combined.
VHF relay lines have become widespread. Between two communication points located at a great distance, several VHF transceivers are installed, located within the line of sight from one another. Intermediate stations work automatically. The organization of retransmission lines allows to increase the communication range on VHF and to carry out multichannel communication (conduct several telephone and telegraph transmissions at the same time).
Much attention is now paid to the use of the VHF band for long-distance radio communications.
The most widely used communication lines operating in the range of 20-80 MHz and using the phenomena of ionospheric scattering. It was believed that radio communication through the ionosphere is possible only at frequencies below 30 MHz (wavelength over 10 m), and since this range is fully loaded and a further increase in the number of channels in it is impossible, the interest in scattered propagation of radio waves is understandable.
This phenomenon consists in the fact that some of the energy of ultrahigh frequency radiation is scattered by irregularities in the ionosphere. These inhomogeneities are created by air currents of layers with different temperatures and humidity, wandering charged particles, ionization products of meteorite tails and other still poorly studied sources. Since the troposphere is always inhomogeneous, scattered refraction of radio waves exists systematically.
Scattered propagation of radio waves is similar to the scattering of light from a searchlight on a dark night. The more powerful the light beam, the more it gives off scattered light.
When studying distant spread of ultrashort waves, the phenomenon of a sharp short-term increase in the audibility of signals was noticed. Such random bursts last from several milliseconds to several seconds. However, in practice, they are observed during the day with interruptions rarely exceeding a few seconds. The appearance of moments of increased audibility is mainly due to the reflection of radio waves from ionized layers of meteorites burning at an altitude of about 100 km. The diameter of these meteorites does not exceed a few millimeters, and their tracks stretch for several kilometers.
From meteorite tracks radio waves with a frequency of 50-30 MHz (6-10 m) are well reflected.
Several billion such meteorites fly into the earth's atmosphere every day, leaving behind ionized trails with a high density of air ionization. This makes it possible to obtain reliable operation of long-distance radio links when using transmitters of relatively low power. An integral part of stations on such lines is an auxiliary direct-printing equipment equipped with a memory element.
Since each meteorite trail exists for only a few seconds, transmission is carried out automatically in short bursts.
At present, communication and television transmissions through artificial earth satellites are widely used.
Thus, according to the mechanism of radio wave propagation, radio communication lines can be classified into lines using:
the process of propagation of radio waves along the earth's surface with bending around it (the so-called earthly or surface waves);
the process of propagation of radio waves within the line of sight ( straight waves);
reflection of radio waves from the ionosphere ( ionospheric waves);
the process of propagation of radio waves in the troposphere ( tropospheric waves);
reflection of radio waves from meteor trails;
reflection or retransmission from artificial earth satellites;
reflection from artificially created formations of gas plasma or artificially created conducting surfaces.
2.4 Features of the propagation of radio waves of various bands
The conditions for the propagation of radio waves in the space between the transmitter and the radio receiver of the correspondents are influenced by the finite conductivity of the earth's surface and the properties of the medium above the earth. This effect is different for different wavelengths (frequencies).
Myriameter and kilometer waves (ADV and DV) can propagate both terrestrial and ionospheric. The presence of an earth wave propagating over hundreds and even thousands of kilometers is explained by the fact that the field strength of these waves decreases rather slowly with distance, since the absorption of their energy by the earth or water surface is small. The longer the wave and the better the conductivity of the soil, the longer radio communication is provided.
Sandy dry soils and rocks absorb electromagnetic energy to a large extent. When propagating due to the phenomenon of diffraction, they bend around the convex earth's surface, obstacles encountered in the way: forests, mountains, hills, etc. Starting from a distance of 300-400 km from the transmitter, an ionospheric wave appears, reflected from the lower region of the ionosphere (from the D or E layer). During the day, due to the presence of the D layer, the absorption of electromagnetic energy becomes more significant. At night, with the disappearance of this layer, the communication range increases. Thus, the passage of long waves at night is generally better than during the day. Global communications in the VLF and LW are carried out by waves propagating in a spherical waveguide formed by the ionosphere and the earth's surface.
Advantage of SDV-, DV- band:
VLF and LW radio waves have the property of penetrating into the water column, as well as propagating in some soil structures;
due to waves propagating in the spherical waveguide of the Earth, communication is provided for thousands of kilometers;
communication range depends little on ionospheric disturbances;
good diffraction properties of radio waves in these ranges make it possible to provide communication for hundreds and even thousands of kilometers with an earth wave;
The constancy of the parameters of the radio link ensures a stable signal level at the receiving point.
disadvantagesSDV-, DV, - ranges:
effective radiation of waves of the considered parts of the range can be achieved only with the help of very bulky antenna devices, the dimensions of which are commensurate with the wavelength. The construction and restoration of antenna devices of this size in a limited time (for military purposes) is difficult;
since the dimensions of the actually manufactured antennas are less than the wavelength, then compensation for their reduced efficiency is achieved by increasing the power of the transmitters to hundreds or more kW;
the creation of resonant systems in this range and at significant powers determines the large sizes of the output stages: transmitters, the complexity of fast tuning to another frequency;
for power supply of VLF- and DV-band radio stations), large power plants are required;
a significant disadvantage of the VLF and LW ranges is their low frequency capacity;
a sufficiently high level of industrial and atmospheric interference;
dependence of the signal level at the receiving point on the time of day.
Scope of practical application of VLF-, DV-band radio waves:
communication with underwater objects;
global backbone and underground communications;
radio beacons, as well as communications in long-range aviation and the Navy.
Hectometer waves(SV) can be propagated by surface and space waves. Moreover, the range of communication with a surface wave is shorter (does not exceed 1000-1500 km), since their energy is absorbed by the soil more than that of long waves. Waves reaching the ionosphere are intensely absorbed by the layer D when it exists, but is well discharged in a layer E.
For medium waves, the communication range is very dependent from time of day. During the day, the middle waves are so strong absorbed in the lower layers of the ionosphere, that the sky wave is practically absent. Night layer D and the bottom of the layer E disappear, so the absorption of medium waves decreases; and space waves begin to play a major role. Thus, an important feature of medium waves is that during the day, communication on them is maintained by a surface wave, and at night - by both surface and space waves simultaneously.
Benefits of the CB band:
at night in summer and during most of the day in winter, the communication range provided by the ionospheric wave reaches thousands of kilometers;
medium-wave antenna devices are quite effective and have acceptable dimensions even for mobile radio communications;
the frequency capacity of this range is greater than that of the VLF and LW ranges;
good diffraction properties of radio waves in this range;
the power of the transmitters is less than that of the VLF and LW bands;
low dependence on ionospheric disturbances and magnetic storms.
Disadvantages of the CB range:
the congestion of the MW band with powerful broadcasting radio stations creates difficulties in widespread use;
the limited frequency capacity of the range makes it difficult to maneuver frequencies;
the communication range on the NE in the daytime in summer is always limited, since it is possible only by an earth wave;
sufficiently high transmitter powers;
it is difficult to use highly efficient antenna devices, the complexity of construction and restoration in a short time;
a sufficiently high level of mutual and atmospheric interference.
The area of practical application of CB-band radio waves; Medium-wave radio stations are most often used in the Arctic regions, as a backup in cases of loss of widely used short-wave radio communications due to ionospheric and magnetic disturbances, as well as in long-range aviation and the Navy.
Decameter waves (KB) occupy a special position. They can propagate both terrestrial and ionospheric waves. With relatively low transmitter powers typical of mobile radio stations, ground waves propagate over distances not exceeding several tens of kilometers, since they experience significant absorption in the ground, which increases with increasing frequency.
Ionospheric waves, due to single or multiple reflections from the ionosphere, under favorable conditions can propagate over long distances. Their main property is that they are weakly absorbed by the lower regions of the ionosphere (layers D and E) and are well reflected by its upper regions (mainly by the layer F2 ... located at an altitude of 300-500 km above the ground). This makes it possible to use relatively low-power radio stations for direct communication over an infinitely wide range of distances.
A significant decrease in the quality of HF radio communication by ionospheric waves occurs due to signal fading. The nature of fading is mainly reduced to the interference of several rays arriving at the receiving site, the phase of which is constantly changing due to a change in the state of the ionosphere.
The reasons for the arrival of several beams at the place of receiving signals can be:
irradiation of the ionosphere at angles at which the rays undergoing
different number of reflections from the ionosphere and the Earth, converge at the point of reception;
the phenomenon of birefringence under the influence of the Earth's magnetic field, due to which two beams (ordinary and extraordinary), reflecting from different layers of the ionosphere, reach the same receiving point;
inhomogeneity of the ionosphere, leading to diffuse reflection of waves from its various regions, i.e. to the reflection of beams of many elementary rays.
Fading can also occur due to polarization fluctuations of waves when reflected from the ionosphere, leading to a change in the ratio of the vertical and horizontal components of the electric field at the receiving point. Polarization fading is observed much less frequently than interference fading and accounts for 10-15% of their total number.
As a result of fading, the signal level at the receiving points can vary over a wide range - tens and even hundreds of times. The time interval between deep fading is a random value and can vary from tenths of a second to several seconds, and sometimes more, and the transition from a high to a low level can be either smooth or very abrupt. Fast level changes often overlap with slow ones.
The conditions for the passage of short waves through the ionosphere vary from year to year, which is associated with an almost periodic change in solar activity, i.e. with a change in the number and area of sunspots (Wolf number), which are sources of radiation that ionize the atmosphere. The recurrence period of the maximum solar activity is 11.3 ± 4 years. During the years of maximum solar activity, the maximum usable frequencies (MUF) increase, and the areas of operating frequency ranges expand.
In fig. 2.10 shows a typical family of daily MUF and least usable frequencies (LUF) plots for a radiated power of 1 kW.
Rice. 2.10 The course of the MUF and NUF curves.
This family of daily charts corresponds to specific geographic areas. It follows from this that the applicable frequency range for communication over a given distance may be very small. It should be borne in mind that ionospheric forecasts may have an error, therefore, when choosing the maximum communication frequencies, they try not to exceed the line of the so-called optimal operating frequency (OPF), passing below the MUF line by 20-30%. It goes without saying that the working width of the range is further reduced from this. The decrease in the signal level when approaching the maximum usable frequency is explained by the variability of the parameters of the ionosphere.
Due to the fact that the state of the ionosphere changes, communication by an ionospheric wave requires the correct choice of frequencies during the day:
DAY using frequencies 12-30 MHz,
MORNING and EVENING 8-12 MHz, NIGHT 3-8 MHz.
It can also be seen from the graphs that with a decrease in the length of the radio communication line, the section of applicable frequencies decreases (for distances up to 500 km at night, it can be only 1-2 MHz).
The conditions of radio communication for long lines are more favorable than for short ones, since there are fewer of them, and the range of suitable frequencies for them is much wider.
Ionospheric and magnetic storms can have a significant effect on the state of HF radio communication (especially in the polar regions), i.e. disturbances of the ionosphere and the Earth's magnetic field under the influence of charged particle streams erupted by the Sun. These streams often destroy the main reflective ionospheric layer F2 in the region of high geomagnetic latitudes. Magnetic storms can manifest themselves not only in the polar regions, but throughout the entire globe. Ionospheric disturbances have a periodicity and are associated with the time of revolution of the Sun around its axis, which is equal to 27 days.
Short waves are characterized by the presence of zones of silence (dead zones). The zone of silence (Fig. 2.8) occurs during radio communication over long distances in areas to which the surface wave does not reach due to its attenuation, and the space wave is reflected from the ionosphere at a greater distance. This occurs when using narrow beam antennas when radiating at small angles to the horizon.
Advantages of the HF band:
Ionospheric waves can travel long distances due to single or multiple reflections from the ionosphere under favorable conditions. They are weakly absorbed by the lower regions of the ionosphere (D and E layers) and are well reflected by the upper ones (mainly by the F2 layer);
the ability to use relatively low-power radio stations for direct communication over an infinitely wide range of distances;
the frequency capacity of the HF band is much greater than that of the VLF, DV, and MW bands, which makes it possible to simultaneously operate a large number of radio stations;
Antenna devices used in the decameter wave range have acceptable dimensions (even for installation on moving objects) and can have pronounced directional properties. They have short deployment times, are cheap, and are easily recoverable in the event of damage.
Disadvantages of the HF band:
radio communication by ionospheric waves can be carried out if the frequencies used are below the maximum values (MUF) determined for each length of the radio communication line by the degree of ionization of the reflecting layers;
communication is possible only if the power of the transmitters and the gains of the antennas used, with the energy absorption in the ionosphere taking place, provide the necessary strength of the electromagnetic field at the point of reception. This condition limits the lower limit of usable frequencies (LUF);
insufficient frequency capacity for the use of broadband modes of operation and frequency maneuvering;
a huge number of simultaneously operating radio stations with a long communication range creates a large level of mutual interference;
long communication range makes it easy for the enemy to use deliberate interference;
the presence of zones of silence when providing communication over long distances;
a significant decrease in the quality of HF radio communication by ionospheric waves due to fading of signals arising due to the variability of the structure of the reflecting layers of the ionosphere, its constant disturbance and multipath propagation of waves.
Practical application of HF radio waves
KB radio stations find the widest practical application for communication with remote subscribers.
Meter waves (VHF) include a number of sections of the frequency range that have a huge frequency capacity.
Naturally, these areas differ significantly from each other in the properties of radio wave propagation. The VHF energy is strongly absorbed by the Earth (in the general case, in proportion to the square of the frequency), so the Earth wave decays rather quickly. For VHF, regular reflection from the ionosphere is unusual, therefore, the communication is calculated on the use of an earth wave and a wave propagating in free space. Space waves shorter than 6-7 m (43-50 MHz), as a rule, pass through the ionosphere without being reflected from it.
VHF propagation occurs in a straight line, the maximum range is limited by the line-of-sight range. It can be determined by the formula:
where Dmax is the line-of-sight range, km;
h1 is the height of the transmitting antenna, m;
h2 - receiving antenna height, m.
However, due to refraction (refraction), the propagation of radio waves is curved. In this case, in the range formula, the coefficient will not be 3.57, but 4.1-4.5. From this formula it follows that to increase the VHF communication range, it is necessary to raise the transmitter and receiver antennas higher.
An increase in the transmitter power does not lead to a proportional increase in the communication range, therefore, low-power radio stations are used in this range. Tropospheric and ionospheric scatter communications require significant transmitters.
At first glance, the VHF terrestrial wave communication range should be very short. However, it should be borne in mind that with an increase in frequency, the efficiency of antenna devices increases, due to which energy losses in the Earth are compensated.
The communication range by terrestrial waves depends on the wavelength. The longest range is achieved on meter waves, especially on waves adjacent to the HF band.
Meter waves have the property diffraction, i.e. the property to bend around the unevenness of the terrain. The increase in the communication range at meter waves is facilitated by the phenomenon of tropospheric refraction, i.e. the phenomenon of refraction in the troposphere, which ensures communication on closed routes.
In the range of meter waves, long-range propagation of radio waves is often observed, which is due to a number of reasons. Long-range propagation can occur with the formation of sporadic ionized clouds ( sporadic layer Fs). It is known that this layer can appear at any time of the year or day, but for our hemisphere - mainly in late spring and early summer in the daytime. A feature of these clouds is a very high ionic concentration, sometimes sufficient to reflect waves of the entire VHF range. In this case, the area of the location of radiation sources relative to the receiving points is most often at a distance of 2000-2500 km, and sometimes closer. The intensity of signals reflected from the Fs layer can be very high even at very low source powers.
Another reason for the long-distance propagation of meter waves in the years of maximum solar activity may be the regular F2 layer. This propagation manifests itself in the winter months at the illuminated time of the reflection points, i.e. when the absorption of wave energy in the lower regions of the ionosphere is minimal. In this case, the communication range can reach global scales.
Long-distance propagation of meter waves can also occur during high-altitude nuclear explosions. In this case, in addition to the lower region of increased ionization, an upper one appears (at the level of the Fs layer). Meter waves penetrate the lower region, experiencing some absorption, are reflected from the upper and return to the Earth. The distances covered in this case are in the range from 100 to 2500 km. Field strength reflected of those waves depends on frequency: the lowest frequencies undergo the greatest absorption in the lower ionization region, and the highest ones experience incomplete reflection from the upper region.
The interface between KB and meter waves passes at a wavelength of 10 m (30 MHz). The propagation properties of radio waves cannot change abruptly, i.e. there must be a region or section of frequencies that is transitional... Such a section of the frequency range is a section of 20-30 MHz. In the years of minimum solar activity (as well as at night, regardless of the phase of activity), these frequencies are practically unsuitable for long-distance communication by ionospheric waves and their use turns out to be extremely limited. At the same time, under the indicated conditions, the properties of wave propagation in this area become very close to the properties of meter waves. It is no coincidence that this section of frequencies is used in the interests of radio communication, oriented to meter waves.
Advantages of the VHF band:
the small dimensions of the antennas make it possible to realize a pronounced directional radiation, which compensates for the rapid attenuation of the radio wave energy;
propagation conditions generally do not depend on the time of day and year, as well as solar activity;
limited communication range allows multiple use of the same frequencies on surface areas, the distance between the boundaries of which is not less than the sum of the range of radio stations with the same frequencies;
lower level of unintentional (natural and artificial) and intentional interference due to narrow directional antennas and og limited communication range;
huge frequency capacity, allowing the use of anti-jamming broadband signals for a large number of simultaneously operating stations;
when using broadband signals for radio communication, the frequency instability of the radio link is sufficient δf = 10 -4;
the ability of VHF to penetrate the ionosphere without significant energy losses made it possible to carry out space radio communications over distances measured in millions of kilometers;
high quality radio channel;
because of the very low energy losses in free space, the communication range between aircraft equipped with relatively low-power radio stations can reach several hundred kilometers;
property of long-range propagation of meter waves;
low power of transmitters and a small dependence of the communication range on the power.
Disadvantages of the VHF range:
short range of radio communication with an earth wave, practically limited by line of sight;
when using narrowly directed antennas, it is difficult to work with several correspondents;
when using antennas with a circular directivity, the communication range, intelligence protection, and noise immunity are reduced.
The area of practical application of VHF-Dianazon radio waves The range is used simultaneously by a large number of radio stations, especially since the range of mutual interference between them, as a rule, is not large. The properties of the propagation of ground waves provide a wide application of ultrashort waves for communication in the tactical control link, including between various types of mobile objects. Interplanetary communication.
Considering the advantages and disadvantages of each band, we can conclude that the most acceptable ranges for low-power radio stations are the decameter (KB) and meter (VHF) wavelengths.
2.5 Influence of nuclear explosions on the state of radio communications
In nuclear explosions, instantaneous gamma radiation, interacting with the atoms of the environment, creates a stream of fast electrons flying at high speed mainly in the radial direction from the center of the explosion, and positive ions that remain practically in place. Thus, in space, for some time, there is a separation of positive and negative charges, which leads to the emergence of electric and magnetic fields. Due to their short duration, these fields are usually called electromagnetic pulse (AMY) nuclear explosion. The duration of its existence is approximately 150-200 milliseconds.
Electromagnetic pulse (the fifth damaging factor of a nuclear explosion) in the absence of special protection measures, it can damage the control and communication equipment, disrupt the operation of electrical devices connected to long external lines.
Communication, signaling and control systems are most susceptible to the effect of an electromagnetic pulse from a nuclear explosion. As a result of the impact of the EMP of a ground or air nuclear explosion on the antennas of radio stations, an electric voltage is induced in them, under the influence of which a breakdown of insulation, transformers, melting of wires, failure of arresters, damage to electronic lamps, semiconductor devices, capacitors, resistances, etc. ...
It was found that when EMP is applied to the equipment, the highest voltage is induced on the input circuits. With regard to transistors, the following dependence is observed: the higher the transistor gain, the lower its dielectric strength.
The radio equipment has a constant voltage dielectric strength of no more than 2-4 kV. Considering that the electromagnetic pulse of a nuclear explosion is short-lived, the ultimate electric strength of equipment without protective equipment can be considered higher - approximately 8-10 kV.
Table 1 shows the approximate distances (in km) at which dangerous voltages for equipment exceeding 10 and 50 kV are induced in the antennas of radio stations at the time of a nuclear explosion.
Table 1
At larger distances, the effect of EMR is similar to the effect of a not very distant lightning discharge and does not cause damage to the equipment.
The impact of an electromagnetic pulse on radio equipment is sharply reduced in the case of the application of special protection measures.
The most affective way to protect radio electronic equipment located in structures is the use of electrically conductive (metal) screens, which significantly reduce the magnitude of voltages induced on internal wires and cables. Protective equipment similar to lightning protection means is used: arresters with drainage and locking coils, fuse-links, decoupling devices, circuits for automatic disconnection of equipment from the line.
A good protective measure is also a reliable grounding of the equipment at one point. The implementation of radio engineering devices is also effective in block-by-block, with the protection of each block and the entire device as a whole. This makes it possible to quickly replace a failed unit with a backup one (in the most critical equipment, the units are duplicated with automatic switching when the main ones are damaged). In some cases, selenium elements and stabilizers can be used to protect against EMP.
In addition, can be applied protective entrance devices, which are various relay or electronic devices that react to overvoltage in the circuit. When a voltage pulse arrives, induced in the line by an electromagnetic pulse, they turn off the power from the device or simply break the working circuits.
When choosing protective devices, it should be borne in mind that the impact of EMP is characterized by massiveness, that is, the simultaneous operation of protective equipment in all circuits caught in the explosion area. Therefore, the applied protection circuits should automatically restore the operability of the circuits immediately after the termination of the electromagnetic pulse.
The resistance of equipment to the effects of voltages arising in lines during a nuclear explosion depends to a large extent on the correct operation of the line and careful monitoring of the serviceability of protective equipment.
TO important operating requirements includes a periodic and timely check of the electrical strength of the insulation of the line and the input circuits of the equipment, the timely identification and elimination of emerging wire grounding, monitoring the serviceability of arresters, fuse-links, etc.
High altitude nuclear explosion accompanied by the formation of regions of increased ionization. In explosions at altitudes up to about 20 km, the ionized region is limited first by the size of the luminous region, and then by the explosion cloud. At altitudes of 20-60 km, the dimensions of the ionized region are somewhat larger than the dimensions of the explosion cloud, especially at the upper boundary of this altitude range.
In nuclear explosions at high altitudes, two regions of increased ionization appear in the atmosphere.
First area is formed in the area of the explosion due to the ionized substance of the ammunition and the ionization of the air by the shock wave. The dimensions of this area in the horizontal direction reach tens and hundreds of meters.
Second area increased ionization occurs below the center of the explosion in the atmosphere at altitudes of 60-90 km as a result of absorption of penetrating radiation by air. The distances at which penetrating radiation produce ionization are hundreds and even thousands of kilometers in the horizontal direction.
Areas of increased ionization arising from a high-altitude nuclear explosion absorb radio waves and change the direction of their propagation, which leads to a significant disruption in the operation of radio equipment. In this case, there are interruptions in radio communication, and in some cases it is completely disrupted.
The nature of the damaging effect of the electromagnetic pulse of high-altitude nuclear explosions is basically similar to the nature of the damaging effect of the EMP of ground and air explosions.
The measures of protection against the damaging effect of the electromagnetic pulse of high-altitude explosions are the same as against the EMP of ground and air explosions.
2.5.1 Protection against ionizing and electromagnetic radiation
high-altitude nuclear explosions (HNE)
Interference with RS can arise as a result of explosions of nuclear weapons, accompanied by the emission of powerful electromagnetic pulses of short duration (10-8 sec) and changes in the electrical properties of the atmosphere.
EMR (radio flash) occurs:
At first , as a result of asymmetric expansion of the cloud of electrical discharges formed under the influence of ionizing radiation from explosions;
Secondly , due to the rapid expansion of a highly conductive gas (plasma) formed from the explosion products.
After an explosion in space, a fireball is created, which is a highly ionized sphere. This sphere is rapidly expanding (at a speed of about 100-120 km / h) above the earth's surface, transforming into a sphere of false configuration, the thickness of the sphere reaches 16-20 km. The concentration of electrons in a sphere can reach 105-106 electrons / cm3, i.e., 100-1000 times higher than the normal concentration of electrons in the ionospheric layer D.
High-altitude nuclear explosions (HNE) at altitudes above 30 km significantly affect the electrical characteristics of the atmosphere for a long time over large areas, and, therefore, have a strong effect on the propagation of radio waves.
In addition, a powerful electromagnetic pulse arising during IYE induces high voltages (up to 10,000-50,000 V) and currents up to several thousand amperes in wire communication lines.
The power of the EMP is so great that its energy is sufficient to penetrate the earth up to 30 m and induce the EMF within a radius of 50-200 km from the epicenter of the explosion.
However, the main effect of the IJW is that the huge amount of energy released during the explosion, as well as intense fluxes of neutrons, X-rays, ultraviolet and gamma rays, lead to the formation of highly ionized regions in the atmosphere and an increase in the density of electrons in the ionosphere, which in turn leads to to the absorption of radio waves and disruption of the stability of the functioning of the control system.
2.5.2 Characteristic signs of IJV
EYE in a given area or near it is accompanied by an instant cessation of reception of distant stations in the HF wave range.
At the moment of termination of communication, a short click is observed in the phones, and then only the receiver's own noises and weak crackles such as thunderbolts are heard.
A few minutes after the termination of communication on HF, interference from distant stations in the meter range of waves on VHF sharply increases.
The range of the radar and the accuracy of coordinate measurement are reduced.
The basis for the protection of electronic means is the correct use of the frequency range and all the factors that arise as a result of the use of IYA
2.5.3 Basic definitions:
reflected radio wave (reflected wave ) Is a radio wave propagating after reflection from the interface between two media or from inhomogeneities of the medium;
direct radio wave (straight wave ) - a radio wave propagating directly from sources to the place of reception;
terrestrial radio wave (earth wave ) - a radio wave propagating near the earth's surface and including a direct wave, a wave reflected from the earth, and a surface wave;
ionospheric radio wave (ionospheric wave ) - a radio wave propagating as a result of reflection from the ionosphere or scattering on it;
absorption of radio waves (absorption ) - a decrease in the energy of a radio wave due to its partial transition into thermal energy as a result of interaction with the environment;
multipath (multipath ) Is the propagation of radio waves from the transmitting to the receiving antenna along several paths;
effective reflection height of the layer (effective height ) Is the hypothetical height of the reflection of the radio wave from the ionized layer, depending on the distribution of the electron concentration over the height and length of the radio wave, determined in terms of the time between the transmission and reception of the reflected ionospheric wave during vertical sounding under the assumption that the propagation speed of the radio wave along the entire path is equal to the speed of light in vacuum;
ionospheric jump (leap ) Is the path of propagation of a radio wave from one point on the Earth's surface to another, the passage along which is accompanied by one reflection from the ionosphere;
maximum usable frequency (MUF) - the highest frequency of radio emission at which there is ionospheric propagation of radio waves between given points at a given time under certain conditions, this is the frequency that is still reflected from the ionosphere;
optimum operating frequency (ORCH) - the frequency of radio emission below the IF, at which stable radio communication can be carried out in certain geophysical conditions. As a rule, the ORF is 15% lower than the MUF;
vertical ionospheric sounding (vertical sounding ) - ionospheric sounding using radio signals emitted vertically upward relative to the Earth's surface, provided that the points of emission and reception are aligned;
ionospheric disturbance - violation in the distribution of ionization in the atmosphere, which usually exceeds the change in the average ionization characteristics for given geographic conditions;
ionospheric storm - long-term ionospheric disturbance of high intensity.
When determining the range of radio systems, it is necessary to take into account the absorption and refraction of radio waves during their propagation in the atmosphere, their reflection from the ionosphere, the influence of the underlying surface along the path along which the radio signal propagates.
The degree of influence of these factors depends on the frequency range and operating conditions of the radio system (time of day, geographic area, transmitter and receiver antenna heights).
The influence of absorption and refraction of radio waves is most significant in the lower main layer of the atmosphere, called the troposphere. The troposphere extends in height up to 8-10 km in the polar regions and up to 16-18 km in the tropical latitudes of the Earth. The main part of water vapor is concentrated in the troposphere, clouds and turbulent flows are formed, which affects the propagation of radio waves, especially the millimeter, centimeter and decimeter ranges, used in radar and short-range radio navigation.
The reflection of radio waves from the ionosphere most strongly affects decameter and longer waves used in navigation and communication systems.
Let's briefly consider the influence of the listed factors.
The influence of the attenuation of radio waves in the troposphere is associated with their absorption by oxygen and water vapor molecules, hydrometeors (rain, fog, snow) and solid particles. Absorption and scattering leads to a decrease in the power flux density of the radio wave with distance according to the exponential law, i.e., the signal power at the input is attenuated by a factor. The value of the attenuation factor depends on the attenuation coefficient and the distance traveled by the radio waves D. If the coefficient along the entire path is constant and the case of an active radar with a passive response is considered, then the signal power at the receiver input decreases due to attenuation from to
If we express, in, then. In the presence of hydrometeors and other particles in the atmosphere, the attenuation coefficient is the sum of the partial attenuation coefficients caused by the absorption of oxygen and water vapor by molecules, as well as the influence of liquid and solid particles. Molecular absorption in the atmosphere occurs mainly at frequencies close to resonance. Resonance lines of all gases in the atmosphere, with the exception of oxygen and water vapor, are located outside the range of radio waves, therefore, only absorption by oxygen and water vapor molecules significantly affects the range of the RTS. Absorption by water vapor molecules is maximal on a wave, and by oxygen molecules - on waves.
Thus, molecular absorption is significant in the centimeter and especially in the millimeter wavelength range, where it limits the range of radio systems, especially radar systems operating on reflected signals.
Another reason for the loss of signal energy during propagation is the scattering of radio waves, primarily by raindrops and fog. The greater the ratio of the radius of the drop , to the wavelength
, to the wavelength , the greater the loss of energy due to its dissipation in all directions. This scattering increases in proportion to the fourth power of the frequency, since the EPR of the drop at
where is the dielectric constant of water.
If the diameter of the droplets and their number per unit volume are known, then the attenuation coefficient can be determined. In reference books, the coefficient for rain is usually indicated depending on its intensity and wavelength. In the centimeter range, the attenuation coefficient varies approximately in proportion to the square of the signal frequency. If at a frequency at mm / h, then at a frequency at the same rain rate.
The attenuation of radio waves in fog is directly proportional to the concentration of water in it. The attenuation of radio waves due to hail and snow is much less than that due to rain or fog and is usually neglected.
The maximum range of the radar, taking into account the attenuation, can be found by the formula
if the range in free space is known. This equation can be solved graphically by presenting it in logarithmic form. After simple transformations, we find
Let us denote the relative decrease in the range and write the equation in a form convenient for a graphical solution:
Figure 9.4 shows the dependence that allows for given and to find, and therefore,.
Influence of radio wave refraction in the atmosphere. Refraction (refraction, curvature) of radio waves is the deviation of the propagation of radio waves from rectilinear when they pass through a medium with changing electrical parameters. The refractive properties of a medium are characterized by the refractive index, which is determined by its dielectric constant. Together with the refractive index in the atmosphere, it changes with altitude. The rate of change with height is characterized by a gradient, the value and sign of which characterize refraction.
When there is no refraction. If, then the refraction is considered negative and the trajectory of the radio wave is bent away from the surface of the Earth. the refraction is positive and the trajectory of the radio wave is bent towards the Earth, which leads to its bending by the radio wave and an increase in the range of radio systems and, in particular, the range of radar detection of ships and low-flying ones.
For the normal state of the atmosphere, i.e., the refraction is positive, which leads to an increase in the range of the radio horizon. The influence of normal refraction is taken into account by the apparent increase in the Earth's radius by a factor of, which is equivalent to an increase in the range of the radio horizon to. The radius of curvature of the trajectory of the radio wave is inversely proportional to the gradient, i.e. When the radius of curvature of the trajectory of the radio wave is equal to the radius of the Earth, and the radio wave, directed horizontally, propagates parallel to the surface of the Earth, bending around it. This is a case of critical refraction, in which a significant increase in the range of the radar is possible.
Under abnormal conditions in the troposphere (a sharp increase in pressure, humidity, temperature), super-refraction is also possible, in which the radius of curvature of the radio wave trajectory becomes less than the radius of the Earth. In this case, waveguide propagation of radio waves over very long distances is possible in the troposphere if the radar antenna and the object are at altitudes within the tropospheric layer that forms the waveguide channel.
Influence of the underlying surface. In addition to atmospheric refraction, bending around the earth's surface occurs due to the diffraction of radio waves. However, in the shadow zone (beyond the horizon), the intensity of radio waves decreases rapidly due to losses in the underlying surface, which rapidly increase with an increase in the frequency of the radio signal. Therefore, only at waves of more than 1000 m, a surface wave, i.e., a wave enveloping the Earth's surface, can provide a long range of the system (several hundred and even thousands of kilometers). Therefore, long-range RNS use waves of long-wave and super-long-wave ranges.
Attenuation of a surface wave depends on the dielectric constant and electrical conductivity of the underlying surface, and for the sea surface and for sandy or mountain deserts; at the same time it varies in the range of 0.0001 - 5 S / m. With a decrease in soil conductivity, attenuation increases sharply, therefore, the greatest range of action is provided when radio waves propagate over the sea, which is essential for maritime radio navigation.
The influence of the underlying surface affects not only the range of the RNS, but also their accuracy, since the phase velocity of radio wave propagation also depends on the parameters of the underlying surface. Special maps of phase velocity corrections are created depending on the parameters of the underlying surface, however, since these parameters change depending on the time of year and day and even the weather, it is practically impossible to completely eliminate positioning errors caused by a change in the phase velocity of radio wave propagation.
Radio waves with a length of more than 10 m can propagate beyond the horizon as a result of single or multiple reflections from the ionosphere.
Influence of reflection of radio waves by the ionosphere. Radio waves that reach the receiving antenna after being reflected by the ionosphere are called spatial.
Such waves provide a very long range, which is used in communication systems of the short-wave (decameter) range. On space waves, ultra-long-range radar detection of certain targets (nuclear explosions and missile launches) is also carried out using signals reflected by the target, which on the propagation path experience one or more reflections from the ionosphere and the Earth's surface. The phenomenon of receiving such signals (the Kabanov effect) was discovered by the Soviet scientist NI Kabanov in 1947. Radars based on this effect are called ionospheric or over-the-horizon. In such stations, operating at wavelengths of 10-15 m, as in conventional radars, the target range is determined by the signal delay time, and the direction is fixed using a directional antenna. Due to the instability of the ionosphere, the accuracy of such stations is low, and the calculation of the operating range is a difficult task due to the difficulty of taking into account the losses due to scattering and absorption of radio waves along the propagation path, as well as when they are reflected from the Earth and the ionosphere. In this case, one must also take into account losses due to a change in the plane of polarization of radio waves.
The dependence of ionospheric altitude for many reasons leads to unpredictable changes in signal delay, which makes it difficult to use sky waves for radio navigation. Moreover, the interference of space and surface waves leads to distortion of the surface signal and reduces the accuracy of location.
In conclusion, let us consider the features of propagation of radio waves of the myriameter (super-long-wave) range with a length of 10-30 km, used in ground-based global navigation systems. These waves are poorly absorbed by the underlying surface and are well reflected from it, as well as from the ionosphere, both at night and during the day. As a result, ultra-long waves propagate around the Earth, as in a waveguide bounded by the Earth's surface and the ionosphere, over very long distances. At the same time, the change in the propagation velocity and phase shifts can be predicted, which provides a positioning accuracy sufficient for navigation in the open sea.
Currently, satellite RNS are used for global navigation, in which, due to the high altitude of the satellite orbits, direct "visibility" is provided at long distances using decimeter waves that freely pass through the ionosphere. a system that, for global SRNS, covers the entire near-earth space.
Write the free-space radar range equation.
How does the range of a radar station depend on its wavelength?
How does the reflection of radio waves from the Earth's surface affect the range of the radar?
What is the peculiarity of detecting low-lying objects?
What are the main causes of propagation attenuation of a radar signal?
Determine the range of a 3-centimeter range radar operating in rain conditions of mm / h (). Radar range in free space.
Under what conditions does the refraction of radio waves lead to an abnormal increase in the range of the radar?
What is the effect of the underlying surface on the operation of the RNS?
What is the "Kabanov effect" and how is it applied in practice?
Why are VLF radio waves used in global ground-based RNS?
