The passage of radio waves in sea water. Radio waves and radio wave propagation
When determining the range of radio systems, it is necessary to take into account the absorption and refraction of radio waves as they propagate in the atmosphere, their reflection from the ionosphere, and 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, geographical area, height of the transmitter and receiver antennas).
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 globe. 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 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 us briefly consider the influence of these factors.
The influence of 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 a radio wave with distance according to an 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 resonant ones. The resonance lines of all gases of the atmosphere, with the exception of oxygen and water vapor, are located outside the range of radio waves, therefore only the absorption of oxygen and water vapor by molecules significantly affects the range of the RTS. Absorption by water vapor molecules is maximum at the wave, and by oxygen molecules - at the waves.
Thus, molecular absorption is significant in the centimeter and especially in the millimeter range, where it limits the range of radio systems, especially radar, 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 droplet radius , to the wavelength
, to the wavelength , the greater the energy loss due to its scattering 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 handbooks, 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 intensity .
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 due to rain or fog, and their influence is usually neglected.
The maximum range of the radar, taking into account attenuation, can be found by the formula
if the range in free space is known. This equation can be solved graphically in logarithmic form. After simple transformations, we find
We denote the relative decrease in range and write the equation in a form convenient for a graphical solution:
Figure 9.4 shows a dependence that allows, for given and , to find , and therefore, .
Influence of refraction of radio waves in the atmosphere. Refraction (refraction, curvature) of radio waves is the deviation of the propagation of radio waves from a straight line when they pass through a medium with changing electrical parameters. The refractive properties of a medium are characterized by the refractive index, determined by its dielectric constant. Together with the refractive index in the atmosphere varies with altitude. The rate of change with height is characterized by a gradient whose value and sign 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 curved towards the Earth, which leads to its envelope 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 aircraft.
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 effect of normal refraction is taken into account by an apparent increase in the Earth's radius by a factor of 1, which is equivalent to an increase in the range of the radio horizon up to . The radius of curvature of the radio wave trajectory is inversely proportional to the gradient , i.e. . When the radius of curvature of the radio wave trajectory is equal to the radius of the Earth, and a 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), superrefraction is also possible, in which the radius of curvature of the radio wave trajectory becomes less than the radius of the Earth. At the same time, waveguide propagation of radio waves over very long distances is possible in the troposphere if the radar antenna and the object are at heights within the troposphere layer forming the waveguide channel.
The influence of the underlying surface. In addition to atmospheric refraction, the rounding of 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 drops rapidly due to losses in the underlying surface, which rapidly increase with increasing frequency of the radio signal. Therefore, only at waves greater than 1000 m, a surface wave, i.e., a wave enveloping the Earth's surface, can provide a large range of the system (several hundreds and even thousands of kilometers). Therefore, in the long-range RNS, waves of the long-wave and ultra-long-wave ranges are used.
The attenuation of a surface wave depends on the dielectric constant and electrical conductivity of the underlying surface, both for the sea surface and for sandy or mountainous deserts; while changing within 0.0001 - 5 S/m. With a decrease in the conductivity of the soil, the attenuation increases sharply, therefore, the greatest range of action is provided by the propagation of radio waves over the sea, which is essential for marine radio navigation.
The influence of the underlying surface affects not only the range of RNS, but also their accuracy, since the phase velocity of radio waves 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 location errors caused by a change in the phase velocity of radio waves.
Radio waves with a length of more than 10 m can also 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 reaching 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 in the shortwave (decameter) range. On sky 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, along 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 N. I. Kabanov in 1947. Radars based on this effect are called ionospheric or over-the-horizon. In such stations operating at waves 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 range of action is a difficult task due to the difficulty of taking into account 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, it is also necessary to take into account losses due to a change in the plane of polarization of radio waves.
The dependence of the height of the ionosphere on many factors leads to unpredictable changes in signal delay, which makes it difficult to use sky waves for radio navigation. Moreover, the interference of spatial and surface waves leads to distortion of the surface signal and reduces the accuracy of the location.
In conclusion, let us consider the features of the 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, superlong waves propagate around the Earth, as in a waveguide limited by the Earth's surface and the ionosphere, over very long distances. At the same time, the change in propagation velocity and phase shifts can be predicted, which provides positioning accuracy sufficient for navigation on the high seas.
At present, satellite RNSs are used for global navigation, in which, due to the high altitude of satellite orbits, direct “visibility” is provided over long distances using decimeter waves that freely pass through the ionosphere. system, which for global SRNS covers the entire near-Earth space.
Write the radar range equation in free space.
How does the range of a radar 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 feature of detecting low-lying objects?
What are the main reasons for the weakening of the radar signal during propagation?
Determine the range of a three-centimeter range radar operating in rain with an intensity of mm / h (). The range of the radar in free space.
Under what conditions does the refraction of radio waves lead to an anomalous increase in the range of the radar?
What is the influence of the underlying surface on the operation of the RNS?
What is the "Kabanov effect" and how is it applied in practice?
Why do global ground-based RNSs use VLF radio waves?
The laws of radio wave propagation 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 both experience and theory show, the surface of the Earth has a strong influence on 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, differences between sea and land) and its geometric shape (the general curvature of the surface of the globe and individual uneven terrain - mountains, gorges, etc.). This influence is different for waves of different lengths and for waves of different lengths and for different distances between transmitter and receiver.
The influence exerted on the propagation of radio waves by the shape of the earth's surface is clear from the foregoing. After all, we have here, in essence, various manifestations of the diffraction of waves coming from the radiator (§ 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 topography affect the propagation of waves of different lengths in different ways.
So, for example, a mountain range casts a “radio shadow” in the case of short waves, while sufficiently long (several kilometers) waves go around this obstacle well and are slightly attenuated on the mountain slope opposite the radio station (Fig. 147).
Rice. 147. The mountain casts a "radio shadow" in the case of short waves. Long waves go around the mountain
As for the globe as a whole, it is extremely large even compared with the longest wavelengths used in radio. Very short waves, for example, meter waves, do not turn at all 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 waves used could not, due to diffraction, wrap so much to go around the globe - from us to the antipodes. If, nevertheless, radio communication is carried out between any points on the globe, and at waves of very different wavelengths, then this is possible not because of 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, high-frequency electric currents arise in the soil and in sea water, 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 hand, on the wavelength. Short waves are attenuated much more strongly 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 much less. As a result, the range of action of the same transmitter turns out to be significantly (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 
During winter nights, stations 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 
position is different. Here, at any time of the day and any time of the year, you can find such wavelengths that reliably overlap any distance. To ensure round-the-clock communication, it is necessary 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 for the 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 grounds 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 wavelengths; for such waves (more) the globe turns out to be surrounded, as it were, by a spherical "mirror", and the propagation of these radio waves occurs between two reflective spherical surfaces - the surface of the Earth and the "surface" of the ionosphere (Fig. 148). That is why radio waves are able to go around the globe.
Rice. 148. The wave goes between the Earth and the ionosphere
Of course, one should not understand the words "surface of the spherical mirror of the ionosphere" literally. The ionized layers do not have any 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 it is greater than in the lower), 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, it varies 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 much greater than at night, when only positive ions and negative electrons are reunited into neutral molecules (recombination). The difference in ionization during the 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 frequently, and recombination proceeds more intensively. During the night, the ionization of the lower layers of the ionosphere can have time to drop to zero. Ionization is also different depending on the time of year, that is, on the height of the Sun's rise above the horizon.
The study of daily 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 various lengths at different times of the day and year (radio forecasts).
The presence of the ionosphere not only makes short-wave communication possible over long distances, but also allows radio waves to sometimes circumnavigate the entire globe, and even several times. Because of this, a peculiar phenomenon arises in radio reception, the so-called radio echo, in which the signal is perceived by the receiver several times: after the signal arrives along the shortest path from the transmitter, repeated signals can be heard that circled the globe.
It often happens that the wave comes from the transmitter to the 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 strengthening each other, depending on the path difference. Since the ionosphere is not an absolutely stable "mirror", but changes over time, the difference between the paths of waves that come along different paths from the transmitter to the receiver also changes, resulting in amplification, etc. We can say that the interference fringes "crawl" over the surfaces of the Earth, and the receiver is now at the maximum, then at the minimum of oscillations. In the received transmission, this results in a change in good audibility and reception fading, in which audibility can drop to zero.
Rice. 149. Different paths of a wave from a transmitter to a 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 aircraft interferes with the wave from the transmitting station, and we see how the image “flashes” due to the fact that the interference “bands” of alternating signal amplification and attenuation 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 shifted horizontally relative to each other to varying degrees. This is the result of the reflection of a radio wave from houses, towers, etc. The reflected waves travel a longer path than the distance between the transmitting and receiving antennas, and therefore are delayed in giving a picture. shifted in the direction of scanning of the electron beam in the kinescope. In essence, here we see 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 less than , made it possible to detect radio emission coming from extraterrestrial sources. It has been around since the 40s. In our century, radio astronomy is developing rapidly, opening 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 continuously increasing.
It turned out that radio waves are emitted by both the Sun and the planets, and outside our solar system - by 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 observable 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 a second of arc) were discovered. Initially, they were considered to be a special kind of stars belonging to our Galaxy, and therefore they were called quasi-stellar sources or quasars. But since 1962, it has become clear that quasars are extragalactic objects with a huge power of radio emission.
Separate, or, as they say, discrete radio sources in our Galaxy emit a wide range of wavelengths. But "monochromatic" radio emission with a wavelength of , emitted by interstellar hydrogen, has also been detected. The study of this radiation made it possible to find the total mass of interstellar hydrogen and to 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 bursts of radio emission observed against a general constant background. The year 1968 was marked by a new radio astronomical discovery of great significance: sources (located mostly within the Galaxy) were discovered emitting strictly periodic pulses of radio waves. These sources are called pulsars. The pulse repetition periods for different pulsars are different and deliver from a few seconds to a few hundredths of a second or even less. The nature of the radio emission of pulsars receives, apparently, the most plausible explanation, if we assume that pulsars are rotating stars, consisting mainly of neutrons (neutron stars). The great scientific significance of this radio astronomical discovery lies in the discovery and possibility of observing such stars.
In addition to receiving their own radio emission from the 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 its axis, and judge (by the intensity of the reflection of radio waves of various lengths) about the properties of the surface and atmosphere of the planet.
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 satellites of the Earth and spacecraft (communication itself, radio control, television, as well as telemetry - transmission to Earth of readings of various measuring instruments). For the same reason, it is now possible to use meter radio waves for communication and television between points on the earth's surface that are very distant from each other (for example, between Moscow and our Far Eastern cities), using a single retransmission of transmissions by special satellites on which receiving and transmitting radio equipment is installed.
In this article we will tell you about radio waves and the properties of their propagation.
Many people, not possessing elementary concepts about the types of energy, their properties, often talk about ways to wirelessly transmit energy over distances. Others, not knowing how radio waves propagate, make antennas for their radio transmitters and receivers in an attempt to achieve maximum transmission and reception characteristics, but they fail. Some read smart books, while others are based on experience, or the advice of an illiterate comrade. In order to dispel at least some of the misconceptions and give an idea about electromagnetic waves and how they look - this article is devoted to radio waves.
As usual, I will not paint the formulas of Maxwell, Faraday and other famous scientists. There are a huge number of them in physics textbooks, reading which, even I, having an education and experience in radio electronics, do not understand why abstruse formulas are given in these textbooks, but the simplest information of useful practical value is missing? Indeed, the next day, or a week after graduation, the student will not remember these formulas, but he will not know simple concepts, just as he did not know.
Let's start with the fact that the great inventor-practitioner of electrical machines Nikola Tesla actively used electromagnetic oscillations in his experiments, which no one knew about before, and as we now know from high school physics textbooks, they generate a type of electromagnetic waves - radio waves. But I repeat, in the days of Tesla, no one knew about the existence of electromagnetic waves. Intuitively, through observations, Tesla understood that as a result of his experiments, some kind of energy appears in the surrounding space. But in those days there was no such science and equipment to reveal the concept of electromagnetic waves. Therefore, this phenomenon was considered as a philosophical category, which Tesla called - ether.
Today they argue that "ether" and electromagnetic waves are different concepts. They are completely wrong only because absolutely all of Tesla's inventions are based on the use of ordinary alternating electric current and electromagnetic fields, which in turn generate not "ether", but the most ordinary electromagnetic waves in the radio frequency range. Exactly what is now called electromagnetic waves, in those days, Nikola Tesla called ether. There are no other possible explanations. You can argue for a long time that these are different concepts. For example, someone foaming at the mouth is trying to prove that the speed of propagation of the ether is greater than the speed of light, but there is no evidence base. What experiment did Nikola Tesla use to measure the speed of the ether? There is no such information anywhere. There is only one conclusion, he did not measure it, but only assumed. You say that the ether carries energy? I will answer, any electromagnetic wave carries energy! I came across practical circuits of radio receivers without batteries, designed not to work on headphones or a dynamic head, but to receive direct electric current “out of thin air” by those residents of megacities who live near powerful television and radio centers.
where: f– frequency, λ is the wavelength, With- the speed of light, equal to 300,000 km / s.
Radio waves are divided into several ranges:
Extra long "SDV"- frequency 3 - 30 kHz, wavelength 100 - 10 km;
Long "DV"- frequency 30 - 300 kHz, wavelength 10 - 1 km;
Medium "SV"- with a frequency of 300 - 3000 kHz, with a wavelength of 1000 - 100 meters;
Short "KV"- with a frequency of 3 - 30 MHz, with a wavelength of 100 - 10 meters;
Ultrashort "VHF", including:
- meter "MV"- a frequency of 30 - 300 MHz, with a wavelength of 10 - 1 meters;
- decimeter "DMV"- a frequency of 300 - 3000 MHz, with a wavelength of 10 - 1 dm;
- centimeter "SMV"- frequency 3 - 30 GHz, wavelength 10 - 1 cm;
- millimeter "MMV"- frequency 30 - 300 GHz, wavelength 10 - 1 mm;
- submillimeter "SMMV"- frequency 300 - 6000 GHz, wavelength 1 - 0.05 mm;
The ranges from decimeter to millimeter waves, because of their very high frequency, are called ultra-high frequencies. "microwave".
Naturally, all the listed radio wave ranges, both domestic and bourgeois, can be divided into sub-bands.
Remember the practical importance of EMW polarization - if the radio transmitter and radio receiver are tuned to the same frequency, but have different polarizations, for example, the transmitter is vertical and the receiver is horizontal, then radio communication will be poor. To this it is worth adding the directional diagram of the whip antenna, and then, using the example of two radiotelephones - portable radio stations (1 and 2) shown in the figure below, we can make a logical conclusion:
Another parameter also affects the radio communication range - the thickness of the vibrator elements, the larger it is, the antenna broadband– the range of well-received frequencies is wider, but the signal level decreases at almost all frequencies. This is due to the fact that the dipole antenna is the same oscillatory circuit, and with the expansion of the resonance frequency response frequency band, the resonance amplitude decreases. Therefore, do not be surprised that a television antenna made from aluminum beer cans in a city where the signal level of the television tower is high receives a television signal from different channels no worse, and often better than a complex professional antenna.
Good professional radio antennas have an indicator - antenna gain. After all, an ordinary half-wave vibrator does not amplify the signal, its action is selective - at a certain frequency, in certain directions and a certain polarization. In order to have less interference in the receiver, to increase the range of reception and transmission, while at the same time narrowing the antenna radiation pattern (the common name is DND), a simple half-wave vibrator is not suitable. The antenna is complicated.
Earlier, I wrote about the influence of various obstacles - their reflective property. If the obstacle is not commensurate in size (an order of magnitude smaller) with the length of the radio wave, then this is not an obstacle for the radio signal, it does not affect it in any way. If the obstacle is in a plane parallel to the electric wave and greater than the wavelength, then this obstacle reflects the radio wave. If the obstacle is a multiple (equal to a quarter, half or whole) wavelength, oriented parallel to the electric wave and perpendicular to the direction of wave propagation, then this obstacle acts as a resonant oscillatory circuit at the whole wavelength or its harmonics, and has the greatest reflective properties.
The next way to further amplify the receiving signal by narrowing and stretching the bottom is to add a passive vibrator - directors. The principle of operation is all on the same in-phase addition. At the same time, the radiation pattern narrows and stretches even more. The figure shows the antenna "wave channel", consisting of a reflector (1), a loop half-wave vibrator (2) and one director (3). Further addition of directors further narrows and stretches the radiation pattern. The length of the directors (B) is chosen slightly less than the length of the active vibrator. To increase the gain of the antenna and its broadband, directors are added in front of the active vibrator with a gradual decrease in their length. Please note that the length of the active vibrator is equal to half the average wavelength of the received signal, the length of the reflector is more than half the wavelength, and the length of the director is less than half the wavelength. The distances between the elements are also chosen to be about half the wavelength.
In professional technology, the method of narrowing the bottom and increasing the amplifying properties of the antenna is often used - phased antenna array, in which several antennas are connected in parallel (for example, simple dipoles, or antennas of the “wave channel” type). As a result, the currents of neighboring channels are added, and as a result, the signal power is increased.
At ultrahigh frequencies, a waveguide is used as an antenna vibrator, and a continuous web is used as a reflector, all points of which are equidistant from the vibrator plane (at the same distance) - paraboloid of revolution, or in the common people - "plate". Such an antenna has a very narrow radiation pattern and a high antenna gain.
Conclusions based on the propagation and complexity of radio wave formation
How and where radio waves propagate can be calculated using clever formulas and transformations only for ideal conditions - in the absence of natural obstacles. To do this, the elements of the antennas, various surfaces must be perfectly flat. In practice, due to the influence of many factors of refraction and reflection, not a single “scientific brain” has yet been able to calculate the propagation of radio waves in natural conditions with high reliability. There are areas of confident reception space and radio shadow zones - where there is no reception at all. Only in the cinema, climbers do not answer a radio call because their hands are busy, or they themselves are busy "saving the world", in fact, radio communication is not a stable business and more often climbers do not answer because there is simply no connection - there is no transmission of radio waves . It was the dependence of radio communication on natural phenomena (rain, low cloudiness, rarefied atmosphere, etc.) that led to the emergence of the concept "ham radio". This is now the concept of "ham radio" - a person who likes to solder radio circuits. About twenty years ago, it was a “shortwave signalman”, who, on a low-power transceiver made by himself, contacted another radio amateur (or, in other words, a radio correspondent) located on the other side of the Earth, for which he received “bonuses”. Previously, there were even competitions in radio communications. Today they are also held, but with the development of technology it has become less important. Among these amateur radio operators there are many who are dissatisfied with the fact that ordinary “solderers”, who do not sit in headphones in search of radio correspondents for organizing radio exchange, call themselves radio amateurs.
If Maxwell had not predicted the existence of radio waves, and Hertz had not discovered them in practice, our reality would have been completely different. We could not quickly exchange information using radio and mobile phones, explore distant planets and stars using radio telescopes, observe aircraft, ships and other objects using radar.
How do radio waves help us with this?
Sources of radio waves
The sources of radio waves in nature are lightning - giant electrical spark discharges in the atmosphere, the current strength in which can reach 300 thousand amperes, and the voltage - billion volts. We see lightning during thunderstorms. By the way, they occur not only on Earth. Lightning flashes have been detected on Venus, Saturn, Jupiter, Uranus and other planets.
Almost all space bodies (stars, planets, asteroids, comets, etc.) are also natural sources of radio waves.
In radio broadcasting, radar, communication satellites, fixed and mobile communications, and various navigation systems, radio waves obtained by artificial means are used. The source of such waves are high-frequency generators of electromagnetic oscillations, the energy of which is transmitted into space with the help of transmitting antennas.
Properties of radio waves
Radio waves are electromagnetic waves whose frequency is in the range from 3 kHz to 300 GHz, and the length is from 100 km to 1 mm, respectively. Spreading in the environment, they obey certain laws. When passing from one medium to another, their reflection and refraction are observed. The phenomena of diffraction and interference are also inherent in them.
Diffraction, or bending, occurs if there are obstacles in the path of radio waves that are smaller than the length of the radio wave. If their sizes turn out to be larger, then radio waves are reflected from them. Obstacles can be of artificial (structures) or natural (trees, clouds) origin.
Radio waves are also reflected from the earth's surface. Moreover, the surface of the ocean reflects them about 50% more strongly than the land.
If the obstacle is a conductor of electric current, then radio waves give off some of their energy to it, and an electric current is created in the conductor. Part of the energy is spent on the excitation of electric currents on the surface of the Earth. In addition, radio waves diverge from the antenna in circles in different directions, like waves from a pebble thrown into water. For this reason, radio waves lose energy over time and decay. And the farther from the source is the receiver of radio waves, the weaker the signal that has reached it.
Interference, or superposition, causes mutual amplification or attenuation of radio waves.
Radio waves propagate in space at a speed equal to the speed of light (by the way, light is also an electromagnetic wave).
Like any electromagnetic waves, radio waves are characterized by wavelength and frequency. The frequency is related to the wavelength by the relation:
f= c/ λ ,
where f is the frequency of the wave;
λ - wavelength;
c is the speed of light.
As you can see, the longer the wavelength, the lower its frequency.
Radio waves are divided into the following ranges: extra long, long, medium, short, ultra short, millimeter and decimillimeter waves.
Propagation of radio waves
Radio waves of different lengths do not propagate equally in space.
Ultra long waves(wavelength of 10 km or more) easily go around large obstacles near the surface of the Earth and are very weakly absorbed by it, so they lose less energy than other radio waves. Consequently, they also decay much more slowly. Therefore, in space, such waves propagate over distances of up to several thousand kilometers. The depth of their penetration into the environment is very large, and they are used to communicate with submarines located at great depths, as well as for various studies in geology, archeology and engineering. The ability of ultralong waves to easily bend around the Earth makes it possible to study the earth's atmosphere with their help.
Long, or kilometer, waves(from 1 km to 10 km, frequency 300 kHz - 30 kHz) are also subject to diffraction, therefore they are able to propagate over distances up to 2,000 km.
Medium, or hectometric, waves(from 100 m to 1 km, frequency 3000 kHz - 300 kHz) they go around obstacles on the surface of the Earth worse, they are absorbed more strongly, therefore they decay much faster. They extend over distances up to 1,000 km.
short waves behave differently. If we tune the car radio in the city to a short radio wave and start moving, then as we move away from the city, the reception of the radio signal will get worse, and at a distance of about 250 km it will stop completely. However, after some time, radio broadcasting will resume. Why is this happening?
The thing is that short-range radio waves (from 10 m to 100 m, frequency 30 MHz - 3 MHz) at the Earth's surface fade very quickly. However, waves leaving at a large angle to the horizon are reflected from the upper layer of the atmosphere - the ionosphere, and return back, leaving hundreds of kilometers of the "dead zone" behind them. Further, these waves are already reflected from the earth's surface and again directed to the ionosphere. Repeatedly reflected, they are able to circle the globe several times. The shorter the wave, the greater the angle of reflection from the ionosphere. But at night, the ionosphere loses its reflectivity, so short-wave communications are worse at night.
BUT ultrashort waves(meter, decimeter, centimeter with a wavelength shorter than 10 m) cannot be reflected from the ionosphere. Spreading in a straight line, they penetrate it and go higher. This property is used to determine the coordinates of air objects: aircraft, flocks of birds, the level and density of clouds, etc. But ultrashort waves cannot also go around the earth's surface. Due to the fact that they propagate within the line of sight, they are used for radio communication at a distance of 150 - 300 km.
In their properties, ultrashort waves are close to light waves. But light waves can be collected into a beam and sent to the right place. This is how a searchlight and a flashlight are arranged. The same is done with ultrashort waves. They are assembled with special antenna mirrors and a narrow beam is sent in the right direction, which is especially important, for example, in radar or satellite communications.
millimeter waves(from 1 cm to 1 mm), the shortest waves of the radio range, are similar to ultrashort waves. They also propagate in a straight line. But a serious obstacle for them is precipitation, fog, clouds. In addition to radio astronomy, high-speed radio relay communication, they have found application in microwave technology used in medicine and in everyday life.
Submillimeter, or decimillimeter, waves (from 1 mm to 0.1 mm) according to the international classification also belong to radio waves. Under natural conditions, they almost do not exist. In the energy spectrum of the Sun, they occupy a negligible fraction. They do not reach the Earth's surface, as they are absorbed by water vapor and oxygen molecules in the atmosphere. Created by artificial sources, they are used in space communications, to study the atmospheres of the Earth and other planets. The high degree of safety of these waves for the human body allows them to be used in medicine for scanning organs.
Submillimeter waves are called "waves of the future". It is quite possible that they will give scientists the opportunity to study the structure of the molecules of substances in a completely new way, and in the future, maybe even allow them to control molecular processes.
As you can see, each range of radio waves is used where the features of its propagation are used with maximum benefit.
Radio waves, and their distribution, are an undeniable mystery for beginners on the air. Here you can get acquainted with the basics of the theory of propagation of radio waves. This article is intended to introduce novice fans of the air, as well as for those who have some idea about it.
The most important introductory, which is often forgotten to be said before introducing the theory of radio wave propagation, is that radio waves propagate around our planet due to reflection from the ionosphere and a beam of light is reflected from the earth as from translucent mirrors.
Peculiarities of medium wave propagation and cross modulation
Medium waves include radio waves with a length of 1000 to 100 m (frequency 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 terrestrial and ionospheric way. Medium waves experience significant absorption in the semiconducting surface of the Earth, the range of propagation of the earth wave 1, (see Fig. 1), is limited to a distance of 500-700 km. Over long distances, radio waves 2 and 3 are propagated by an ionospheric (spatial) wave.
At night, medium 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, on the path of wave propagation, layer D is located, which extremely strongly absorbs medium waves. Therefore, at ordinary transmitter powers, the electric field strength is insufficient for reception, and during the daytime, the propagation of medium waves occurs practically only by the earth wave over relatively short distances, on the order of 1000 km. In the medium wave range, the longer waves experience less absorption and the electric field strength of the skywave is greater at longer wavelengths. Absorption increases in summer months and decreases in winter. Ionospheric disturbances do not affect the propagation of medium waves, since the E layer is little disturbed during ionospheric-magnetic storms.
At night, see fig. 1, at some distance from the transmitter (point B), the simultaneous arrival of spatial 3 and surface waves 1 is possible, and the length of the path of the spatial wave varies with the change in the electron density of the ionosphere. A change in the phase difference of these waves leads to fluctuations in the electric field strength, called near field fading.
At a considerable distance from the transmitter (point C), waves 2 and 3 can arrive by one or two reflections from the ionosphere. A change in the phase difference of these two waves also results in a fluctuation in the electric field strength, called far field fading.
To combat fading at the transmitting end of the communication line, antennas are used, in which the maximum of the 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 zone of near fading moves away from the transmitter, and at large distances the field of the wave that arrived by way of two reflections is weakened.
Unfortunately, not all novice broadcasters operating in the 1600-3000 kHz frequency range are aware 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 the signals from powerful stations. This phenomenon is called cross modulation. The depth of the modulation coefficient reaches 5-8%. From the reception side, the impression is made of a poorly executed transmitter, with all sorts of hums and wheezing, this is especially noticeable in the AM modulation mode.
Due to cross modulation, intense lightning noise often penetrates 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 operate more often at higher frequencies. Foreign radio transmitters of CB stations amplify them and compress modulating signals, and for undistorted operation on the air, they use inverse frequencies.
The phenomena of demodulation and cross modulation in the ionosphere are observed only in the range of medium waves (MW). In the short wave (SW) range, 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. Extra long distance connections, over 10,000 km, are usually possible at sunset and sunrise. In the daytime, communication is possible at a distance of up to 300 km. Free FM radio broadcasters can only envy such large radio routes.
In the summer, this band is often interfered with by interference from 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 (frequency 3-30 MHz). The advantage of short wavelength operation over longer wavelength operation is that directional antennas can be easily created in this range. Short waves can propagate as 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 greatly. Therefore, at ordinary transmitter powers, short-wave terrestrial waves propagate over distances not exceeding several tens of kilometers. On the sea surface, this distance increases significantly.
Short waves can be propagated by an ionospheric wave over many thousands of kilometers, and this does not require high-power transmitters. Therefore, at present, short waves are mainly used for communication and broadcasting over long distances.
Short waves propagate over long distances by reflection from the ionosphere and the Earth's surface. This method of propagation is called hopping, see fig. 2 and is characterized by hop distance, number of hops, exit and arrival angles, maximum usable frequency (MUF), and lowest usable frequency (LFF).
If the ionosphere is uniform in the horizontal direction, then the wave trajectory is also symmetrical. Usually, 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). To reflect the wave, it is necessary that the operating frequency be no higher than the value of the maximum usable 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 by the concept of the maximum applicable frequency (MUF), taking into account its reduction by 15-20% of the MUF. Anti-aircraft radiation antennas are used for broadcasting in the near zone by the method of single-hop reflection from the ionosphere.
The second condition limits the operating range from below: the lower the operating frequency (within the shortwave range), the stronger the absorption of the wave in the ionosphere. The lowest applicable frequency (LFC) is determined from the condition that at 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 ionospheric layers should not be more than permissible. The electron density of the ionosphere varies during the day, during the year, and during the period of solar activity. This means that the boundaries of the operating range also change, which leads to the need to change the operating 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 evaluate the HF range by sections.
Frequency range 5-8 MHz, in many ways similar to the 3 MHz band, 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 over any distance, with the exception of ZM, which increases to several hundred kilometers. During the hours of changing the time of day (sunset/sunrise), the most convenient for long-distance communications. Atmospheric noise is less pronounced than in the range of 1.5-3 MHz.
In the frequency range 10-15 MHz during periods of solar activity, communications are possible during the daytime with almost any point on the globe. In summer, the duration of radio communications 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 topic of MUF, here the angle of the reflected wave must be small with respect to the ionosphere, otherwise it has a large attenuation in the ionosphere, or a simple escape into space. Small radiation angles correspond to large jumps and correspondingly large silence zones. During periods of maximum solar activity, communication is also possible at night.
In addition to the above models, cases of anomalous propagation of radio waves are possible. Anomalous propagation can occur when a sporadic layer appears on the path of a wave, from which shorter waves, up to meter wavelengths, can be reflected. This phenomenon can be observed in practice by passing 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.
In the VHF FM band, Except in rare cases of anomalous radio wave propagation, 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 it is impossible to speak of any visual and line of sight, but radio waves pass through urban development with some attenuation. The higher the frequency, the higher the attenuation in urban areas. The frequency range 88-108 MHz is also subject to some attenuation in urban conditions.
Fading of HF radio signals
The reception of short radio waves is always accompanied by a measurement of the level of the received signal, and this change is random and temporary. This phenomenon is called fading (fading) of the radio signal. On the air, fast and slow signal fading is observed. The fading depth can reach up to several tens of decibels.
The main cause of fast signal fading is multipath propagation of radio waves. In this case, the cause of fading is the arrival at the receiving point of two beams propagating by one and two reflections from the ionosphere, wave 1 and wave 3, see Fig. 2.
Since the rays travel different paths in distance, their phases of arrival 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 a wave by 180°, it is enough that the path length changes by only ½. It should be recalled that when rays of one signal arrive at the receiving point with the same strength and with a phase difference of 180 °, they are completely subtracted according to the vector law, and the strength of the incoming signal in this case can be equal to zero. Such small changes in path length can occur continuously, therefore, fluctuations in the electric field strength in the short wave range are frequent and deep. The interval of their observation in 3-7 minutes can be at low frequencies of the HF band, and up to 0.5 seconds at frequencies closer to 30 MHz.
In addition, signal fading is caused by the scattering of radio waves on the inhomogeneities of the ionosphere and the interference of scattered waves.
In addition to interference fading, at short wavelengths, polarization fading occurs. The cause of polarization fading is the rotation of the polarization plane of the wave relative to the received antenna. This occurs when the wave propagates in the direction of the Earth's magnetic field lines, and with a change in the electron density of the ionosphere. If the transmitting and receiving antennas are horizontal vibrators, then the radiated horizontally polarized wave, after passing through the ionosphere, will undergo a rotation of the polarization plane. This leads to fluctuations. d.s., induced in the antenna, which has an additional attenuation of up to 10 dB.
In practice, all these 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 are observed with a period of 40-60 minutes in the low-frequency part of the HF band. The reason for these fading is a change in the absorption of radio waves in the ionosphere. The distribution of the envelope amplitude of the signal during slow fading obeys a normally logarithmic law with a decrease in the signal to 8-12 dB.
To combat fading, on short waves, the antenna diversity method is used. The fact is that the increase and decrease in the strength of the electric field 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, spaced apart by several wavelengths, and the signals are added after detection. It is effective to separate antennas by polarization, i.e., simultaneous reception on vertical and horizontal antennas with subsequent addition of signals after detection.
I would like to note that these control measures are effective only for eliminating fast fading, slow signal changes are not eliminated, since this is due to a change in the absorption of radio waves in the ionosphere.
In amateur radio practice, the antenna diversity method is used quite rarely, due to the constructive 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. The use of diversity reception requires an increase in the number of antennas at least twice.
Another thing is when an amateur lives in a rural area, while having enough space to accommodate an anti-fading structure, he can simply use two broadband vibrators for this, covering all, or almost all, of the required ranges. One vibrator must be vertical, the other horizontal. It is not necessary to have several masts for this. It is enough to place them on the same 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 by a radio signal in the VHF / FM bands
The frequencies of the meter range are distributed within the line of sight. The range of radio wave propagation within the line of sight, without taking into account the radiation power of the transmitter and other natural phenomena that reduce communication efficiency, looks like this:
r = 3.57 (√h1 + √h2), km,
Calculate line-of-sight radii when installing a receiving antenna at different heights, where h1 is a parameter, h2 = 1.5 m. We 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 of the possibility of line of sight, taking into account a perfectly round earth.
Let's make a calculation the required level of the radio signal together with the reception for a wavelength of 3 m.
Since on the routes between the transmitting station and the moving object there are always such phenomena as reflections, scattering, absorption of radio signals by various objects, etc., corrections should be made to the level of signal attenuation, which was proposed 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 introduce a factor of 2, which will be a 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. This ratio is unacceptable for high-quality broadcasting, so we will introduce an additional correction of another 12–20 dB, and take 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 20dB (10 times). Then, with a receiver sensitivity of 1.5 μV. at the place of reception, a field with a strength 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; P - 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 the band pass filter.
Answer: With a power of 10 W, radiation height h1 = 27 meters and h2 = 1.5 m, really high-quality radio reception with a radius in urban areas 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 on a remote antenna, then the range will be calculated in tens of kilometers.
73! UA9LBG & Radio-Vector-Tyumen
Student of a pedagogical university: life and professional prospects: Monograph Professional activity of a young teacher
Features of competence-based education Preparing for the lesson
Psychological and pedagogical approaches to assessing the results of competence-based education III