Radio relay communication

Radio relay tower

Radio relay communication (from the English. Relay - transmit, broadcast) - one of the types of radio communication formed by a chain of transmitting and receiving (relay) radio stations. Terrestrial radio relay communication is usually carried out at deci - and centimeter waves (from hundreds of megahertz to tens of gigahertz).

By design, radio relay communication systems are divided into three categories, each of which has its own frequency ranges on the territory of Russia:

• local links 0.39 GHz to 40.5 GHz
• intra-zone links 1.85 GHz to 15.35 GHz
• trunk links 3.4 GHz to 11.7 GHz

This division is associated with the influence of the propagation environment on ensuring the reliability of radio relay communications. Up to a frequency of 12 GHz, atmospheric phenomena have a weak effect on the quality of radio communication, at frequencies above 15 GHz this influence becomes noticeable, and above 40 GHz, it is decisive, in addition, at frequencies above 40 GHz, attenuation in the Earth's atmosphere has a significant effect on the quality of communication.

Atmospheric losses mainly consist of losses in oxygen atoms and in water molecules. Almost complete opacity of the atmosphere for radio waves is observed at a frequency of 118.74 GHz (resonant absorption in oxygen atoms), and at frequencies above 60 GHz, the linear attenuation exceeds 15 dB / km. Attenuation in atmospheric water vapor depends on its concentration and is very large in humid warm climates and dominates at frequencies below 45 GHz.

Also, hydrometeors, which include raindrops, snow, hail, fog, etc., negatively affect radio communication. The influence of hydrometeors is noticeable already at frequencies above 6 GHz, and in unfavorable environmental conditions (in the presence of metallized dust, smog, acids or alkalis in atmospheric precipitation ) and at much lower frequencies.

Principles of constructing RRL equipment

RRL equipment is usually built on a modular basis. Functionally, a module of standard interfaces is distinguished, usually including one or more PDH (E1, E3), SDH (STM-1), Fast Ethernet or Gigabit Ethernet interfaces, or a combination of the above interfaces, as well as control and monitoring interfaces for radio relay links (RS-232 and etc.) and synchronization interfaces. The task of the standard interface module is to switch interfaces between themselves and other radio relay modules. Structurally, the module of standard interfaces can be a single unit or consist of several units installed in a single chassis. In the technical literature, the standard interface module is usually called an indoor unit (since usually such a unit is installed in a line equipment room or in a telecommunication trailer). Data streams from several standard interfaces are combined into a single frame in the I / O unit. Further, service channels are added to the received frame, which are necessary for RRL control and monitoring. In total, all data streams form a radio frame. The radio frame from the indoor unit, as a rule, is transmitted at an intermediate frequency to another functional unit of the RRL - the radio module. The radio module performs noise-immune coding of the radio frame, modulates the radio frame according to the type of modulation used, and also converts the total data stream from the intermediate frequency to the RRL operating frequency. In addition, the radio module often performs the function of automatically adjusting the power gain of the RRL transmitter. Structurally, the radio module is one sealed unit with one interface that connects the radio module to the indoor unit. In the technical literature, a radio module is usually called an outdoor unit, because in most cases, the radio module is installed on a radio relay tower or mast in the immediate vicinity of the radio relay antenna. The location of the radio module in the immediate vicinity of the radio relay antenna is usually due to the desire to reduce the attenuation of the high-frequency signal in various transition waveguides (for frequencies above 6 - 7 GHz) or coaxial cables (for frequencies below 6 GHz).

In the currently outdated analog radio relay lines, as well as main digital radio relay lines, both units with standard interfaces and radio modules are usually installed in the line equipment room. This is due to the implementation of complex N + 1 redundancy schemes, when it is impossible to locate a power divider from one antenna to several radio modules in the immediate vicinity of the antenna due to the bulkiness of the power divider. In this case, the radio modules and the antenna are connected by a waveguide laid from the line equipment room to the place where the antenna is attached to the radio relay tower.

The form of digital radio relay lines is also widespread, in which a standard interface module and a radio module are structurally combined in the form of one sealed unit with several standard interfaces, a power connector and a waveguide connector for direct attachment to the antenna.

Configurations and methods of redundancy

In the most important areas, in order to reduce the unavailability of RRL intervals, various methods of RRL equipment backup are used. Typically configurations with redundant RRL equipment are denoted as the sum "N + M", where N denotes the total number of RRL trunks, and M - the number of reserved RRL trunks. After the sum, the abbreviation HSB, SD or FD is added, which denotes the method of reserving RRL trunks.

Reducing the unavailability ratio is achieved by duplicating the functional blocks of the RRL or using a separate backup RRL trunk.

Configuration 1 + 0

Configuration of RRL equipment with one barrel without redundancy.

Configuration N + 0

Configuration of RRL equipment with N trunks without redundancy. The N + 0 configuration consists of several RRL frequency trunks or trunks with different polarizations, operating through one antenna. In the case of using several frequent shafts, the shafts are separated using a power divider and frequency band-pass filters. In the case of using RRL trunks with different polarizations, the separation of the trunks is carried out using special antennas that support the reception and transmission of signals with different polarizations (for example, cross-polarized antennas with the same gain for a signal with horizontal and vertical polarization).

The N + 0 configuration does not provide RRL redundancy; each trunk is a separate physical data transmission channel. This configuration is usually used to increase the throughput of radio relay links. In RRL equipment, individual physical data transmission channels can be combined into one logical channel.

Configuration N + 1 HSB (Hot StandBy)

Configuration of RRL equipment with N trunks and one reserve trunk in the "hot" reserve. In fact, redundancy is achieved by duplicating all or part of the RRL functional blocks. In case of failure of one of the RRL units out of order, the units in the "hot" standby replace the inoperative units.

Configuration N + M HSB (Hot StandBy)

Configuration of RRL equipment with N barrels and M reserve barrel in "hot" reserve.

Configuration N + 1 SD (Space Diversity)

Configuration N + M SD (Space Diversity)

Configuration N + 1 FD (Frequency Diversity)

Configuration N + M FD (Frequency Diversity)

Ring topology building RRL

Constructed RRL intervals on a ring topology is one of the most reliable methods of redundancy, even if all RRL intervals in the ring operate in a 1 + 0 configuration. Nevertheless, there are several rules for constructing a ring topology of RRL intervals: the number of spans in a ring must be at least four, and the angle between adjacent RRL intervals must be greater than 90 ° (in order to reduce the influence of hydrometeors on adjacent RRL intervals).

As a rule, in real networks consisting of radio relay links, various methods of redundancy are combined in order to increase the reliability of the network.

Technologies used in RRL

Digital radio relay links are used not only for organizing PDH and SDH communication lines, but also for organizing Ethernet lines with a transmission rate of up to 2.5 Gbit / s without using such technologies as EoPDH, PoSDH. Transmission of Ethernet frames without the need to encapsulate their TDM frames (E1 or E3 streams, SDH frames, etc.) is possible due to the use of a packet radio frame instead of a TDM radio frame in the radio channel. According to the technologies used to organize radio frames, the following types of digital radio relay links are distinguished:

• packet RRL
• hybrid RRL
• TDM RRL

Packet includes digital radio relay links with a packet radio frame. For transmission of TDM streams, pseudowire data transmission technologies are used. Due to the use of a packet radio frame, it is possible to use QoS mechanisms over data streams transmitted through packet radio relay links. Also, in packet RRL, adaptive modulation is most often used, usually combined with QoS.

Energy and quality indicators

The main documents for calculating the energy and quality indicators of RRL line of sight on the territory

In RRSP line-of-sight, to increase the distance between stations of radio-relay lines, repeater antennas are suspended on tall structures (masts, supports, high-rise buildings, etc.). In conditions of flat terrain, the height of raising the antennas 60 ... 100 meters allows you to organize reliable communication at distances of 40 ... 60 kilometers.

The chain of the radio relay line consists of radio relay stations of three types: terminal radio relay stations (ORS), intermediate radio relay stations (RRS), nodal radio relay stations (URS). A conventional radio relay communication line is schematically shown in Figure 8.1.

Figure: 8.1 Radio relay communication line

At the terminal radio relay station, the transmission path begins and ends. The OPC equipment converts signals coming from different sources of information (telephone signals from a long-distance telephone exchange, television signals from a long-distance television control room, etc.) into signals transmitted over a radio relay line, as well as reverse conversion of signals arriving via an RRL into broadcasting or telephony signals. The OPC radio signals are emitted by the transmitter and antenna towards the next, usually intermediate, radio relay station.

Intermediate radio relay stations are designed to receive signals from the previous radio relay station, amplify these signals and radiation in the direction of the next radio relay station.

At each intermediate radio relay station, two antennas are installed, oriented to the neighboring RRSP. Each of the antennas is a transceiver, that is, it is used for both receiving and transmitting signals. One of the advantages of operating a radio relay communication line in the microwave (microwave) range is the possibility of using highly directional antennas with small dimensions. The small size of the antennas makes them easy to install on tall buildings. The good directional properties of microwave antennas make it possible to alleviate the requirements for the characteristics of the transceiver path.

To eliminate such phenomena, repeaters of the radio-relay communication line are placed not in a straight line, but in a zigzag, so that the main directions of neighboring sections of the route using the same frequencies do not coincide. In this case, the directional properties of the antennas are used. Radio-relay stations are spaced from the general direction of the radio-relay communication line in such a way that the minimum levels of the antenna radiation pattern correspond to the direction to the station spaced three spans away. Figure 8.4 shows three spans of the RRL route section. On the outermost spans, the same frequencies are used. On such a path, even with a strong refraction of radio waves, the signals from the stations with the numbers PRS i and PRS i + 2 practically do not affect each other. It is noticeable in the figure that the antennas practically do not perceive radio waves coming from a direction lying on a straight line connecting these stations. Figure: 8.4 Layout of repeaters on the route of the radio relay communication line

Tropospheric radio relay transmission systems use local volumetric inhomogeneities in the atmosphere caused by various physical processes occurring in near-earth space. These inhomogeneities are capable of reflecting and scattering electromagnetic oscillations as they propagate in the atmosphere. Since the irregularities are located at a considerable height, then the radio waves scattered by them can propagate over long distances, significantly exceeding the line-of-sight distance.

Due to the irregular structure of tropospheric irregularities, signals from tropospheric lines are subject to deep fading.

Satellite communication systems can be considered as a special type of radio relay communication lines, if the repeater antenna is suspended on a support, the height of which is equal to the height of the satellite's orbit. In such a communication system, the line-of-sight area of \u200b\u200bthe Earth's surface, viewed from a satellite, and, accordingly, the size of the earth's territory from which the satellite is visible at the same time instant is significantly increased.

The radio equipment of a satellite communication system located on a satellite is called a space radio station, and radio equipment located on Earth is called a ground radio station. The channel for transmitting the radio signal from the ground station to the satellite is called the upstream, and the channel for transmitting signals in the opposite direction is called the downstream. On satellites, in addition to relay equipment, power sources (solar batteries) are also placed. In addition, the satellites have equipment that ensures the stabilization of the position of the satellites in orbit and its orientation in space (the repeater antennas are directed towards the Earth, the solar batteries - towards the Sun).

The performance of satellite communication systems is highly dependent on the parameters of the satellite's orbit. The satellite's orbit is the trajectory of the satellite in space.

Radio relay communication provides high-quality duplex communication channels, which practically do not depend on the time of year and day, weather conditions and atmospheric interference.

When organizing radio relay communication, it is necessary to take into account its dependence on the terrain, which necessitates a careful choice of the communication line route, the impossibility of working or a significant reduction in the range of radio relay stations in motion, the possibility of intercepting transmissions and creating radio interference by the enemy.

Radio relay communication can be organized in direction, along the network and along the axis. The application of this or that method in each individual case depends on the specific conditions of the situation, the peculiarities of the organization of management, the terrain, the importance of this connection, the need for exchange, the availability of funds and other factors.

Direction of radio relay communication - this is a way of organizing communication between two control points (commanders, headquarters) (Fig. 19).

Figure 19. Organization of radio relay communication by directions

This method provides the most reliable operation of the direction of communication and its greater throughput, but in comparison with other methods, it usually requires an increased consumption of frequencies and radio relay stations at the headquarters organizing communication. In addition, when organizing communications in directions, difficulties arise in placing a large number of radio relay stations without mutual interference at the communications center of the senior headquarters and the possibility of maneuvering channels between directions is excluded.

Radio relay network - this is a method of organizing communications, in which the connection of the senior command post (commander, headquarters) with several subordinate command posts (commanders, headquarters) is carried out using one radio relay semi-set (Fig. 20).

Figure 20. Organization of a radio relay network

During network operation, the transmitters of the radio relay stations of the slave correspondents are constantly tuned to the frequency of the receiver of the main station. It should be borne in mind that in the absence of exchange, all stations on the network must be in simplex mode, that is, in standby mode. The call right is granted primarily to the main station. After the main station calls one of the correspondents, the conversation between them can continue in full duplex mode. At the end of the conversation, the stations again switch to simplex mode. The number of radio relay stations in the network should not exceed three or four.

Network communication is possible mainly when the master station operates on a non-directional (whip) antenna. Depending on the situation, subordinate correspondents can use both whip and directional antennas. If the subordinate correspondents are located relative to the main station in any one direction or within the directional radiation sector of the main station antenna, then communication between the senior commander and the subordinates can be provided via the network and when working on a directional antenna having a relatively large directional angle (60 - 70 ° ).

Radio relay axis is a method of organizing radio relay communications, in which the communication of the senior control point (commander, headquarters) with several subordinate control points (commanders, headquarters) is carried out along one radio relay line deployed in the direction of movement of its control point or one of the control points 1 of subordinate headquarters (Fig. . 23).

Figure 21. Organization of the axis of radio relay communication

Communication of the control center of the senior headquarters with the control points is carried out through the support (auxiliary) communication centers, where telephone and telegraph channels are distributed between the control centers.

Compared with communication by directions, the organization of radio relay communication along the axis reduces the number of radio relay stations at the communication center of the control center of the senior headquarters and thereby simplifies the assignment of frequencies to these stations without mutual interference, makes it possible to maneuver channels, ensures their more efficient use, reduces the time for selection and calculation of routes, facilitates the management of radio relay communications and requires fewer personnel required for the protection and defense of intermediate stations. The disadvantages of this method are the dependence of the entire radio relay communication on the operation of the center line and the need for additional channel switching at the reference (auxiliary) communication nodes. The capacity of the axis is determined by the capacity of the center line, therefore, the organization of radio relay communication along the axis is advisable only if multichannel stations are used on the center line, and low-channel stations are used on the reference lines. The use of low-channel stations for the axis does not give the desired effect, since it requires a significant number of these stations and frequencies.

Radio relay communication is carried out directly or through intermediate (relay) radio relay stations. These stations are deployed in cases where communication directly between terminal stations is not ensured due to their remoteness from each other or due to terrain conditions, as well as if it is necessary to allocate channels at an intermediate point.

Basic principles of radio relay communication

The structure of the radio relay transmission system. Basic concepts and definitions. Radio relay trunk. Multilateral RRSU. Frequency bands used for radio relay communications. Frequency plans.

Under radio relay understand radio communication based on the retransmission of radio signals of decimeter and shorter waves by stations located on the surface of the Earth. The set of technical means and environment for the propagation of radio waves to provide radio relay communication forms radio relay communication line.

Terrestrial called a radio wave propagating near the earth's surface. Terrestrial radio waves shorter than 100 cm propagate well only within line of sight. Therefore, a radio-relay communication line for long distances is built in the form of a chain of receiving and transmitting radio relay stations (RRS), in which neighboring RRS are placed at a distance that provides line-of-sight radio communication, and they call it line-of-sight radio relay (RRL).

Figure 1.1 - To an explanation of the principle of constructing the RRL

A generalized block diagram of a multichannel RSP is shown in Fig. 1.3.

Figure: Generalized block diagram of a multichannel radio transmission system:

1.7 - channel-forming and group equipment;

2.6 - connecting line;

3, 5 - trunk terminal equipment;

4 - radio channel

Span (interval) RRL is the distance between the two nearest stations.

Section (section) RRL is the distance between the two nearest serviced stations (URS or OPC).

The channel-forming and group equipment provides the formation of a group signal from a plurality of primary telecommunication signals to be transmitted (at the transmitting end) and the reverse transformation of the group signal into a plurality of primary signals (at the receiving end). This equipment is usually located at network stations and switching nodes of the primary EASC network.

RSP stations, including those at which the allocation, introduction and transit of transmitted signals are made, as a rule, are geographically remote from network stations and switching nodes, therefore, most RSPs include wired connecting lines.

To form a radio signal and transmit it over a distance by means of radio waves, various radio communication systems are used. A radio communication system is a complex of radio technical equipment and other technical means designed to organize radio communication in a given frequency range using a certain mechanism of radio wave propagation. Together with the medium (path) of propagation of radio waves, the radio communication system forms linear pathor trunk.The RSP barrel consists of the terminal equipment of the barrel and the radio channel. The trunk equipment is located at terminal and relay stations.

In the terminal equipment of the trunk at the transmitting end, a linear signal,consisting of group and auxiliary service signals (service communication signals, pilot signals, etc.), which modulate high-frequency oscillations. At the receiving end, the reverse operations are performed: the high-frequency radio signal is demodulated and the group signal is extracted, as well as auxiliary service signals. The trunk terminal equipment is located at the RSP terminal stations and at special relay stations.

The purpose of the radio channel is to transmit modulated radio signals over a distance using radio waves. A radio channel is called simple if it includes only two terminal stations and one radio wave propagation path, and composite if, in addition to two terminal radio stations, it contains one or more relay stations that provide reception, conversion, amplification and retransmission of radio signals. The need to use composite radio channels is due to a number of factors, the main of which are the length of the RSC, its capacity and the mechanism of radio wave propagation.

The structural diagram of the barrel of a bilateral RSP is shown in the figure

Figure: 1.4. Block diagram of the trunk of a two-way radio transmission system:

1 - terminal equipment;

2 - transmitting equipment;

3 - the reception is equipped;

4 - transmitter;

5 - receiver;

6 - feeder path;

7 - antenna;

8 - path of propagation of radio waves;

9 - interference (internal and external)

From terminal transmitting equipment 2 barrels ^ 1, a high-frequency radio signal modulated by a linear signal arrives at the input of the radio channel. In the radio transmitter 4 the power of the radio signal is increased to the nominal value, and its frequency is converted to transfer the spectrum to the specified frequency range. Through the feeder path 6, the transmitted radio signals are directed to the antenna 7, which ensures the radiation of radio waves into the open space in the desired direction. At the same time, in most modern two-sided RSPs, a common antenna-feeder path is used for transmitting and receiving radio signals in opposite directions. In open space (propagation path 8) radio waves propagate at a speed close to the speed of light c \u003d 3 * 10 8 m / s. Part of the energy of radio waves coming from the radio station 1, is picked up by antenna 7 located at the terminal radio station 2. Energy of the received radio signal from antenna 7 along the feeder path 6 is sent to the radio receiver 5, where the frequency selection of the received radio signals, the reverse frequency conversion and the necessary amplification are carried out. From the output of the radio channel, the received radio signal enters the terminal equipment of the barrel 1. Similarly, radio signals are transmitted in the opposite direction from terminal radio station 2 to radio station 1. As seen from Fig. 1.4, a two-way RSP radio channel consists of two radio channels, each of which provides the transmission of radio signals in one direction. Thus, the radio channel equipment (including radio transmitters, radio receivers and antenna-feeder paths) is, in fact, equipment for interfacing the terminal equipment of the RSP trunk with the radio wave propagation path.

Frequency ranges

Frequency plans

For RRL operation, frequency bands with a width of 400 MHz in the range 1.2 GHz (1.7 ... 2.1 GHz), 500 MHz in the ranges 4 (3.4 ... 3.9), 6 (5.67 ... .6.17) and 8 (7.9 ... 8.4) GHz and 1 GHz wide in the 11 and 13 GHz bands and higher. These bands are allocated to the HF trunks of the radio-relay system in a specific plan called a frequency allocation plan. Frequency plans are designed to ensure minimal mutual interference between the trunks sharing the antenna.

In the 400 MHz band, 6 duplex HF trunks can be organized, in the 500 MHz band - 8 and in the 1 GHz-12 band.

In terms of frequencies (Fig. 1.3), the average frequency f0 is usually indicated. The reception frequencies of the trunks are located in one half of the allocated band, and the transmission frequencies in the other. With such a division, a sufficiently large shift frequency is obtained, which provides sufficient isolation between the receive and transmit signals, since the receive RF (or transmit RF) will work only in half of the entire system frequency band. In this case, you can use a common antenna for receiving and transmitting signals. If necessary, additional isolation is obtained between the receiving and transmitting waves in one antenna due to the use of different polarizations. RRL uses waves with linear polarization: vertical or horizontal. Two variants of polarization distribution are used. In the first version, on each PRS and EOS there is a change in polarization so that waves of different polarization are received and transmitted. In the second variant, one polarization of waves is used in the direction "there", and in the direction "backward" - another.

Figure 1.3. Frequency allocation plan for the KURS radio relay system for an NV type station in bands 4 (f0 \u003d 3.6536), 6 (f0 \u003d 5.92) and 8 (f0 \u003d 8.157)

The station at which the receiving frequencies are located in the lower (H) part of the allocated band, and the transmitting frequencies in the upper (B) part are designated by the index "HB". At the next station, the receiving frequency will be higher than the transmitting frequency and such a station is designated by the index "BH".

For the reverse direction of communication of a given trunk, one can take either the same pair of frequencies as for the forward one, or another. Accordingly, they say that the frequency plan allows you to organize work on a two-frequency (Fig. 1.4) or four-frequency (Fig. 1.5) systems. In these figures through f1n, f1v, ... f5n, f5v the average frequencies of the trunks are indicated. The frequency indices correspond to the designations of the wells in Fig. 1.3. With a two-frequency system, the same frequency must be taken on the PRS and U PC for reception from opposite directions. Antenna WA1 (Fig. 1.4, a) will receive radio waves at a frequency f1н from two directions: main A and return B. A radio wave coming from direction B creates interference. The attenuation of this interference by the antenna depends on the protective properties of the antenna. If the antenna attenuates the backward wave by at least 65 dB compared to the wave coming from the main direction, then such an antenna can be used in a dual-frequency system. A dual-frequency system has the advantage that it allows organizing 2 times more RF trunks in a dedicated frequency band than a four-frequency system, but it requires more expensive antennas.

On trunk radio relay lines, as a rule, two-frequency systems are used. The frequency plan does not provide for guard frequency intervals between adjacent receive (transmit) shafts. Therefore, signals from adjacent wellbores are difficult to separate using RF. To avoid mutual interference between adjacent barrels, either even or odd barrels work on one antenna. In terms of frequencies, indicate the minimum frequency separation between the transmit and receive trunks connected to the same antenna (98 MHz in Fig. 1.3). As a rule, even trunks are used on main radio relay lines, and odd ones - on branches from them. In this case, the frequencies of reception and transmission between the trunks of the main RRL are distributed according to Fig. 1.4, c, and between the trunks of the zone RRL with a four-frequency system - according to Fig. 1.5, c.

In practice, the frequency plan implemented on the RRL based on a two-frequency (four-frequency) system is called a two-frequency (four-frequency) plan.

On RRL, there is a repetition of transmission frequencies through the span (see Fig. 1.1). At the same time, in order to reduce mutual interference between RRS operating at the same frequencies, the stations are placed in a zigzag manner relative to the direction between the end points (Fig. 1.6). Under normal propagation conditions, the signal from PPC1 at a distance of 150 km is strongly weakened and practically cannot be received at PPC4. However, in some cases, favorable conditions arise for the era of spread. In order to reliably mitigate such interference, the directional properties of the antennas are used. On the path between the direction of maximum radiation of the transmitting antenna PPC1, i.e. That is, the direction to PPC2, and the direction to PPC4 (the direction of the AC in Fig. 1.6) provide for a protective path bend angle a1 of several degrees, so that in the direction of the AC the transmitting antenna gain at the PPC1 is sufficiently small.

PPC classification, composition of terminal station equipment. Equipment composition and layouts of intermediate stations. Equipment and features of circuit designs of nodal radio relay stations.

The development of antennas, like the entire development of radio engineering, has gone a long and difficult way from the first antenna of A.S. Popov in the form of a long wire suspended above the ground to complex structures such as modern radar and radio relay antennas. Whole teams of scientists and engineers are currently working on their design and research.

The creation of broadband systems in radio engineering, be it antennas, amplifiers, etc., is always fraught with significant difficulties. Anyone who has a TV at home knows that for high-quality reception, for example, the third TV channel, a different antenna with different dimensions is needed compared to the antenna for the first channel. And it is very difficult to create television antennas that are equally effective for receiving all television programs. At centimeter and decimeter waves, however, these difficulties were overcome. Microwave links use very broadband antennas that work equally well in the frequency band occupied by several high frequency trunks. On the other hand, these antennas are highly directional.

Let's see how you can get a highly directional antenna, what difficulties you have to overcome for this.

First of all, we note one of the basic principles of antenna technology, which consists in the fact that the properties of the antenna when radiating radio waves, i.e. directivity, broadband, and others, remain unchanged when using the same antenna to receive radio waves. Based on this principle, in what follows we will only talk about transmitting antennas, assuming that the receiving antennas are the same in design and therefore work just as effectively. In practice, in microwave links, transmitting and receiving antennas are always the same.

A typical broadcasting or television station antenna emits radio waves evenly in all directions. This means that the power - of the transmitter is equally distributed in all directions and only a small part of the radiated energy is distributed in any one direction.

Let on the receiving side we receive the signals of the transmitting station. If the transmitter emits radio waves through a non-directional antenna, then on the receiving side we will receive a signal of a certain magnitude. Now let's change the transmitter antenna to a directional one and "aim" the direction of maximum radiation on the receiving antenna. On the receiving side, there will be a sharp increase in the received signal, although the transmitter power remains unchanged. It turns out that the antenna, as it were, amplifies the signal.

On radio relay lines, pointed antennas are used, which have a gain (in power) of the order of a thousand or even tens of thousands and a radio beam width of about 1-2 degrees. The latter means that the antenna radiates almost nothing in all directions that differ from the main one by more than 0.5-1 degrees.

Thus, due to the "amplification" of the antennas, the power of the transmitters can be reduced by several thousand times compared with the power that would be required if the antennas were omnidirectional. On the other hand, due to the directivity of the antennas, the interference of one radio relay line is sharply reduced

On the other, even if they are close to each other and operate at the same frequencies.

The "gain" of a directional antenna is explained by the fact that it does not distribute the energy emitted by the transmitter equally in all directions, but directs it in one direction, that is, as if it collects the transmitter energy from all directions in one direction. The word "amplification" is put in quotation marks because the antenna does not convert the energy of an external source into the energy of a radio signal, as is the case in the transmitter and receiver, ^ where the energy of the power sources is converted into high-frequency energy in the radio tubes and where only due to energy power supplies, the useful signal is amplified.

The most common on microwave links are parabolic and lens antennas.

Figure: 17 explains how a parabolic antenna works. Its appearance is shown in Fig. fourteen.

It has an irradiator either of a special design, or in the form of an open end of a waveguide, which directs the emitted energy to a parabolic metal reflector (most often in the form of a paraboloid of revolution). The irradiator emitting a diverging beam of radio waves (rays AB and AB "in Fig. 17) is located at the focus of the paraboloid, that is, at a certain point A on its axis of rotation. If the irradiator were very small or, as they say, point-like, then the rays reflected from the paraboloid would be parallel and directed towards the receiving antenna (in Fig. 17, the BV beam is parallel to the B "C" beam), that is, almost
all radio wave energy emitted by the transmitter would propagate in the direction we need.

But since the feed has finite dimensions and is not strictly in focus, the rays reflected from the paraboloid are not quite parallel: they diverge somewhat.

Numerous studies of highly directional antennas, and in particular parabolic ones, have shown that the larger the diameter of the parabolic surface in comparison with the wavelength, the narrower the radio wave beam emitted by it, the higher its directivity.

Paraboloids of radio relay stations on centimeter waves have a diameter of 3-4 meters and have a power amplification from one thousand to ten thousand. At meter wavelengths, the directivity of the antennas is less, and the gain is only 50 - * - 500, since we cannot increase the size of the antennas in proportion to the increase in wavelength when switching from centimeter waves to meter waves. Otherwise, we would have to have parabolic mirrors tens of meters in size. Their installation would require very cumbersome and expensive antenna supports.

The lens antennas are based on the principle of refraction of radio waves at the boundary of two media, i.e., a change in the direction of the beam when passing from one medium to another.

If a lens for light waves, that is, an optical lens, is a glass or some other transparent body of a certain convex or concave shape (glasses, camera lens, etc.), then a lens for radio waves usually has a completely different view. For example, it can be a set of parallel to each other metal plates of a special shape (Fig. 18), separated by air gaps. The shape of the plates is chosen so that the diverging beam of radio waves incident on the lens from the waveguide, passing through the lens, becomes parallel. And here the larger the size of the lens exit aperture in comparison with the wavelength, the higher the directivity of the antenna.

The horn in front of the lens is used to ensure that all the high-frequency energy exiting the waveguide reaches the lens.

Sometimes purely horn antennas are used on radio relay lines. Structurally, they are simpler and much lighter than horn-lens ones, however, with the same hole dimensions, the former have a slightly lower gain. In addition, the length of the horn here has to be taken in 1.5-

2 times more than with lenses.

In addition to directivity, the requirement for the absence of mutual influences between the receiving and transmitting antennas located at the same intermediate station is imposed on the antennas of radio relay lines.

It turns out that the antennas described above do not radiate all the energy in the main direction. Insignificant part

A / Justin Lenses

In fig. 20 shows the construction of another repeater station antenna system used on "local" radio links. Thanks to the ingenious use of flat reflectors, the construction of this station is much cheaper than the stations shown in Fig. 12 and fig. sixteen.

The principle of operation of such an antenna system is as follows: antennas with high gain are installed very close to the transceiver on the roof of a one-storey building of a relay station than

A small length of waveguides or cables is achieved, and, if necessary, a small amount of losses in them. The radiation from the transmitting antenna is directed vertically upward. On light steel masts, perforated (i.e., with holes to reduce wind load) metal sheets, inclined at an angle of 45 degrees to the horizon, are fixed at the required height. A vertically directed radio beam, like light from a mirror, is reflected from the sheets towards the next relay station. The receiving antenna is arranged in a similar way.

Note also that quite often at intermediate stations of radio relay lines, instead of four antennas, only two are used. The transmission and reception of one direction is carried out on one antenna. it
possible only on relatively low-channel lines, where the number of high-frequency trunks does not exceed three. To ensure that the emitted signal does not affect the received signal, their frequency bands are spaced approximately

At 100 megahertz (remember the channel multiplexing system at frequency). In this case, using filters, the transmitted and received frequency bands can be fairly well separated.