Principles of radio communication. Channel and radio link

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General schemes for organizing radio communications

An information transmission system in which telecommunication signals are transmitted by means of radio waves in open space is called a radio system. Radio systems are divided into radio links and radio networks.

According to the method of organizing radio links, one-way and two-way radio communications are distinguished. Radio communication, in which one of the radio links only transmits, and the other only receives, is called one-way. One-way radio communication, in which the radio transmission of one (main) radio station can be received simultaneously by several correspondents, is called circular. Examples of one-way broadcast messaging are public address systems, messaging services from press centers to newspapers, magazines, etc. Television and sound broadcasting networks are also typical examples of the circular way of organizing radio communications. In this case, the radio transmitting station, the radio signal propagation medium (open space) and each radio receiver located in the coverage area of ​​the station form a one-way radio link, and the totality of such radio links is a broadcasting network.

Two-way radio communication implies the possibility of transmitting and receiving information by each radio station. This requires two sets of one-way communication equipment, ie. each point must have both a transmitter and a receiver. Two-way communication can be simplex and duplex (Fig. 1). With simplex radio communication, transmission and reception at each radio station are carried out in turn. The radio transmitters at the end points of the communication line in this case operate at the same frequency, and the receivers are also tuned to the same frequency.

Fig.1 Functional diagrams of the organization of two-way radio communication: a-simplex radio communication, b-duplex communication

With duplex radio communication, radio transmission is carried out simultaneously with reception. Two different frequencies must be allocated for each duplex radio link. This is done so that the receiver receives signals only from the transmitter from the opposite point and does not receive signals from its own radio transmitter. The radio transmitters and receivers of both duplex radio correspondents are on during the entire time the radio link is in operation.

Simplex communication is used, as a rule, in the presence of relatively small information flows. For transmission systems with a large information load, duplex communication is typical.

If it is necessary to have radio communication with a large number of correspondents, then a radio network is organized (Fig. 2). In this case, one radio station, called the main one, can transmit messages for one or several subordinate correspondents. Its radio operator controls the mode of operation in the radio network and directly sets the order for the transmission of slave stations. The latter, with the appropriate permission, can exchange information not only with the main radio station, but also among themselves. This version of the radio network organization can be built on the basis of both a complex simplex (see Fig. 2, a) and a complex duplex (see Fig. 2, b). In the first case, it is possible to use radio stations (radio transmitters) operating on the same (common) radio wave (frequency). In the second "case, the main radio station transmits on one frequency, and receives on several (according to the number of slave radio stations).

Fig.2 Functional schemes for organizing a radio network: a-complex simplex, b-complex duplex

Any information transmission radio link (communication, sound or television broadcasting) contains radio transmitting and receiving devices equipped with antennas at the ends. The transmitting antenna radiates the electrical signal of the transmitter in the form of a radio wave. The receiving antenna picks up the radio wave, and from its output an electrical signal is fed to the input of the receiver. Electromagnetic energy transmission lines connecting the antenna to a radio transmitter or receiver are called feeders. Antenna-feeder devices are very important elements of a radio link. In practice, antennas with directional action are very often used. When transmitting, a directional antenna radiates radio wave energy in a specific direction. The more directivity of the antenna, the lower the power of the transmitter, radio communication is possible. Receiving directional antennas increase the signal-to-noise ratio at the input of the receiving device, which also makes it possible to reduce the required power of the radio transmitter.

The successful operation of radio links depends not only on the design features and manufacturing quality of radio equipment. During the construction and operation of radio links, it is necessary to take into account the features of the propagation of radio waves on the way from the transmitting to the receiving antenna. These features are different depending on the frequency range. The division of radio waves into ranges in accordance with the Radio Regulations is given in Table. 1. Radio waves propagate on radio lines under natural conditions, and these conditions are varied and changeable. First of all, it must be taken into account that the Earth is round. On the way from the transmitting to the receiving antenna, the radio waves must go around the bulge of the Earth.

Table 1. Classification of the division of radio waves into ranges

By themselves, electromagnetic oscillations do not carry information. To transmit information, it is necessary to imprint a message on electromagnetic oscillations, i.e. use high-frequency electromagnetic oscillations only as a carrier of a message containing information. To this end, it is necessary to change one or more parameters of the carrier wave (for example, amplitude, frequency, phase, and other parameters) in accordance with changes in the message. Then a high-frequency oscillation is obtained with time-varying parameters according to the law of the transmitted message. The considered process is called modulation.

Thus, any radio transmitting device must consist of an electrical oscillation generator connected to a transmitting antenna and a modulator with which modulation is carried out.

At the receiving point there must be a device that converts the energy of electromagnetic waves into the energy of electrical oscillations, i.e. receiving antenna. The antenna picks up electromagnetic waves emitted by different transmitters operating at different frequencies. In order to receive signals from only one station, it is necessary to have a selective device capable of separating from the oscillations of various frequencies only those oscillations that are transmitted by the desired radio station. To solve this problem, electrical oscillatory circuits are used that are tuned to the frequency of the received radio station.

The high-frequency oscillations selected with the help of the oscillatory circuit must be subjected to inverse transformation, i.e. obtain from them currents or voltages that change in accordance with the law of modulation of electrical oscillations in the radio transmitter. To solve this problem, the receiver must have a special device called a detector.

Finally, the selected signal must be fed to some terminal device that will record it or allow a person to perceive it in the form of sound or light (image).

Consider the structure of radio communication (Fig. 2.15).

The microphone (M) converts the sound vibrations of speech into electrical current vibrations of sound (low) frequency. One of the main blocks of the radio transmitter is a master oscillator (MG) (or a high frequency generator), which converts direct current energy (a special power source) into the energy of high frequency (HF) current fluctuations. The audio frequency current amplified in the low frequency amplifier (ULF) enters the modulator (Mod), affecting one of the parameters (amplitude, frequency or phase) of the high frequency current. generated by the master oscillator. As a result, high frequency (radio frequency) currents are fed into the transmitter antenna, varying in amplitude, frequency, or phase in accordance with the transmitted sound vibrations (transmitted by the original message). The process of influencing one of the parameters of the RF signal according to the law of changing the transmitted initial message is called modulation , respectively amplitude, frequency or phase.

Figure 2.15 - Structural diagram of radio communication

High-frequency currents, passing through the transmitter antenna, form an electromagnetic field around it. Electromagnetic waves (radio waves) are separated from the antenna and propagate in space at a speed of 300,000 km/s.

In the receiving antenna, radio waves (an electromagnetic field) induce an EMF of radio frequency, which creates a modulated RF current that exactly repeats all current changes in the transmitting antenna. The high frequency currents from the receiving antenna are transmitted via the feeder line to a selective high frequency amplifier (UHF). Selectivity is provided by a resonant circuit, most often consisting of an inductor and a capacitor connected in parallel, forming a parallel oscillatory circuit having a current resonance at the frequency of the electromagnetic oscillations transmitted by the transmitter. To transmitters of radio stations operating at other frequencies, this radio receiver is practically insensitive.

The amplified signal is fed to the detector (Det), which converts the received RF signals into sound vibration currents, which change like the sound frequency currents created by the microphone at the transmitter. Such a transformation is called detection (demodulation). The audio or low frequency (LF) current received after detection is usually still amplified in the ULF and transmitted to the loudspeaker (speaker or headphones), which converts this LF current into sound vibrations.

Radio communication is one-way and two-way. With one-way radio communication, one of the radios only transmits, and the other (or others) only receives. In two-way radio communication, radios transmit and receive at the same time.

Simplex radio- this is a two-way radio communication, in which each subscriber only transmits or only receives in turn, turning off his transmitter for the duration of the reception (Fig. 2.16). For simplex communication, one radio frequency is sufficient (single-frequency simplex radio communication). Each radio station has one antenna, which, when receiving and transmitting, switches respectively to the input of the radio receiver or to the input of the radio transmitter.

Figure 2.16 - Structural diagram of simplex radio communication

Simplex radio is usually used when there are relatively small traffic flows. For radio networks with a large load, duplex communication is typical.

duplex radio- This is a two-way radio communication in which reception and transmission are carried out simultaneously. Two-way radio communication requires two different carrier frequencies, and transmitters and receivers must have their own antennas (Figure 2.17). In addition, a special filter is installed at the input of each receiver ( duplexer), which does not transmit vibrations of the radio frequency of its own transmitter. The advantages of duplex radio communication are its high efficiency and bandwidth of the radio network.

Figure 2.17 - Structural diagram of duplex radio communication

Radio communication has the following advantages over wired communication:

Ø rapid deployment on any terrain and in any conditions;

Ø high efficiency and survivability of radio communications;

Ø the ability to send various messages to any number of subscribers circularly, selectively or to a group of subscribers;

Ø the ability to communicate with moving objects.

Radio transmitting devices

In a functional sense, a radio transmitting device is understood as a set of equipment designed to form and emit a radio frequency signal (radio signal). As functional units, the radio transmitter includes a carrier generator and a modulator. In addition, radio transmitting devices (especially powerful ones) contain a lot of other equipment: power supplies, means of cooling, automatic and remote control, signaling, protection and blocking, etc.

The main indicators of radio transmitting devices can be conditionally divided into 2 groups: energy and electromagnetic compatibility indicators.

The most important energy indicators of a radio transmitting device are the rated power and industrial efficiency. Under rated power (P) understand the average value of the energy supplied to the antenna over the period of the radio frequency oscillation. Industrial efficiency factor (COP) is the ratio of the rated power P to the total P total consumed from the AC mains by the radio transmitter: η = P/P total 100%.

The main indicators of electromagnetic compatibility are the operating frequency range, oscillation frequency instability and out-of-band emissions.

Operating frequency range called the frequency band in which the radio transmitting device provides operation in accordance with the requirements of the standard.

Under frequency instability radio transmitter understand the deviation of the oscillation frequency at its output for a certain period of time relative to the set frequency. Small instability (high stability) of the frequency allows you to reduce interference to radio reception.

out of band call such radiation, which are located outside the band allocated for the transmission of useful messages. Out-of-band emissions are a source of additional interference to radio reception. When suppressing out-of-band emissions, the signal transmission quality does not deteriorate.

According to their purpose, radio transmitting devices are divided into communication devices. Broadcasting and television. According to the operating frequency range, radio transmitting devices are divided in accordance with the classification of types of radio waves. Depending on the rated power, radio transmitting devices are divided into low-power (up to 100 W), medium power (from 100 to 10000 W), powerful (from 10 to 500 kW) and heavy-duty (over 500 kW).

The specifics of operation makes it possible to distinguish between stationary and mobile radio transmitting devices (automobile, aircraft, portable, etc.).

radio receivers

radio reception is the separation of signals from radio emission. In the place where radio reception is conducted, there are simultaneously radio emissions from many natural and artificial sources. The power of a useful radio signal is a very small fraction of the power of the total radio emission at the place of radio reception. The task of the radio receiver is to isolate a useful radio signal from a variety of other signals and possible interference, as well as to reproduce (restore) the transmitted message.

The main (in the sense of universality) indicators of radio receivers are: operating frequency range, sensitivity, selectivity, noise immunity.

Operating frequency range determined by the range of possible tuning frequencies. In other words, this is the tuning frequency range within which the radio receiver can smoothly or hop from one frequency to another.

Sensitivity is a measure of the ability of a radio receiver to receive weak radio signals. It is quantitatively estimated by the minimum value of the electromotive force (EMF) of the signal at the input of the radio receiver, at which the required signal-to-noise ratio at the output takes place in the absence of external interference.

Selectivity is called the property of a radio receiver that allows you to distinguish a useful radio signal from radio interference according to certain features inherent in the radio signal. In other words: it is the ability of the radio receiver to isolate the desired radio signal from the spectrum of electromagnetic waves at the receiving site, reducing interfering radio signals. Distinguish between spatial and frequency selectivity. Spatial selectivity It is achieved by using an antenna that provides reception of the desired signals from one direction and attenuation of radio signals from other directions from extraneous sources. Frequency selectivity quantitatively characterizes the ability of the radio receiver to select from all radio frequency signals and radio interference acting at the input, the signal corresponding to the tuning frequency of the radio receiver.

Noise immunity radio receiver is called its ability to counteract the interfering effect of interference. Quantitatively, the noise immunity is estimated by the maximum value of the interference level in the antenna, at which the reception of radio signals is still ensured.

Radio receivers can be classified according to various criteria. According to the purpose, broadcasting (usually called radio receivers or receivers), television (TV sets), professional, special radio receivers can be distinguished. The professional ones include trunk radio receivers of the decameter range, radio relay and satellite communication lines. Among special-purpose radio receivers, for example, radar, radio navigation, aircraft, etc. should be mentioned.

Antennas and feeders

Antenna represents an interface element between the transmitting or receiving equipment and the radio wave propagation medium. Antennas, which look like wires or surfaces, emit electromagnetic waves when transmitting, and when receiving, they "collect" the incident energy. Antennas consisting of wires with a small cross-section compared to the wavelength and longitudinal sections are called wire. Antennas that radiate through their aperture are called aperture. Sometimes they are called diffraction, reflex, mirror. The electric currents of such antennas flow along conductive surfaces having dimensions commensurate with the wavelength or much larger than it.

The electrical circuit and accessories by which RF signal energy is conducted from a radio transmitter to an antenna or from an antenna to a radio receiver is called feeder. The following requirements are imposed on feeders: energy losses of high-frequency signals in it must be minimal; they should not have an antenna effect, i.e. must not emit or receive electromagnetic waves; have sufficient electrical strength, i.е. transmit the required power without the danger of electrical breakdown of the insulation.

Transmitting antennas used in the kilometer and hectometer radio wave bands are connected to the radio transmitter using multi-wire coaxial feeders. In the decameter range, feeders are usually made in the form of wire two- or four-wire lines. To antennas of meter radio waves, energy, as a rule, is conducted using a coaxial cable. At shorter wavelengths, in particular in the centimeter range, the feeder is made in the form of a hollow metal pipe - a waveguide of rectangular, elliptical or circular cross section.

The classification and propagation methods of radio waves are given in the tables below.

Any type of communication is designed to transmit information over a distance. Information is a collection of information about events in the surrounding world. The form of information presentation is a message, which can be speech, text, a sequence of numbers, etc.

To transmit a message from the source of information to the recipient, it is necessary to use any physical process that can propagate at a certain speed from the source to the recipient of information, for example: sound vibrations, electric current in conductors, light, electromagnetic field, etc. The physical quantity that determines this process, changing in time and displaying the transmitted message (current strength, intensity of the electromagnetic field, brightness of light, etc. is called a signal. Signals are not a transmitted message, but only display it. Often, the signal obtained as a result of message conversion is called the primary electrical signal. Depending on the nature of the message, the primary electrical signals may be continuous or discrete.

Continuous signals take on any values ​​in terms of states in a certain interval. Such signals are described on some sufficiently large time interval by continuous functions of time. A typical example of a continuous signal is a speech signal, its amplitude changes continuously with time within ±Umax. When transmitting such a telephone signal, it is necessary first of all to take into account its frequency spectrum.

It is known that the spectrum of sounds perceived by the human ear occupies a frequency band ranging from 16 to 20,000 Hz. However, the transmission of such a wide range of frequencies over communication channels is associated with certain difficulties associated with an increase in the frequency band occupied by the communication channel, and, consequently, with a decrease in the number of communication channels provided in a certain frequency range. Therefore, in telephone communication, the speech signal spectrum is limited to a frequency band from 300 to 3400 Hz, in which the main frequency components and the main energy of human speech sounds are located (Fig. 2.1).

At the same time, such a limitation of the frequency spectrum of the telephone signal does not lead to a noticeable distortion of the signal. The spectrum width of 0.3¸3.4 kHz was called the standard telephone channel.

Discrete signals take on a finite number of well-defined values ​​by state. The most common example of discrete signals is telegraph signals that display the text of a message using a specific alphabet (code). In this case, each letter or digit of the code is expressed by a well-defined discrete state of the signal. In Fig.2.2. Discrete states are shown that the signal receives when transmitting the letter "Ж" using Morse code.

The transmission of telegraph signals can be carried out at different telegraphy speeds. The speed of telegraphy is determined by the number of elementary impulses transmitted per unit of time (1s) and is measured in Bauds (B). 1 B = 1 imp / 1 s. For most direct-printing telegraph machines, the telegraphy speed is 50 baud. The primary electrical signal, regardless of its type, is of a low-frequency nature. It can be directly transmitted over wired communication lines, but it cannot be effectively radiated into the propagation medium, since it is practically impossible to create antennas whose geometric dimensions would be commensurate with the signal wavelength.

For example, at F=1kHz, the wavelength is l=300(km), and the antenna length is L=l/4 = 75(km), which is practically not feasible. Therefore, for radio transmission, the primary electrical signal must be converted into a high-frequency signal capable of being efficiently radiated into the surrounding space. Such a signal is called a radio signal. The conversion of primary low-frequency electrical signals into radio signals is carried out in radio transmitters, which are the main part of radio transmitting devices. The process of converting continuous primary signals into radio signals is called modulation, and discrete ones are called manipulations.

A radio signal formed and radiated into the environment in the form of radio waves, propagating at a certain speed, reaches the location of the recipient of information. When a radio signal passes through a propagation medium, it is affected by other signals determined both by the properties of the propagation medium itself and by other sources of electrical signals. At the point of receiving the transmitted information, it is necessary to convert the radio signal into a message.

The conversion of radio signals arriving at the receiving point into the original message is carried out by the radio receiver. The task of converting a received radio signal into a message is more complicated than converting a message into a radio signal, since not only the transmitted radio signal is converted, but its mixture with other signals (interference) that can distort the transmitted message.

The source of information, the radio transmitter, the propagation medium of radio waves, the radio receiver and the recipient of information form a radio communication line (Fig. 2.3). The block diagram of the radio link, shown in Fig. 2.3., ensures the transmission of a message in only one direction - from the source of information to the recipient, i.e. one way radio. To ensure two-way radio communication, it is necessary to have a radio transmitting radio receiver at each end of the radio link. In this case, the source of information and the recipient of information periodically change the functions performed in the radio link, so it is customary to combine them with one concept of a correspondent.

For two-way radio communication, the mode of operation of the radio link can be simplex or duplex. A radio link in which the transmission and reception of messages are carried out alternately is called simplex, but if the radio link provides simultaneous transmission and reception of information, then such a radio link is called duplex. A radio link that allows simultaneous transmission of several signals displaying independent messages is called multi-channel (two-channel, three-channel, etc.), but if the radio link is designed to transmit only one signal corresponding to one message, then it is called single-channel. Thus, a radio communication channel is understood as a part of the line that provides signal transmission and reception.

In the general case, a radio communication channel is understood as a part of a radio transmitting device, a medium for the propagation of radio waves and a part of a radio receiving device. Which parts of the radio transmitting and receiving device are included in the concept of a radio channel is negotiated separately. Most often, a radio communication channel (radio channel) is limited only by the propagation medium of radio waves. This is explained by the fact that the most characteristic features of the radio channel, which distinguish it from other communication channels, are determined precisely by the propagation medium. In the future, unless otherwise stated, by a radio channel we will understand the propagation medium of radio waves.

Thus, any radio transmitting device must provide the following three functions:

1. Converting a message into a primary electrical signal, which is carried out by the terminal transmitting equipment (microphone, telegraph key, telegraph machine, transmitting television tube, etc.).

2. Converting the primary electrical signal by modulating (manipulating) high-frequency oscillations into a radio signal that can be efficiently emitted and propagated in the form of radio waves over a given distance. This function is performed by the radio transmitter itself.

3. Emission of radio signals generated by a radio transmitter in the form of electromagnetic waves, carried out by a transmitting antenna-feeder device (AFD).

At the receiving end of the radio link, using a radio receiver, the radio signals are converted back into a message. The radio receiver also performs the following three main functions:

1. The receiving antenna-feeder device (AFD) captures the energy of electromagnetic waves and converts it into a radio signal.

2. The selection of the received radio signal from the set of signals induced in the antenna, and its conversion into a primary low-frequency signal of the required power, carried out by the radio receiver.

3. Converting the primary signal into a message performed by the receiving terminal equipment (headphones, speaker, receiving telegraph machine, television receiver, etc.). To ensure two-way radio communication, it is necessary to have a radio transmitting and receiving device at each end of the radio link, which are organizationally, and often structurally, together with control devices, combined into a single complex-radio station.

Figure 2.4 shows a generalized block diagram of the radio link between correspondents A and B.

The main properties of the radio channel, which distinguish it from other communication channels, are determined mainly by the properties of the propagation medium. Therefore, when considering this issue, the concept of a radio channel will be limited to the propagation medium of radio waves.

In radio communication, the space surrounding the earth's surface is used as a propagation medium. Such a medium does not have directional properties, as is the case, for example, in wired and cable communication lines. In radio communication lines, those emitted by the transmitting antenna propagate in almost all directions from the emitter, and only a small part of their energy is radiated towards the radio receiver of the correspondent. Radio wave energy is scattered in the propagation medium. In addition, due to the absorption of radio wave energy in the earth's surface and ionosphere, as well as due to the refraction of radio waves, there is an additional decrease in the energy of radio waves arriving at the receiving point. In cases where the energy of the radio waves arriving at the receiving point is insufficient to convert it into a primary signal, radio communication is impossible.

First property radio channel and lies in the fact that in the process of propagation of radio waves due to their scattering and absorption in the earth's surface and the ionosphere, there is a sharp decrease in the power of radio signals at the input of radio receivers. Therefore, the radio channel, unlike other communication channels, is considered as a channel with high attenuation.

The large attenuation of the radio channel leads to the fact that the level of the radio signal at the input of the radio receiver is commensurate with the level of fluctuation currents (intrinsic noise) of the radio receiver, which makes it difficult, and in some cases impossible, to recognize the received signals and separate them from noise.

It is possible to “reduce” the attenuation of the radio channel by choosing the optimal operating frequencies for a given time of the required radio communication range, as well as by more directional and efficient transmitting and receiving antenna devices.

The second property radio channel is the change in attenuation over time in a very wide range, so the radio channel is considered to be a communication channel with variable parameters. The change in the attenuation of the radio channel can occur for various reasons. The amount of attenuation in the radio channel is affected by changes in the relative position of radio stations on the ground and the distances between them, which is especially noticeable when radio communication is carried out by earth waves. Since the strength of the electromagnetic field decreases almost in proportion to the square of the path length traveled by the wave during propagation, any change in the distance between operating radio stations leads to a change in the power of the radio signal at the receiving point.

Obviously, these changes have a particularly strong impact on the provision of radio communications between moving objects. But even in cases where the distance between the operating radio stations remains constant, and only their relative position on the ground changes, rather sharp changes in the attenuation in the radio channel can occur, caused by changes in soil parameters, and, consequently, its absorbing properties. The parameters of dry soil differ from the parameters of wet soil and from the parameters of the water surface, and also depend on the type of soil itself - sand, clay, etc.

In the meter wave range, the absorbing properties of the propagation medium are strongly influenced by the terrain and local objects - hills, mountains, vegetation, buildings, etc. All this leads to a change in the attenuation of the radio channel, which can reach hundreds of decibels.

Third property radio channel is its public availability, i.e. the possibility of using the same propagation medium by any radio devices. The general availability of the propagation medium enables the simultaneous operation of a large number of radio links.

Thus, at the input of the receiving device, in addition to the received radio signal, there will always be interference that distorts it, but. hence the primary signal, directly displaying the transmitted message. The degree of distortion of the primary signal determines the correctness of the received message, i.e. its credibility.

So, to improve the reliability of radio communications and ensure high reliability of the received message, it is necessary to take the following measures:

To carry out radio communication at frequencies optimally selected according to radio forecasts, free from interference;

Use such types of radio signals that provide the required reliability of radio communications with the lowest possible values ​​of the degree of excess of the signal over interference;

Use efficient and directional transmitting and receiving antennas;

Reduce the bandwidth of the radio receiver to the smallest possible values ​​determined by the spectrum of the received radio signal.

Structural diagram and principle of construction of transceiver radio stations.

Any kind of radio communication is carried out using electromagnetic waves propagating in space at the speed of light. Electromagnetic waves are generated around an antenna device that is powered by high frequency alternating current. High frequency currents are generated (generated) by the radio transmitter. A radio transmitter is a device designed to perform two main functions:

1. generating high-frequency oscillations, i.e., converting the energy of power supplies into high-frequency electromagnetic oscillations;

2. modulation of these oscillations in accordance with the signals to be transmitted.

The modulated high-frequency oscillations received in the radio transmitter are transmitted to the antenna and then radiated in the form of free electromagnetic waves. Depending on the purpose, range of operating waves, power, type of control of transmitter oscillations, their design and schemes may be different.

Each radio transmitter consists of several stages that perform a specific role. The block diagram of the radio transmitter is shown in fig. 1.1.

The main element of the radio transmitter is an exciter designed to generate high-frequency oscillations in a given range with their high stability. As an exciter, a low-power tube generator with self-excitation (oscillator) is usually used.

The highly stable high-frequency oscillations obtained in the exciter are fed to the next element - an intermediate amplifier. In this cascade, high-frequency oscillations are pre-amplified to a value that ensures the normal operation of the next cascade - the power amplifier cascade. In the power amplifier, the high frequency signal is amplified to the required power. The amplified signal is transmitted to the transmitting antenna. In the antenna, high-frequency current is converted into electromagnetic waves propagating in space.

In low-power transmitters, there may be no intermediate stage, and high-frequency oscillations from the exciter are fed directly to the power amplifier. In transmitters of medium and high power, there may be several intermediate stages. In this case, in the intermediate stages, not only amplification of high-frequency oscillations can be carried out, but also the frequency of the exciter oscillations can be multiplied. Frequency multiplication makes it possible to expand the frequency range of the transmitter with a narrow-band exciter. The block diagram of such a transmitter is shown in fig. 1.2. This transmitter is four-stage. It consists of: an exciter, the first intermediate stage (doubling amplifier), the second intermediate stage (doubling amplifier) ​​and a power amplifier.

Exciter frequency range 1.5 - 3.0 MHz, the frequency range of the transmitter is 1.5-12.0 MHz. Such a wide transmitter frequency range is obtained due to frequency multiplication in the intermediate stages. The entire transmitter range is divided into three subbands. In the first subrange, both intermediate stages work as amplifiers of the exciter frequency oscillations, i.e., they amplify the high-frequency oscillations of the exciter in the range of 1.5 - 3.0 MHz. On the second subband, the first intermediate stage operates as a frequency doubler of the exciter.

The remaining stages act as amplifiers. So it turns out the second subrange 3-6 MHz. Finally, on the third sub-band, both intermediate stages work as frequency doublers, forming the third sub-band of the transmitter 6-12 MHz.

The transmitter power amplifier in all cases operates only in amplification mode. The principle of formation of operating frequencies of such a transmitter is illustrated in Table. 1.1.

To transmit messages, it is necessary to superimpose the oscillations of these messages on high-frequency oscillations generated by the transmitter and called carrier frequency oscillations. The process of controlling carrier frequency oscillations by the transmitted signal is called modulation. It is carried out by a special device - a modulator (Mod.). In addition to the listed elements, each transmitter has power supplies. The radio receiver (radio receiver) is the last link in the radio link.

The radio receiver is designed to isolate the correspondent's high-frequency signal from a variety of signals from various radio stations, amplify the selected weak signal, convert the high-frequency signal into an audio frequency signal and amplify the audio frequency signal to a value that ensures normal operation of the output device (telephones, loudspeakers). According to the principle of operation, there are several types of radio receivers. The most common of these are direct amplification receivers and superheterodyne type receivers.

In direct amplification radio receivers, the simplest in design, the main selectivity and signal amplification are carried out at a high frequency of the received signal. The high frequency signal amplified to the desired value is then converted into a low audio frequency voltage and, after appropriate amplification, drives telephones or loudspeakers. A block diagram of such a radio receiver is shown in fig. 1.26

Direct amplification radio receivers are simple in design, but do not provide the necessary selectivity and sufficient amplification. Therefore, such receivers are not currently used in military radio stations. More advanced, although much more complex, are superheterodyne-type radio receivers. In superheterodyne type radio receivers, the received high-frequency oscillations are converted in a special device into intermediate frequency oscillations. The main amplification of the signal and ensuring high selectivity are carried out at the intermediate frequency. Only then is the amplified modulated intermediate frequency signal converted into an audio frequency voltage.

A modern communication radio receiver must provide good audibility of weak signals in the desired wavelength range, provide good selectivity and not distort the received signal. Therefore, certain requirements are imposed on the radio receiver.

To receive weak signals, the radio receiver must be highly sensitive. Quantitatively, the sensitivity of the receiver is estimated by the smallest EMF of the signal that must be applied to the input of the radio receiver, at which the normal volume of the signal at the output of the receiver is ensured for a given ratio of the useful signal voltage and the noise voltage. The lower the input voltage required for the normal operation of the radio receiver, the higher the sensitivity of the radio receiver.

Modern military radio receivers have a sensitivity equal to units and even fractions of a microvolt.

In modern conditions, many thousands of radio stations operate simultaneously, and many of them operate at close frequencies. To receive a signal under such conditions, it is necessary that the radio receiver has good selectivity, i.e., the ability to select the desired signal from a variety of signals. In other words, the radio receiver must allocate a certain frequency band occupied by the desired signal, and not miss (suppress) all signals that lie outside this band.

Typically, selectivity is expressed as the amount of signal attenuation when detuned by a certain number of kilohertz, depicted graphically in the form of a selectivity curve. On fig. 1.27 shows the selectivity curves of two receivers: the curve a expresses the selectivity of a bad receiver, the curve b- a good receiver. It follows from the curves that the signal of the interfering station operating at a frequency of 1020 kHz, compared to the signal of the received station operating at a frequency of 1000 kHz, will be attenuated by the second receiver (curve b) by almost 10,000 times, and by the first receiver (curve a) almost not weakened. In the above example, the signal of the interfering station in the second receiver is practically inaudible (suppressed), while in the first receiver it is received in the same way as the signal of the correspondent.

Modern military radios have very good selectivity.

Military radios operate over a wide range of waves, with high sensitivity and good selectivity over the entire range. All these requirements are most fully met by superheterodyne-type radio receivers.

A block diagram of a superheterodyne-type radio receiver is shown in fig. 1.28. The radio receiver consists of the following main elements:

• input circuit;
• high frequency voltage amplifier;

Frequency converter consisting of a mixer and a local oscillator;

• intermediate frequency voltage amplifier;
• detector;
• low frequency voltage amplifier.

If the radio receiver is designed to receive telegraph signals with amplitude keying, then in this case it has an additional element - the second local oscillator. We will consider the principle of operation of a superheterodyne receiver using the example of receiving a telephone signal (Fig. 1.28). Radiotelephone signal frequency 2000 kHz, received by the receiving antenna is highlighted by the input circuit of the receiver (Fig. 1.28, a).

The signal selected by the input circuit is very weak. For amplification, the signal from the input circuit is fed to a high-frequency voltage amplifier. The gain of this amplifier is low, especially at high frequencies. Usually it is units or tens of times. But even this small gain is very important for obtaining a high sensitivity of the radio receiver, as it allows more successful signal conversion and, most importantly, to create a predominance of the useful signal over the mixer's own noise at the converter input. In addition, a high-frequency voltage amplifier improves the selectivity of the radio receiver, since the oscillatory circuits included in the anode circuits of the amplifier lamps are also tuned to the signal frequency and, together with the circuits of the input circuit, form a high-frequency selectivity curve. To improve the sensitivity and selectivity of the radio receiver, especially at high frequencies, high-frequency voltage amplifiers are made in two or three stages.

The signal selected and amplified by the input circuit and a high-frequency voltage amplifier (Fig. 1.28.6) is fed to the mixer. At the same time, an auxiliary frequency voltage is supplied to the mixer from a special low-power generator - a local oscillator operating at a frequency of 2460 kHz(Fig. 1.28, c). As a result of the operation of the converter, an intermediate frequency voltage is generated on the mixer load, equal to the difference between the frequencies of the generator and the signal 460 kHz(Fig. 1.28, G) and constant over the entire range of the receiver. The nature of the modulation of the high-frequency signal does not change during the conversion. From the mixer load, the selected intermediate frequency signal is fed to the intermediate frequency voltage amplifier. In superheterodyne radio receivers, the main amplification of the signal is carried out in the intermediate frequency path. Therefore, amplifiers to obtain a large gain are made multi-stage. The main amplification, regardless of the frequency of the received signal, is carried out at one intermediate frequency, which makes it possible to use high-quality oscillatory systems in such an amplifier. Along with amplifying the intermediate frequency voltage, the amplifier provides high receiver selectivity. Amplified intermediate frequency signal (Fig. 1.28, e) is then fed to the detector. In the detector, the amplitude-modulated intermediate frequency signal is converted into an audio frequency voltage. Voltage (Fig. 1.28, e) released on the load of the detector is amplified by a low (sound) frequency voltage amplifier and fed to telephones or a loudspeaker (Fig. 1.28, g).

When receiving a telegraph amplitude-shift keyed signal, the passage of the signal to the detector does not differ from the passage of a telephone amplitude-modulated signal. The receiver uses a second local oscillator to "voice" telegraph messages. With the help of oscillations of the second local oscillator, telegraph messages in the detector are converted into an audio frequency voltage, which is then amplified in an audio frequency voltage amplifier.

Depending on the type and purpose of the radio receiver, its block diagram may be modified, but the listed main elements are mandatory for each superheterodyne radio receiver.

Principles of radio communication. Channel and radio link

The word "radio" comes from the Latin radiare, to radiate or emit rays, and is a general term used for any practical application of radio waves. In this case, radio waves are understood as electromagnetic waves propagating through an open space (radio wave propagation medium) without artificial guiding media, such as wires or pipes - waveguides. When using electromagnetic waves as a material carrier for transmitting information over a distance, we come to radio communication as one of the telecommunication methods that uses electrical transmission systems to exchange information. Thus, radio communication is telecommunication carried out by means of radio waves.

In a broad sense, radio communication is represented by several types of communication that use various mechanisms for the transmission of radio waves to transmit messages: along the earth's surface, using reflection in different layers of the atmosphere, or through space relays. Each type of radio communication is characterized by its own principles, determined mainly by the characteristics of the ranges used to transmit radio waves. In the future, speaking of radio communications, we will mean such a kind of it that makes it possible to directly communicate between spatially separated points on the earth's surface without the use of intermediate points of communication that carry out re-reception (retransmission) of signals. In this case, relaying, in principle, can be used to increase the communication range or in other cases, for example, to improve communication efficiency in difficult conditions of interference. Another distinctive feature of the kind of radio communication, which will be discussed below, is the possibility of transmitting and receiving messages on the move.

All messages coming from the source for transmission via radio waves are converted in the transmitting end device into a primary electrical signal u (t), which is a time-varying voltage (current) that displays the messages. Depending on the nature of the messages and the type of conversion, the primary electrical signal can be discrete or continuous. As a transmitting terminal device, a microphone of a microphone-telephone headset (MTG) or handset, a telegraph key, a telegraph apparatus and other technical means can act.

A characteristic feature of primary electrical signals is their relatively slow change in time, i.e., low oscillation frequency. The spectra of most primary electrical signals are limited to a maximum frequency not exceeding a few kilohertz. Such low-frequency signals cannot be effectively radiated into the radio wave propagation medium, since this requires emitters with geometric dimensions commensurate with the signal wavelength. Therefore, further in the radio transmitter, the primary electrical signal is converted into a radio signal uc(t) convenient for transmission. The conversion process is called modulation for continuous primary signals or keying for discrete ones. In the process of modulation (manipulation), the primary electrical signal acts as a modulating signal that changes one of the parameters (amplitude, frequency, phase) of the high-frequency harmonic oscillation of the carrier frequency.

In the general case, the process of modulation of the primary electrical signal is preceded by the operation of its encoding, as a result of which the sequence of message elements is replaced by a sequence of code symbols according to a certain rule.

Radio signals, by analogy with the primary electrical signals that they display, can be continuous (analogue) or discrete. In some cases, discrete signals are called digital, since they can be represented in digital form - in the form of numbers with a finite number of digits. In radio communications, digital signals, which have only two discrete values, have found the greatest use. Discrete signals can be used to transmit not only discrete, but also continuous messages, and vice versa, continuous signals - to transmit discrete messages.

The radio signal from the output of the radio transmitter is fed to the transmitting antenna with the help of a connecting line, which is called a feeder, and is radiated in the form of radio waves into open space. The speed of propagation of radio waves depends on the properties of the medium, while the maximum speed takes place in free space (vacuum), and it coincides with the speed of light in vacuum, equal to 3 × 108 m/s. In other media, the speed of radio waves is less and is determined by the relative permittivity and permeability of the medium.

At the receiving point, the radio waves are converted by the receiving antenna into a high-frequency signal, which is then fed through the feeder to the radio receiver, where the transmitted primary electrical signal u (t) is restored. To do this, operations are performed that are the opposite of those that were carried out in the radio transmitter - demodulation (detection) and signal decoding. In the receiving end device (for example, MTG telephones, telegraph apparatus, loudspeaker), the primary signals are converted into messages and fed to their recipient.

The task of converting received signals into messages is more complicated than converting messages into a radio signal, since not only the transmitted radio signal is converted, but its mixture with other signals (interference) that can distort the transmitted message. The presence of interference in the transmission of messages is due to the fact that the propagation medium of radio waves is common to many sources of electromagnetic radiation, that is, it has free access.

A set of technical devices and a radio wave propagation medium that ensures the transmission of messages from a source to a recipient using radio waves is called a radio link (radio link). At the same time, sources and recipients using radio communication lines for transmitting and receiving messages are radio communication subscribers. Subscribers can transmit messages on their own or with the help of radio operators (radio telegraph operators). Radio subscribers and radio operators who directly transmit messages over the radio line are commonly called correspondents.

A block diagram of a radio link designed to transmit messages between subscribers (correspondents) A and B is shown in fig. 2.1. In it, a radio transmitter (transmitter) and a transmitting antenna are usually combined into a radio transmitter, and a radio receiver (receiver) and a receiving antenna into a radio receiver. In addition, the transmitting antenna and the feeder connecting it to the transmitter are called the transmitting antenna-feeder device (AFD) or path, and the receiving antenna and the feeder connecting it to the receiver are called the receiving AFU or path.

In a general sense, a radio link can be considered one of the types of telecommunication channel (communication channel), which is understood as a path for the passage of telecommunication signals, which, when connected to its ends, subscriber terminal devices, transmit messages from the source to the recipient (recipients). Depending on the type of communication network, telecommunication channels are assigned names, for example, telephone channel, telegraph channel, data transmission channel, sound broadcasting channel.

The radio link may be single-channel or multi-channel. In the latter case, it owns several simultaneously operating communication channels through which signals are transmitted that display different (sometimes identical) messages. Unlike a single-channel radio link, a multi-channel radio link can include several transmitting and receiving terminal devices that convert messages from different sources into primary electrical signals and vice versa. In addition, devices that perform the functions of combining and separating signals from different subscribers should be provided in a multichannel radio communication line.

Radio links can be direct, connecting subscribers directly, without the use of intermediate points (repeaters of radio signals), or composite, passing through such points (in this case, the radio link includes technical devices of the repeater that provide reception, conversion, amplification and subsequent transmission of radio signals received from both correspondents).

The part of the radio link that creates the path of radio signals is called the radio channel (radio channel). Radio channel boundaries
connections, depending on the tasks to be solved or the issues under study, can be chosen arbitrarily, as long as radio signals displaying messages pass through the channel. In some cases, a radio communication channel is understood as a set of technical devices that ensure the formation of a radio signal and its emission in a radio transmitter, as well as the reception of a radio signal and its reverse transformation in a radio receiver, and the propagation medium of radio waves. In other cases, for example, when considering the properties of telecommunication channels, only the propagation medium of radio waves is called a radio communication channel.

A radio communication channel, similar to a radio link, is a special case of a transmission channel, which is understood as a set of technical means and a propagation medium that ensures the transmission of telecommunication signals in a certain frequency band or at a certain speed between network nodes and stations. A radio channel is a transmission channel in which telecommunication signals are transmitted by means of radio waves. Depending on the methods of transmitting telecommunication signals, the transmission channel can be analog or digital (discrete). The type of radio channel, in addition, is determined by the type of radio waves used to transmit messages.

A transmission channel whose parameters comply with accepted standards is called a typical transmission channel. Typical transmission channels in radio communications will be discussed in Chapter 7.

Shown in fig. 2.1, the radio link implements two-way radio communication, since its composition allows both correspondents to both transmit and receive messages. With one-way radio communication, one of the correspondents only transmits messages, and the other (or others) only receives.

Two-way radio communication can be simplex or duplex. In the first case, the transmission and reception of information between correspondents are carried out alternately, while radio exchange is possible at the same frequency or at separated frequencies of reception and transmission. In this case, the radio communication is simplex single-frequency (or simply simplex), and in the second - simplex two-frequency. When conducting duplex radio communication, the transmission and reception of information are carried out simultaneously. Moreover, if the correspondents' transmitters are always on, regardless of whether information is being transmitted or not, radio communication is called duplex, and if the transmitters are turned on only for the time of information transmission, and when there is no transmission, they are turned off - half-duplex.

To transmit messages over radio channels, a part of the spectrum of electromagnetic oscillations is used, which is in the range from 3 kHz to 3000 GHz. This part of the spectrum is called the radio frequency spectrum (radio spectrum), and the frequencies of the radio spectrum are called radio frequencies. According to the international document - the Radio Regulations, the radio spectrum contains 9 bands (bands), starting with the fourth. The division of the spectrum into ranges is made in such a way that the ratio of the upper limit frequency of the range to its lower limit frequency is 10. In this case, the upper limit frequency of any range is included in it, and the lower one is excluded. Within the same range, the propagation properties of radio waves are almost the same. In table. 2.1 shows the names corresponding to the Radio Regulations, letter designations (international and Russian) and the boundaries of the frequency bands that make up the radio spectrum.

Waves in the range from 10 m to 1 cm are often referred to as ultrashort waves (VHF), and UHF, SMW and MMW are understood as ultrahigh frequencies. The first is explained by the fact that each of the bands with numbers from 8 and above, having distribution features, has some properties common to all VHF bands; and the second is that in microwave technical devices, to obtain and isolate high-frequency oscillations in resonant circuits, instead of traditional capacitors and inductors for lower frequencies, other designs are used: short segments of wire lines, metal strips, waveguides and box-shaped cavity resonators. In addition, radio waves in bands 9 and above are often referred to as microwaves.

Radio waves have laws and phenomena common to electromagnetic waves, the most important of which are:

rectilinear propagation of radio waves - the propagation of radio waves in a homogeneous (or slightly inhomogeneous) medium directly from the source to the place of reception along rectilinear or close to them trajectories;

reflection of radio waves - a change in the direction of propagation of radio waves due to reflection from the interface between two media or from inhomogeneities of the medium;

diffraction of radio waves - a change in the structure of the wave field under the influence of obstacles, which are spatial inhomogeneities of the propagation medium, in particular, leading to the rounding of these obstacles by the radio wave;

refraction of radio waves - a change in the direction of propagation of radio waves due to a change in the speed of their propagation when passing through an inhomogeneous medium;

absorption of radio waves - a decrease in the energy of a radio wave due to its partial conversion into thermal energy as a result of interaction with the environment;

scattering of radio waves - the transformation of radio waves propagating in one direction into radio waves propagating in different directions;

multipath propagation - the propagation of radio waves from a transmitting to a receiving antenna along several paths;

interference fading of radio waves - quasi-periodic changes in the field level due to the arrival at the place of reception of many radio waves with phases changing in time relative to each other.

Table 2.1

Classification of radio frequency bands and radio waves

Band number	Frequency limits	Name of frequencies	Borders wavelengths	Name of waves
		Very low		Myriameter, or extra long (MIMV, SDV)
				kilometers or long
	300…3000 kHz			Hectometric, or average
				Decameter, or short (DKMV, KV)
		Very high		Meter
	300…3000 MHz	ultra high		decimeter
		Ultra high		centimeter
				Millimeter
	300…3000 GHz	hyper high		Decimilli- meter

In radio communications, radio signals can be transmitted in two ways: along the earth's surface and with radiation into the ionosphere and from it back to the earth's surface.

Based on this, terrestrial and ionospheric radio waves are distinguished.

Radio waves propagating in the immediate vicinity (on a wavelength scale) of the earth's surface are called terrestrial radio waves. Ground radio waves include direct waves (propagating in a straight line), ground-reflected waves, and surface radio waves (propagating along an interface). Ionospheric are called radio waves propagating in free space by reflection from the ionosphere or scattering in it. Radio communication using ionospheric waves is also defined as ionospheric.

The ionosphere is formed by an ionized region of the atmosphere located at altitudes from 60...80 to 1000...1200 km above the Earth. The main source of atmospheric ionization, under the influence of which neutral molecules and atoms of gases that are part of the ionosphere, are split into positively charged ions and free electrons, is the ultraviolet and X-ray radiation of the Sun, as well as corpuscular streams, mainly of solar origin. In addition, the ionization of the atmosphere occurs under the action of cosmic rays from distant stars and cosmic dust, which continuously enter the Earth's atmosphere.

The degree of ionization, characterized by electron density, varies in height due to the inhomogeneity of the atmosphere. Therefore, the ionosphere acquires a complex multilayer structure, ionized clouds are formed in it, the electron concentration of which depends both on the height of the cloud and on the degree of solar activity, the thickness of the atmosphere, and some other reasons. The height distribution of ionization intensity in a real atmosphere has several maxima. There are three regions D, E, F (in ascending order of height above the Earth's surface), within which there are three ionized layers of the same name. In the daytime, the ionized F layer breaks up into two layers F1 and F2. The degree of ionization depends on the time of year, day and geographic location, and these dependencies are different for different layers. The average heights of the layers and the degree of their ionization (electron density) are shown in Table 1. 2.2.

Each layer has its own critical frequency fcr, defined as the highest frequency of the radio signal at which a vertically directed radio wave is reflected from this layer. At a frequency above the critical one, the radio wave is not reflected, but passes through the ionized layer of the ionosphere.

Simultaneously with the appearance of new electrons in the ionosphere, some of the electrons present in it disappear, joining positive ions and neutral molecules. The process of recombination of charged particles and the formation of molecules in the atmosphere is called recombination.

Ionization, in addition to the Sun, is created by meteors invading the earth's atmosphere at speeds of several tens of kilometers per second. When meteoric matter enters the dense layers of the atmosphere, it heats up and evaporates, and the particles of matter, being ionized, ionize the surrounding air. Due to this, the average level of ionization of the atmosphere increases. In addition, a column of ionized air is formed behind the meteor, shaped like a cylinder, which creates local ionization. The trail of the meteor quickly expands and dissipates, existing in the atmosphere from one to several seconds. Such ionized meteor trails are formed at an altitude of 80...120 km above the earth's surface approximately between the D layer and the E layer. Radio communication based on the use of radio wave reflection from ionized meteor layers is called meteor radio communication. In the meteor radio communication lines, intermittent operation is used with the preliminary accumulation of information and its subsequent transmission during the occurrence of meteor trails.

An information transmission system in which telecommunication signals are transmitted by means of radio waves in open space is called a radio engineering system. Radio systems are divided into radio links and radio networks.

Depending on the purpose, radio engineering systems are divided into groups.

RTS classification

1. RTS information transmission 2. RTS information retrieval

Radio communication

radio navigation

Broadcasting

Fax - still image transmission

Television is the transmission of moving images.

Below are schemes for organizing radio communication between ships and coastal radio stations, depending on the distance between them.

radio transmitting device

The radio transmitting device is designed to create high-frequency oscillations, their modulation and excitation of electromagnetic waves in space. Accordingly, it contains the following main elements. This refers to the transmitter with amplitude modulation.

master oscillator high frequency oscillations. Such a generator converts the energy of a DC voltage source into high-frequency harmonic oscillations. U hf = U m COS (ω mt) frequency ω m these fluctuations is called carrier frequency.

The main elements of the master oscillator are an electron tube, a transistor and an oscillatory circuit. The inductance and capacitance of the oscillatory circuit determine the frequency of the generated oscillations; by changing these parameters, it is possible to rebuild the master oscillator (and, consequently, the entire transmitter) from one carrier frequency to another. An electronic tube and a transistor are non-linear devices, they play the role of a kind of key that regulates the flow of energy into the circuit from a constant voltage source, which ensures the maintenance of oscillations in the circuit.

Message converter into an electrical signal used to modulate high frequency oscillations. The type of transducer depends on the physical nature of the transmitted message: in the case of an audio message, the transducer is a microphone, in the transmission of light images (television) - a transmitting television tube, in the transmission of measurement results of non-electrical quantities - sensors of one kind or another.

The electrical signal received at the output of the message converter is often very weak, and before being used for modulation, it is amplified in a special cascade (modulator), which in Fig. 2 is not shown.

modulation stage. The main elements of the modulation stage are an electron tube, a transistor and an oscillatory circuit. High-frequency oscillations are simultaneously fed to the input of the cascade U HF = Umo COS (ω mt) from the output of the master oscillator and the modulating electrical signal U M(t), changing according to the law of the transmitted message. As a result of the nonlinear transformation of the oscillations supplied to the modulation cascade U HF and U M(t) (carried out by means of a vacuum tube or transistor) in the output circuit of this stage, amplitude-modulated high-frequency oscillations are formed.

In the modulation stage, there is also an amplification of the power of oscillations - therefore it is often called simply a power amplifier.

Output stage (power amplifier). In radio transmitters with a short range, the output stage may be absent, while modulated high-frequency oscillations are fed to the antenna directly from the output of the modulation stage, which acts as a power amplifier. However, in radio stations with a long range, modulated oscillations of high power must be supplied to the antenna, for which cascades of amplifying the power of the modulated oscillations are placed between the modulating stage and the antenna. The law of change in the amplitude of modulated oscillations with power amplification must be preserved.

The main elements of a power amplifier are a lamp, a transistor and an oscillatory circuit.

Transmitting antenna, designed to excite electromagnetic waves in space. High-frequency oscillations received in the power amplifier are fed to the antenna and create a high-frequency current in it. I a1 = I 1 M ( t ) COSω mt , whose amplitude I 1 M ( t ) varies like the amplitude of the modulated oscillations supplied to the antenna. Current I a1 is the cause that causes the excitation of a propagating electromagnetic field in the surrounding space

(electromagnetic waves). The electromagnetic field is characterized by interconnected electric and magnetic components E and H . The nature of the change in the strength of the electric and magnetic fields in time at a certain point in space

is determined by the nature of the current change in the exciting antenna. Therefore, at the considered point in space, the strength of the electric (magnetic) field will have the character of high-frequency oscillations, the amplitude of which changes according to the law of the transmitted message.

radio receiver

The radio receiver is designed to capture part of the energy of the electromagnetic field (excited in space by the transmitter antenna), select the signals of the received radio station, amplify the received high-frequency oscillations, restore the useful signal and reproduce it. In accordance with this, the receiving device contains the following main elements (Fig. 2).

Receiving Antenna. The electromagnetic field, reaching the receiving antenna, excites emf in it. ea1 , proportional to the instantaneous value of the electric field strength. As a result, emf. ea1 is a modulated high frequency oscillation ea1 = E1 m (t )COS ω mt, where the amplitude E1 m (t) changes in time according to the law of the transmitted message.

With the simultaneous operation of several transmitting radio stations, the receiving antenna is exposed to the action of electromagnetic fields created by each of the radio stations. Therefore, several emfs are induced simultaneously in the antenna, each of which is a modulated high-frequency oscillation that differs from the other in the carrier frequency and the modulation law (the law of amplitude change).

input circuit, designed to select the signal of any one (received) radio station from the totality of all signals induced in the antenna by the fields of many radio stations. The main element of the input circuit is the oscillatory circuit. To implement the selection, the property of the oscillatory circuit is used to respond well to oscillations whose frequency is close to the resonant frequency of the circuit, determined by its parameters, and to respond poorly to oscillations with a frequency that is significantly different from the resonant one. By changing the parameters of the circuit (inductance or capacitance), it is possible to ensure that its resonant frequency is equal to one of the emf carrier frequencies induced in the antenna. If the difference between the carrier frequencies is large enough, then with the simultaneous action of all emfs on the circuit. only that emf will be effective, the frequency of which is equal to the resonant frequency of the circuit. As a result, oscillations will occur in the circuit, corresponding only to the received radio station; the voltage removed from the circuit represents high-frequency oscillations modulated in amplitude in accordance with the law of the transmitted message:

U= U 1M(t )COS ω mt.

High Frequency Oscillation Amplifier. The magnitude of the emf induced in the antenna and the high-frequency voltage taken from the input circuit loop is very small. Therefore, before extracting a useful signal from high-frequency oscillations, they are amplified in high-frequency oscillation amplifiers. (UHF).

The main elements of UHF are an electron tube, a transistor (semiconductor triode) and an oscillatory circuit. Thanks to the oscillatory circuits, UHF, like the input circuit, has selective properties.

Detector, designed to restore from modulated high-frequency oscillations a low-frequency electrical signal proportional to the modulating voltage and changing in accordance with the law of the transmitted message. The main element of the detector is an electron lamp or a semiconductor device.

Low Frequency Voltage Amplifier designed to amplify a very weak low-frequency signal received at the output of the detector.

The main element of a low-frequency voltage amplifier is an electron tube or a semiconductor triode.

playback device, designed for such a conversion of the amplified low-frequency signal, in which the received message is reproduced in a form convenient for registration. When transmitting sound signals, the reproducing device is a telephone, a loudspeaker; in television receivers, the message is reproduced on the screen of the receiving television tube in the form of a light image; when receiving data on a certain measured value, the received message is reproduced either using cathode ray tubes or using special recording devices.

Main technical characteristics of RPM:

Sensitivity RPM - the minimum value of the input signal, which ensures the normal operation of the terminal device. In modern RPMs, the sensitivity is a few microvolts.

RPM selectivity - the ability to receive separately signals from stations adjacent in frequency. Selectivity is determined by the bandwidth of the RPM.

The output power of the RPM is the maximum possible undistorted power of the audio frequency amplifier.

Depending on the principle of construction, RPMs are distinguished as detector, direct amplification and superheterodyne types.

Structural diagram of a detector radio receiver.

In a direct amplification RPM, the received signal is selected using a selective DUT (a system of two coupled oscillatory circuits that act as a band pass filter). The RF amplifier is tuned to the same frequency. URF serves to increase the level of the signal induced in the antenna. Detector D extracts from the modulated radio signal a low-frequency component containing a message. After amplifying the ultrasonic signal, the signal is sent to the receiving terminal, which generates a message (speaker, printer). Despite the simplicity of the technical implementation of direct amplification RPM, it is practically not used at present. Its main disadvantages are low selectivity and sensitivity.

Superheterodyne radio receiver

The block diagram of the superheterodyne type RPM, which consists of a local oscillator and a mixer, is shown in the figure below.

The local oscillator is a generator of harmonic signals f g, the frequency of which can be changed. In the Mixer, the frequencies f c and f g are mixed, as a result of which the total f + and difference (intermediate) f _ frequencies are obtained: f + = fc + f g, f _ = fc - f g. (difference frequency f-, and the sum frequency f+ is filtered). The Get frequency changes when the RPM is tuned to the frequency f, simultaneously with the change in the frequency of the DUT and the URCH so that f _ remains constant (in domestic broadcast RPM f _ = 465 kHz). Thus, a signal at an arbitrary frequency f c in a superheterodyne-type RPM is converted into a signal at a constant intermediate frequency. At this intermediate frequency, the oscillatory circuit of the intermediate frequency amplifier of the IF is tuned, in which the main selection and amplification of the useful signal is carried out. Since the frequency of the oscillatory circuit does not change, the bandwidth and selectivity of the RPM are constant over the entire frequency range.

PASSIVE ELEMENTS

Resistor.

The most used element in radio engineering devices is the resistor (the old name is resistance).

Resistor R (constant, adjustable and tuning) is an element of an electrical circuit in which an irreversible conversion (loss) of electromagnetic energy into heat occurs, the main characteristic of a resistor is its electrical resistance R, which relates the voltage U to the current I: U = I R.

The main characteristic of a resistor is resistance, measured in ohms. Two types of resistors are available: stable and general purpose. Production of stable resistors is expensive and therefore they are used in expensive high-precision equipment.

One of the main characteristics is the dissipated power. Power dissipation is the power that a resistor can dissipate without damage. Measured in watts. It is found according to the formula P= I 2 · R.

Every substance has its own resistance. The resistance depends on the material (for gold, it will be less than that of aluminum), on the length of the conductor (the dependence is direct: the longer the greater the resistance) and on the cut area of ​​the conductor (the larger the area, the lower the resistance).

Designation of fixed resistors on circuit diagrams:

	Standard designation

Resistors, especially low power ones, are extremely small parts, a 0.125 W resistor has a length of several millimeters and a diameter of the order of a millimeter. It is impossible to read the denomination with a decimal point on such a part.

Therefore, when specifying the denomination, instead of a decimal point, they write a letter corresponding to the units of measurement (K - for kiloohms, M - for megaohms, E or R for ohm units). For example, 4K7 means a resistor with a resistance of 4.7 kOhm, 1R0 - 1 Ohm, 120K - 120 kOhm, etc. However, it is difficult to read the ratings in this form. Therefore, for especially small resistors, marking with colored stripes is used. For resistors with an accuracy of 20%, use the marking with three stripes, for resistors with an accuracy of 10% and 5%, the marking with four stripes, for more accurate resistors with five or six stripes.

There are also variable resistors that have the ability to change their resistance. They are used to change the current, voltage, etc. (for example: changing the volume and tone). Most often, the circuit diagram is displayed as follows: Variable resistors are: 1) single and double 2) single and multi-turn 3) with and without a switch

By the nature of the change in resistance: 1) Linear i.e. Proportional to the angle of rotation of the axis (group A) 2) Inversely logarithmic (group B) 3) Logarithmic (group C) There are wire and non-wire (film) variable resistors. Wire-wound ones are distinguished by high stability, relatively low level of their noise and low TCR.