Examples of the practical use of radio communications. Electromagnetic waves and their properties

05.11.2019 Iron

Ministry of Education of the Republic of Belarus

Department of Radio Electronics

Abstract on the topic:

Bibliography

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. 1.1). 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. In 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. 1.2). 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 option for organizing a radio network can be built on the basis of both a complex simplex (see Fig. 1.2, a) and a complex duplex (see Fig. 1.2, 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).

Any radio link for transmitting information (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.

Radio waves on radio lines propagate 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.

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. About 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 the signals of 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).

Propagation of radio waves in terrestrial conditions

Radiation of radio waves

Any oscillating electric charge is a source of an alternating electromagnetic field that radiates into the surrounding space. The emission of an electromagnetic wave by a charge can be explained as follows. Consider two conducting balls located at a distance L from each other (Fig. 1.3). Such a system is called an electric dipole. After turning off the generator, the balls will be charged and discharged. In this case, the charging and discharging currents of the capacitance formed by the balls flow through the wire L. The capacitance of the balls is much greater than the capacitance of the segments ab and cd of the wire L, so the displacement current between the wire segments can be neglected. We can assume that the conduction current flowing in the wire L is closed only through the displacement current flowing in the space between the balls. In this case, the amplitude of the current along the wire L remains constant. Such an electric dipole is called a Hertzian dipole.

On fig. 1.3 graphically shows the distribution of the current amplitude along the dipole wire. The same figure shows the lines of force of the electric field of the dipole for the moment when the balls are charged. The displacement current lines are located in N space in the same way as the electric field lines. When the generator r is operating, the alternating displacement current causes the appearance of an alternating magnetic field, the lines of force of which surround the displacement current lines. In turn, the alternating magnetic field, according to the law of electromagnetic induction, causes the appearance of an alternating electric field and the corresponding displacement current in the surrounding space, etc. The considered process propagates in the environment self-sustaining. If, for example, the generator that feeds the dipole is turned off, then the resulting electromagnetic wave continues to propagate in the environment - the displacement current causes an alternating magnetic field, which, in turn, creates an alternating electric field and a displacement current in neighboring areas of space. If the generator that excites the dipole generates a voltage that varies according to the harmonic law U = L/msincof, then the electromagnetic field also changes in time according to the harmonic law with

the same frequency.

The structure of the Earth's atmosphere

Under terrestrial conditions, radio waves propagate in the atmosphere. The atmosphere is divided by height into three regions: the troposphere, stratosphere and ionosphere. The lower region - the troposphere - extends to a height of 7 ... 10 km in the polar regions and up to 16 ... 18 km above the equator. The troposphere passes into the stratosphere, the upper boundary of which is at an altitude of about 50...60 km. The stratosphere differs from the troposphere in the almost complete absence of water vapor, precipitation is formed only in the troposphere. The troposphere and stratosphere only affect the propagation of VHF.

Ticket number 20

electromagnetic waves and

their properties. Principles of radio communication and

examples of their practical

use

Response Plan

1. Definition. 2. Condition of occurrence. 3. Properties of electromagnetic waves. 4. Open oscillatory circuit. 5. Modulation and detection.

The English scientist James Maxwell, based on a study of Faraday's experimental work on electricity, hypothesized the existence in nature of special waves that can propagate in a vacuum.

Maxwell called these waveselectromagnetic waves.According to Maxwell:with any change in the electric field, a vortex magnetic field arises and, conversely,with any change in the magnetic field, a vortex electric field arises.Once begun, the process of mutual generation of magnetic and electric fields must continue continuously and capture more and more new areas in the surrounding space (Fig. 31). The process of mutual generation of electric and magnetic fields occurs in mutually perpendicular planes. An alternating electric field generates a vortex magnetic field, an alternating magnetic field generates a vortex electric field.

Electric and magnetic fields can exist not only in matter, but also in vacuum. Therefore, it should be possible to propagate electromagnetic waves in a vacuum.

The condition for the occurrenceelectromagnetic waves is the accelerated movement of electric charges. So, a change in the magnetic field occurs when the current in the conductor changes, and a change in current occurs when the speed of the charges changes, that is, when they move with acceleration. The propagation velocity of electromagnetic waves in vacuum, according to Maxwell's calculations, should be approximately equal to 300,000 km/s.

For the first time, the physicist Heinrich Hertz experimentally obtained electromagnetic waves, using a high-frequency spark gap (Hertz vibrator). Hertz also experimentally determined the speed of electromagnetic waves. It coincided with the theoretical definition of wave speed by Maxwell. The simplest electromagnetic waves These are waves in which the electric and magnetic fields make synchronous harmonic oscillations.

Of course, electromagnetic waves have all the basic properties of waves.

They obey reflection law waves:

the angle of incidence is equal to the angle of reflection.When moving from one medium to another, they refract and obeythe law of refraction waves: the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for two given media and equal to the ratio of the speed of electromagnetic waves in the first medium to the speed of electromagnetic waves in the second medium and called refractive indexsecond environment relative to the first.

The phenomenon of diffraction of electromagnetic waves, i.e., the deviation of the direction of their propagation from a rectilinear one, is observed at the edge of an obstacle or when passing through a hole. Electromagnetic waves are capable of interference. Interference - this is the ability of coherent waves to superimpose, as a result of which the waves in some places amplify each other, and in other places extinguish. (Coherent waves these are waves that are identical in frequency and phase of oscillation.) Electromagnetic waves have dispersion, i.e., when the refractive index of the medium for electromagnetic waves depends on their frequency. Experiments with the transmission of electromagnetic waves through a system of two gratings show that these waves are transverse.

When an electromagnetic wave propagates, the intensity vectors E and magnetic induction B are perpendicular to the direction of wave propagation and are mutually perpendicular to each other (Fig. 32).

The possibility of practical application of electromagnetic waves to establish communication without wires was demonstrated by May 7, 1895 Russian physicist A. Popov. This day is considered the birthday of radio. For the implementation of radio communication, it is necessary to provide the possibility of radiation of electromagnetic waves. If electromagnetic waves arise in a circuit of a coil and a capacitor, then an alternating magnetic field is associated with the coil, and an alternating electric field concentrated between the plates of the capacitor. Such a circuit is called closed (Fig. 33, but). A closed oscillatory circuit practically does not radiate electromagnetic waves into the surrounding space. If the circuit consists of a coil and two plates of a flat capacitor, then the greater the angle of these plates, the more freely the electromagnetic field enters the surrounding space (Fig. 33b). The limiting case of an open oscillatory circuit is the removal of plates to opposite ends of the coil. Such a system is calledopen oscillatory circuit(Fig. 33, in). In reality, the circuit consists of a coil and a long wire- antennas.

The energy of electromagnetic oscillations emitted (using a generator of continuous oscillations) with the same amplitude of current oscillations in the antenna is proportional to the fourth power of the oscillation frequency. At frequencies of tens, hundreds and even thousands of hertz, the intensity of electromagnetic oscillations is negligible. Therefore, for the implementation of radio and television communications, electromagnetic waves with a frequency of several hundred thousand hertz to hundreds of megahertz are used.

When transmitting speech, music and other sound signals by radio, various types of modulation of high-frequency (carrier) oscillations are used. The essence of modulation lies in the fact that the high-frequency oscillations generated by the generator change according to the law of low frequency. This is one of the principles of radio transmission. Another principle is the reverse process detection.During radio reception, low-frequency sound oscillations must be filtered out from the modulated signal received by the receiver antenna.

With the help of radio waves, not only sound signals are transmitted over a distance, but also an image of an object. Radar plays an important role in the modern navy, aviation and astronautics. Radar is based on the property of reflection of waves from conducting bodies. (Electromagnetic waves are weakly reflected from the surface of a dielectric, and almost completely from the surface of metals.)

The transmission and reception of information by means of electromagnetic waves is called radio communication. Radio links are used, for example, for radiotelephony, telegram transmission, facsimile(s), broadcast and television programs.

Radio communication is a rather complex process. Therefore, we will consider only the most general principles of one of its types - radiotelephone communication, i.e., the transmission of sound information, such as speech and music, using electromagnetic waves. To get a holistic view of this process, let's turn to the flowchart shown in Figure 139.

Rice. 139. Block diagram of the process of radio communication

Figure 139, a shows a transmitting device, consisting of a high-frequency oscillation generator, a microphone, a modulating device and a transmitting antenna.

Sound vibrations (speech, music, etc.) enter the microphone. They are converted by a microphone into electrical vibrations of the same form as sound waves. From the microphone, low-frequency electrical vibrations enter the modulating device. High-frequency oscillations of constant amplitude are fed there from the generator.

In the modulating device, the amplitude of high-frequency oscillations is changed (modulated) using electrical oscillations of sound frequency. As a result, the amplitude becomes variable, and it changes in the same way as the electrical vibrations coming from the microphone. Such high-frequency amplitude-modulated oscillations carry information about the shape of the audio signal. Therefore, the frequency of high-frequency oscillations is called the carrier.

The process of changing the amplitude of high-frequency oscillations with a frequency equal to the frequency of the audio signal is called amplitude modulation.

Under the influence of high-frequency modulated oscillations, a high-frequency alternating current arises in the transmitting antenna. This current generates an electromagnetic field in the space around the antenna, which propagates in space in the form of electromagnetic waves and reaches the antennas of radio receivers.

You already know that the power of an electromagnetic wave is proportional to the fourth power of its frequency: Р ~ v 4 .

Electromagnetic waves of sound, i.e. low, frequencies (from 16 to 20,000 Hz) have low power and decay very quickly after radiation. This is the reason for the need to use modulated radio waves, which, due to the high carrier frequency, propagate over long distances and at the same time contain information about the shape of the transmitted sound vibrations.

As can be seen from Figure 139, b, the radio receiver consists of a receiving antenna, a receiving resonating oscillatory circuit and a detector - an element that passes alternating current in only one direction.

The receiving antenna receives waves from many radio stations. But each radio station broadcasts only on a strictly defined carrier frequency assigned to it.

By tuning your radio receiver to the frequency of the desired radio station, you change the natural frequency of the oscillatory circuit available in the receiver so that it is equal to the carrier frequency of the given radio station, i.e., so that the circuit is tuned into resonance with the oscillations generated at this radio station. In this case, the amplitude of oscillations of the selected radio station in the circuit of your receiver will be maximum compared to the amplitudes of oscillations received from radio stations broadcasting on other carrier frequencies. This is the second purpose of the carrier frequency - it provides the ability to tune to the frequency of the desired radio station.

Alexander Stepanovich Popov (1859-1906)
Russian physicist, electrical engineer, inventor of radio. Designed a generator of electromagnetic oscillations. Invented a receiving antenna, built the world's first radio receiver

The received vibrations are first amplified. Then, to convert high-frequency modulated oscillations into sound, detection is carried out, i.e., the process is the reverse of modulation. Detection is carried out in two stages: first, with the help of a detector (which is an element with one-way conduction), a high-frequency pulsating current is obtained from high-frequency modulated oscillations (Fig. 140, a), and then in dynamics this current is smoothed and converted into sound frequency oscillations (Fig. 140b). The possibility of using electromagnetic waves to transmit radio signals 1 was first pointed out in 1889 by Alexander Stepanovich Popov. In 1896, with the help of a transmitter and receiver of radio signals designed by him, he transmitted the world's first radiogram, consisting of two words "Heinrich Hertz".

Rice. 140. Graphs of high-frequency modulated vibrations and sound vibrations

When transmitting television programs, high-frequency oscillations are modulated not only by audio, but also by video. This is done using a television transmission tube, which converts the optical image into electromagnetic waves. High-frequency oscillations modulated in this way contain information about both sound and image.

Television uses higher (on the order of billions of hertz) carrier frequencies.

Questions

What is radio communication?
Give 2-3 examples of using radio links.
Using figures 139 and 140, describe the principles of radiotelephone communications.
What frequency is called a carrier frequency?
What is the process of amplitude modulation of electrical oscillations?
Why are electromagnetic waves of sound frequencies not used in radio communications?
What is the vibration detection process?

Exercise 43

The period of charge oscillations in an antenna emitting radio waves is 10 -7 s. Determine the frequency of these radio waves.

1 Radio signals - electromagnetic waves emitted for short periods of time in the frequency range from 104 to 1010 kHz.

The possibility of practical application of electromagnetic waves to establish communication without wires was demonstrated on May 7, 1895 by the famous Russian physicist Alexander Stepanovich Popov (1859-1906). This day is considered the birthday of radio.

The receiver of A. S. Popov consisted of an antenna 1, a coherer 2, an electromagnetic relay 3, an electric bell 4 and a direct current source 5 (Fig. 245). Electromagnetic waves caused forced oscillations of current and voltage in the antenna. An alternating voltage from the antenna was applied to two electrodes, which were located in a glass tube filled with metal filings. This tube is the coherer. An electromagnetic relay and a direct current source were connected in series with the coherer.

Due to poor contacts between sawdust, the resistance of the coherer is usually high, so the electric current in the circuit is small and the relay does not close the bell circuit. Under the action of a high-frequency alternating voltage in the coherer, electric discharges occur between individual sawdust, the sawdust particles are sintered and its resistance decreases by 100-200 times. The current strength in the coil of the electromagnetic relay increases, and the relay turns on the electric bell. This is how the reception of an electromagnetic wave by the antenna is recorded.

The blow of the bell hammer on the coherer shook the sawdust and returned it to its original state, the receiver was again ready to register electromagnetic waves.

Open oscillatory circuit.

For the implementation of radio communication, it is necessary to provide the possibility of radiation of electromagnetic waves. If electromagnetic oscillations occur in a circuit of a coil and a capacitor, then an alternating magnetic field turns out to be associated with the coil, and an alternating electric field is concentrated in the space between the capacitor plates (Fig. 246, a). Such a circuit is called closed. A closed oscillatory circuit practically does not radiate electromagnetic waves into the surrounding space.

If the circuit consists of a coil and two flat capacitor plates that are not parallel to each other, then the greater the angle of these plates,

the more freely the electromagnetic field enters the surrounding space (Fig. 246, b).

The limiting case of opening the oscillatory circuit is the removal of the capacitor plates to opposite ends of the straight coil. Such a system is called an open oscillatory circuit (Fig. 246, c). The image of the capacitor plates at the ends of the coil of an open oscillatory circuit in Figure 246 is only a convention. In reality, the circuit consists of a coil and a long wire - an antenna. One end of the antenna is grounded, the other is raised above the ground.

The antenna coil has an inductive connection with the coil of the oscillatory circuit of the generator of undamped electromagnetic oscillations. Forced high-frequency oscillations in the antenna create an alternating electromagnetic field in the surrounding space. With the speed of electromagnetic waves propagate from the antenna.

The energy of radiated electromagnetic waves with the same amplitude of current oscillations in the antenna is proportional to the fourth power of the oscillation frequency. At frequencies of tens, hundreds and even thousands of hertz, the intensity of the radiation of electromagnetic waves is negligible. Therefore, for the implementation of radio and television communications, electromagnetic waves with a frequency of several hundred thousand hertz to hundreds of thousands of megahertz are used.

Amplitude modulation.

When transmitting speech, music and other sound signals by radio, various types of modulation of high-frequency harmonic oscillations are used.

For the implementation of amplitude modulation of electromagnetic oscillations of high frequency

(Fig. 247, a) a transformer coil is connected in series with the oscillatory circuit in the electrical circuit of the transistor generator (Fig. 248). An alternating audio frequency voltage is supplied to the second coil of the transformer, for example, from the microphone output after the necessary amplification. An alternating current in the second coil of the transformer causes an alternating voltage to appear at the ends of the first coil of the transformer. The alternating voltage of the audio frequency (Fig. 247, b) is added to the constant voltage of the current source; voltage changes between the emitter and the collector of the transistor lead to changes with an audio frequency in the amplitude of oscillations of the high frequency current in the generator circuit (Fig. 247, c). Such high frequency oscillations are called amplitude modulated.

The radio transmitter antenna is inductively connected to the oscillatory circuit of the generator. Forced high current fluctuations

the frequencies occurring in the antenna create electromagnetic waves.

Radio.

Electromagnetic waves emitted by the antenna of a radio transmitter cause forced oscillations of free electrons in any conductor. The voltage between the ends of the conductor, in which the electromagnetic wave excites forced oscillations of the electric current, is proportional to the length of the conductor. Therefore, to receive electromagnetic waves in the simplest detector radio receiver, a long wire is used - receiving antenna 1 (Fig. 249). Forced oscillations in the antenna are excited by electromagnetic waves from all radio stations. In order to listen to only one radio transmission, the voltage fluctuations are not directed directly to the input of the amplifier, but are first fed to an oscillating circuit 2 with a varying natural oscillation frequency. A change in the natural frequency of oscillations in the receiver circuit is usually carried out by changing the electric capacitance of a variable capacitor. When the frequency of forced oscillations in the antenna coincides with the natural frequency of the circuit oscillations, resonance sets in, while the amplitude of the forced oscillations of the voltage on the circuit capacitor plates reaches its maximum value. Thus, from a large number of electromagnetic oscillations excited in the antenna, oscillations of the desired frequency are distinguished.

With the oscillatory circuit of the receiver modulated oscillations

high frequencies are fed to detector 3. As a detector, you can use a semiconductor diode that passes alternating current of high frequency in only one direction. After passing through the detector, the current in the circuit changes in time according to the law shown in Figure 250, a. During each high-frequency half-cycle, current pulses charge capacitor 4, while the capacitor slowly discharges through resistor 5. If the values of the capacitance of the capacitor and the electrical resistance of the resistor are chosen correctly, then a current will flow through the resistor, varying in time with the sound frequency used in the modulation vibrations in the radio transmitter (Fig. 250, b). To convert electrical vibrations into sound, an alternating voltage of sound frequency is applied to the telephone 6.

The detector radio receiver is very imperfect. It has very low sensitivity and therefore can only successfully receive radio transmissions from powerful radio stations or from nearby radio transmitters.

To increase the sensitivity in modern radio receivers, the signal from the oscillatory circuit is fed to the input of a high-frequency amplifier (UHF), and from the output of the amplifier, high-frequency electrical oscillations are fed to the detector. To increase the power of the audio signal at the output of the radio receiver, electrical oscillations of sound frequency from the output of the detector are fed to the input of a low-frequency amplifier (ULF).

The alternating voltage of the audio frequency from the ULF output is supplied to the winding of the electrodynamic loudspeaker - the speaker. The speaker converts audio frequency AC energy into sound vibration energy.

To amplify electrical oscillations of high and low frequencies, circuits with electronic tubes or transistors can be used.

The diagram of the device of the simplest radio receiver with amplifiers of high and low frequencies is shown in Figure 251.

To tune in to receive only one station in modern radios, rather complex electronic circuits are used, including generators of electromagnetic oscillations. The addition of electrical oscillations from the internal generator of the receiver with oscillations excited in the receiver circuit by electromagnetic waves from transmitting radio stations allows you to tune the receiver to a very narrow range of received frequencies. The internal oscillator in the receiver is called a local oscillator, and a receiver with such an oscillator is called a superheterodyne radio receiver.

A television.

With the help of radio waves, not only sound signals, but also images of an object are transmitted over a distance. The principle of transmission of moving black-and-white and color images with

using television transmitters and receivers is as follows.

To transmit one frame of a television image using a lens in a television camera, an image of an object is obtained on the screen of a special electrovacuum device - a transmitting tube (Fig. 252). Under the action of light, parts of the screen acquire positive charges. An electron beam is directed to the screen inside the transmitting tube, moving periodically from left to right along 625 horizontal lines - lines. During the run of the beam along the line, the neutralization of electric charges takes place in separate sections of the screen and in the electrical circuit connecting the electron gun and the screen; current pulse flows. Changes in the current strength in the pulse correspond to

changes in the illumination of the screen in the path of the electron beam.

High-frequency electromagnetic oscillations in a television transmitter are modulated by a pulse signal received at the output of the transmitting tube and fed to the transmitter antenna. An antenna emits electromagnetic waves.

In a television receiver - a television - there is an electrovacuum tube called a kinescope. In a kinescope, an electron gun creates an electron beam. Electrons under the action of an electric field move inside the tube to a screen covered with crystals that can glow under the impact of fast moving electrons. On their way to the screen, the electrons fly through the magnetic fields of two pairs of coils located outside the tube.

The magnetic field of one pair of coils causes the electron beam to deviate horizontally, the second - vertically. Periodic changes in the current strength in the coils cause changes in the magnetic fields, as a result of which the electron beam runs 625 times over the screen from left to right in seconds and once from top to bottom (Fig. 253).

During the movement of the beam along the first line, the current in the electron beam is controlled by the signal received by the receiver from the transmitter during the movement of the beam in the transmitting tube along the first line; when the beam moves along the second line, the current in the beam is controlled by the signal from the second line, etc. As a result, the beam “draws” the same image on the TV screen as built by the lens on the screen of the transmitting tube. Frames follow each other at a frequency of 25 frames per second, a sequence of successive frames at a high frame rate is perceived by the human eye as a continuous movement.

Television broadcasts are conducted in the range from 50 MHz to 230 MHz. In this range, electromagnetic waves propagate almost only within the line of sight. Therefore, to ensure the transmission of television signals over long distances, high antennas are built. The transmitting antennas of the studios of the Central Television of the USSR are installed on the top of the Ostankino tower. This height ensures the reception of television broadcasts at distances up to 120 km from Moscow.

The transmission of television signals to any point in our country is carried out with the help of relay artificial Earth satellites in the Orbita system.

The transmission and reception of color images require the use of more complex television systems. Instead of one transmitting tube, it is required to use three tubes transmitting signals of three single-color images - red, blue and green.

Unlike a black-and-white TV, the kinescope screen of a color TV is covered with three types of phosphor crystals. Some crystals, when hit by an alexron beam, glow red, others blue, and still others green. This crystals are located on the tap in a strict order. Signals are sent from a television transmitter to three cathode-beam guns.

On a color TV screen, three beams create three images of red, green and blue at the same time. The overlay of these images, consisting of small luminous dots, is perceived by the human eye as a multi-color image with all shades of colors. The simultaneous glow of crystals in one place with blue, red and green light is perceived by the eye as white; therefore, black and white images can also be displayed on a color TV screen.

Propagation of radio waves.

Radio communication is carried out on long medium short and ultrashort waves. Radio waves with different wavelengths propagate differently at the Earth's surface.

Long waves due to diffraction propagate far beyond the visible horizon; Longwave radio transmissions can be received over long distances beyond the direct line of sight of the antenna.

Medium waves experience less diffraction at the Earth's surface and propagate by diffraction over shorter distances beyond the line of sight. Short waves are even less capable of being diffracted near the Earth's surface, but they can be received at any point on the Earth's surface. The propagation of short radio waves over long distances from a transmitting radio station is explained by their ability to be reflected from the ionosphere.

The ionosphere is the upper part of the atmosphere, starting at a distance of about 50 km from the Earth's surface and

passing into interplanetary plasma at distances of 70-80 thousand km. A feature of the ionosphere is the high concentration of free charged particles in it - ions and electrons. The ionization of the upper layers of the atmosphere is created by ultraviolet and x-ray radiation from the sun. The maximum values of the number of free electrons in the ionosphere of electrons in a cubic centimeter are reached at altitudes of 250-400 km from the Earth's surface.

The conductive layer of the earth's atmosphere - the ionosphere - is capable of absorbing and reflecting electromagnetic waves. Long radio waves are well reflected from the ionosphere. This phenomenon, along with diffraction, increases the range of propagation of long waves. Short radio waves are also well reflected by the ionosphere. Multiple reflections of short radio waves from the ionosphere and the earth's surface make short-wave radio communication possible between any points on Earth (Fig. 254).

Ultrashort waves (UHF) are not reflected by the ionosphere and do not go around the Earth's surface as a result of diffraction (Fig. 255). Therefore, communication on VHF

carried out only within the line of sight of the transmitter antenna.

Radar.

Radar communications play an important role in the modern navy, aviation and astronautics. Radar is based on the property of reflection of radio waves from conducting bodies.

If the radio transmitter is turned on for a very short time and then turned off, then after a while, with the help of a radio receiver, the return of radio waves reflected from conductive bodies far from the radio station can be registered.

By measuring the duration of the time interval between the moments of departure and return of electromagnetic waves with the help of electronic equipment, it is possible to determine the path traveled by radio waves: where c is the speed of the electromagnetic wave. Since the waves traveled the path to the body and back, the distance to the body that reflected the radio waves is equal to half of this path:

To determine not only the distance to the body, but also its position in space, it is necessary to send radio waves in a narrow beam. A narrow beam of radio waves is created using an antenna having a shape close to spherical. In order for the radar antenna to create a narrow beam of radio waves, ultrashort waves are used in radar

To determine, for example, the location of an aircraft, the radar antenna is pointed at the aircraft and the generator of electromagnetic waves is turned on for a very short time. Electromagnetic waves bounce off the aircraft and return to the radar. The reflected radio signal is picked up by the same antenna, disconnected from the transmitter and connected to the receiver (Fig. 256). The angle of rotation of the radar antenna determines the direction to the aircraft. The radar installed on the aircraft makes it possible to measure the height at which the aircraft is located by the time of passage of radio waves to the surface of the Earth and back.

Water and land, dry and wet soil, urban buildings and transport communications reflect radio waves in different ways. This allows using radar instruments on the aircraft not only to measure the distance to

surface of the Earth, but also to receive a kind of radar map of the area over which the plane flies. The aircraft pilot receives this map day and night, in clear weather and in overcast conditions, since clouds are not an obstacle to electromagnetic waves.

Radar methods have made the most accurate measurements of the distances from the Earth to Louisa and to the planets Mercury, Venus, Mars and Jupiter.

The English scientist James Maxwell, based on a study of Faraday's experimental work on electricity, hypothesized the existence in nature of special waves that can propagate in a vacuum. Maxwell called these waves electromagnetic waves. According to Maxwell's ideas: with any change in the electric field, a vortex magnetic field arises and, conversely, with any change in the magnetic field, a vortex electric field arises. Once begun, the process of mutual generation of magnetic and electric fields must continue continuously and capture more and more new areas in the surrounding space (Fig. 42). The process of mutual generation of electric and magnetic fields occurs in mutually perpendicular planes. An alternating electric field generates a vortex magnetic field, an alternating magnetic field generates a vortex electric field.

Electric and magnetic fields can exist not only in matter, but also in vacuum. Therefore, it should be possible to propagate electromagnetic waves in a vacuum.

The condition for the emergence of electromagnetic waves is the accelerated movement of electric charges. Thus, a change in the magnetic field occurs

When the current in the conductor changes, and the change in current occurs when the speed of the charges changes, that is, when they move with acceleration. The propagation speed of electromagnetic waves in vacuum, according to Maxwell's calculations, should be approximately equal to 300,000 km/s.

For the first time, the physicist Heinrich Hertz experimentally obtained electromagnetic waves, using a high-frequency spark gap (Hertz vibrator). Hertz also experimentally determined the speed of electromagnetic waves. It coincided with the theoretical definition of wave speed by Maxwell. The simplest electromagnetic waves are waves in which the electric and magnetic fields make synchronous harmonic oscillations.

Of course, electromagnetic waves have all the basic properties of waves.

They obey the law of wave reflection: the angle of incidence is equal to the angle of reflection. When passing from one medium to another, they refract and obey the law of refraction of waves: the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for two given media and equal to the ratio of the speed of electromagnetic waves in the first medium to the speed of electromagnetic waves in the second medium and is called the refractive index of the second environment relative to the first.

The phenomenon of diffraction of electromagnetic waves, i.e., the deviation of the direction of their propagation from a rectilinear one, is observed at the edge of an obstacle or when passing through a hole. Electromagnetic waves are capable of interference. Interference is the ability of coherent waves to superimpose, as a result of which the waves amplify each other in some places, and cancel each other out in other places. (Coherent waves are waves that have the same frequency and phase of oscillation.) Electromagnetic waves have dispersion, that is, when the refractive index of the medium for electromagnetic waves depends on their frequency. Experiments with the transmission of electromagnetic waves through a system of two gratings show that these waves are transverse.

The possibility of practical application of electromagnetic waves to establish communication without wires was demonstrated on May 7, 1895 by the Russian physicist A. Popov. This day is considered the birthday of radio. For the implementation of radio communication, it is necessary to provide the possibility of radiation of electromagnetic waves. If electromagnetic waves arise in a circuit of a coil and a capacitor, then an alternating magnetic field is associated with the coil, and an alternating electric field is concentrated between the plates of the capacitor. Such a circuit is called closed (Fig. 44, a).

A closed oscillatory circuit practically does not radiate electromagnetic waves into the surrounding space. If the circuit consists of a coil and two plates of a flat capacitor, then the greater the angle at which these plates are deployed, the more freely the electromagnetic field enters the surrounding space (Fig. 44, b). The limiting case of an open oscillatory circuit is the removal of plates to opposite ends of the coil. Such a system is called an open oscillatory circuit (Fig. 44, c). In reality, the circuit consists of a coil and a long wire - an antenna.

When transmitting speech, music and other sound signals by radio, various types of modulation of high-frequency (carrier) oscillations are used. The essence of modulation lies in the fact that the high-frequency oscillations generated by the generator change according to the law of low frequency. This is one of the principles of radio transmission. Another principle is the reverse process - detection. During radio reception, low-frequency sound oscillations must be filtered out from the modulated signal received by the receiver antenna.

With the help of radio waves, not only sound signals are transmitted over a distance, but also images of objects. Radar plays an important role in the modern navy, aviation and astronautics. Radar is based on the property of reflection of waves from conducting bodies. (Electromagnetic waves are weakly reflected from the surface of a dielectric, and almost completely from the surface of metals.)

electromagnetic wave is an electromagnetic field that changes over time and propagates in space.

Properties of electromagnetic waves:

1. Occur during the accelerated movement of charges.

2. Are transverse.

3.Have speed in vacuum 3 ٠ 10 8 m/s.

4. Carry energy

5. Penetration and energy is frequency dependent.

6. Reflected.

7. Possess interference and diffraction.

The property of reflection of electromagnetic waves is used in radar.

Radar is the detection and location of objects using radio waves.

Radar installation (radar) consists of transmitting and receiving parts.

An electromagnetic wave comes from the transmitting antenna, reaches the object and is reflected.

Radars are used for military purposes and also by the weather service to monitor clouds. Radar is used to study the surfaces of the Moon, Venus and other planets.

Ticket number 13

Mechanical work. Power. Energy; kinetic energy; potential energy of a body in a uniform gravitational field and the energy of an elastically deformed body; law of energy conservation; the law of conservation of energy in mechanical processes; limits of applicability of the law of conservation of mechanical energy, work as a measure of change in the mechanical energy of the body.
Principles of radio communication: radiation of electromagnetic waves by a charge moving with acceleration; amplitude modulation; detection; development of means of communication; radar.
The task of applying the equation of state of an ideal gas.

Question 1. Mechanical work. Power. Kinetic and potential energy. The law of conservation of energy of mechanical processes.

Work is a quantity equal to the product of the force applied to the body by the amount of displacement.

A= F*s, where BUT- Job, J

F- strength, H

s- movement, m

Mechanical energy is the sum of the potential and kinetic energy of the body: W=W kin *W p

W kin- Kinetic energy is the energy of motion. Any body that is in motion possesses this energy: , where m- body weight (kg), V- speed (m / s 2)

W p - potential energy (J) is the interaction energy, depends on the body mass ( m) and its height above the ground ( h):

If a body or system of bodies can do work, then it has energy.

Energy is a physical quantity showing how much work a body can do.

Energy is denoted by the letter E and is measured in Joules (J).

Mechanical energy is of two types: kinetic and potential.

Kinetic energy is called a value equal to half the product of the mass of the body and the square of its speed.

Kinetic energy is the energy of motion. For example, a moving car, a flying balloon, etc. have kinetic energy.

Potential energy determined by the position of the body in relation to other bodies or by the mutual arrangement of parts of the same body.

The value equal to the product of the mass of the body times the acceleration of free fall and the height of the body above the surface of the Earth is called potential energy of the interaction of the body and the Earth.

A value equal to half the product of the coefficient of elasticity and the square of the deformation is called potential energy of an elastically deformed body.

For example, a ball thrown to a height or a compressed spring has potential energy.

For a closed system of bodies, the law of conservation of energy is fulfilled: the total mechanical energy of a body or a closed system of bodies remains constant (if friction forces do not act).

Question 2. Principles of radiotelephone communications. Amplitude modulation and detection. The simplest radio receiver.

For radio communication, electromagnetic waves with a frequency of several hundred thousand hertz to hundreds of thousands of megahertz are used. Such waves are well radiated by transmitter antennas, propagate in space and reach the receiver antenna.

The transmitter's microphone converts the sound waves into low frequency electrical vibrations that are not emitted by the antenna. These oscillations are added to the oscillations that the high-frequency generator produces, and are obtained amplitude modulated oscillations. They are high-frequency, but changed in amplitude in accordance with sound vibrations.

Amplitude modulated oscillations are radiated by the transmitting antenna and reach up to receiving antenna. The receiver is detection– selection of the sound frequency signal from high-frequency modulated oscillations.

The simplest receiver consists of a receiving antenna, an oscillatory circuit, a detector, a capacitor, an amplifier, and a speaker.

The receiver antenna oscillates at the same frequency as the transmitter. To tune the radio receiver to the frequency of a radio station, they usually use variable capacitor. With a change in its capacitance, the natural frequency of the receiver circuit changes. When this frequency coincides with the frequency of some radio station, a resonance occurs - a sharp increase in current strength.

Then, from the oscillatory circuit, the modulated oscillations arrive at detector which only allows current to flow in one direction. After the detector, the current becomes pulsating. The current pulses are divided: part charges the capacitor, the other part goes to the speaker. Between pulses, when no current flows through the detector, the capacitor discharges through the speaker. As a result, an audio-frequency current flows through the load, and music or speech is heard from the speaker.

Scale of electromagnetic radiation. The use of electromagnetic radiation in practice.

The scale of electromagnetic waves extends from long radio waves (λ>1 km) to γ-rays (λ<10 -10 м) . Электромагнитные волны различной длины условно делят на диапазоны по различным признакам (способу получения, способу регистрации, характеру взаимодействия с веществом).

It is customary to single out the following seven radiation: low frequency radiation, radio radiation, infrared rays, visible light, ultraviolet rays, x-rays and gamma rays.

low frequency radiation has the smallest frequency and the longest wavelength. Its sources are alternating currents and electrical machines. This radiation is weakly absorbed by air and magnetizes iron. It is used for the manufacture of permanent magnets, in the electrical industry.

radio waves are in the frequency range from 10 3 to 10 11 Hz. They are emitted by transmitter antennas and also by lasers. Radio waves propagate well in the air, are reflected from metal objects, clouds. Radio waves are used for radio communication and radar.

Infrared radiation has an even higher frequency than radio waves (up to 10 14 Hz) and is radiated by all heated bodies. It passes well through fog and other opaque bodies, and acts on thermoelements. It is used for melting, drying, in night vision devices, in medicine.

visible light has a frequency of about 10 14 Hz, a wavelength of 10 7 m. This is the only visible radiation. Sources: Sun, lamps. Light makes visible surrounding objects, decomposes into rays of different colors, causes a photoelectric effect and photosynthesis.

Used for lighting.

Ultraviolet radiation has a frequency from 10 14 to 10 17 Hz. Its sources: Sun, quartz lamps. This radiation causes photochemical reactions, a tan forms on the skin, kills bacteria, and is absorbed by ozone. It is used in medicine, in gas-discharge lamps.

X-rays are formed in the X-ray tube during a sharp deceleration of electrons. They have a great penetrating ability, actively affect the cells, emulsion. They are used in medicine, in radiography.

Gamma rays (γ rays) have the highest frequency (10 19 -10 29 Hz). They are formed during radioactive decay, during nuclear reactions. They have the highest penetrating ability, are not deflected by fields, and destroy living cells. They are used in medicine, military affairs.

Ticket number 14

The main provisions of the molecular-kinetic theory and their experimental substantiation. Mass and size of molecules.
Light is like an electromagnetic wave. The speed of light. Interference of light, Young's experience; thin film colors.
Experimental task: "Measuring the density of a solid substance."

Question 1. The main provisions of the molecular-kinetic theory and their experimental substantiation. Mass and size of molecules.

Molecular Kinetic Theory(MKT) is the doctrine of the structure and properties of matter, using the concept of the existence of atoms and molecules as the smallest particles of matter.

The ICT is based on three main principles:

1. All substances consist of the smallest particles: atoms and molecules.

2. These particles move randomly.

3. Particles interact with each other.

The main provisions of the MKT are confirmed by experimental facts.

The existence of atoms and molecules has been proven experimentally, photographs have been taken using electron microscopes.

The ability of gases to expand indefinitely and occupy the entire volume is explained by the continuous chaotic movement of molecules. It is also explained by diffusion and Brownian motion.

The elasticity of gases, solids and liquids, the ability of liquids to wet some solids, the processes of coloring, gluing, maintaining the shape of solids indicate the existence of forces of attraction and repulsion between molecules.

The masses and sizes of molecules are very small, and it is convenient to use not absolute values of masses, but relative ones. The relative atomic masses of all chemical elements are indicated in the periodic table (in comparison with the mass of a carbon atom).

The amount of a substance containing as many particles as there are atoms in 0.012 kg of carbon is called by one mole.

One mole of any substance contains the same number of atoms or molecules. This number is called the Avogadro constant: .

The mass of one mole is called molar mass: .

The amount of a substance is equal to the ratio of the mass of the substance to its molar mass: .