Antennas are devices that match an artificial electromagnetic wave (EMW) canalization system with the natural environment of their propagation.

Antennas are an integral part of any radio communication system that uses electromagnetic waves for technological purposes. In addition to coordinating the artificial and natural environments of EMW propagation, antennas can perform a number of other functions, the most important of which is the spatial and polarization selection of the received and emitted EMW.

Reference:

Matched systems are systems that transmit to each other the maximum amount of electromagnetic power intended for transmission.

Distinguish between receiving and transmitting antennas.

Transmitting antennas

Structural scheme

1 - antenna input, to which the supply waveguide from the transmitter is connected;

2 - a matching device that provides the traveling wave mode in the supply waveguide;

3 - a distribution system that provides the required spatial amplitude-phase distribution of the radiating fields;

4 - the emitting system (emitter), provides the specified polarization and directional EMW radiation.

Receiving antennas

Structural scheme

1 - the antenna output, to which the waveguide is connected, which connects the antenna to the receiver;

2 - matching device;

3 - integrator - a device that provides weighted coherent-in-phase summation of spatial electromagnetic fields;

4 - the receiving system, provides polarization and spatial selection of EMW entering the antenna from its natural environment.

Reference:

The structural elements of the transmitting and receiving antennas, designated by the same numbers, can have identical designs, as a result of which, apart from the system in which the antennas operate, it is impossible to distinguish the transmitting antenna from the receiving one and vice versa.

There are transmitting and receiving antennas.

Antenna classification

To systematize various types of antennas, they are combined according to a number of common features. Classification signs can be:

working wave range;

generality of design;

the principle of robots;

appointment.

Classes can be subclassed, etc.

By design, all antennas are divided into two large classes:

transmitting;

reception rooms.

These two classes include subtypes:

standing wave antennas;

traveling wave antennas;

aperture antennas;

signal processing antennas;

active antenna arrays;

scanning antenna arrays.

The main tasks of antenna theory

There are two tasks:

the problem of analyzing the properties of specific antennas;

the task of designing antennas according to the given initial requirements for them.

The analysis problem should be solved proceeding from the conditions: the sought EME must satisfy Maxwell's equations, boundary conditions at the interface between the media, and the Sommerfeld radiation conditions.

In such harsh conditions of formulating the solution of problems, the analysis is possible only for some special cases (for example, for a symmetric electric vibrator).

Approximate methods for solving analysis problems are widespread, according to which these problems are divided into two parts:

Internal task;

An external challenge.

The internal task is designed to determine the distribution of currents in the antenna, real or equivalent. The external task is to determine the radiation field of the antenna from the known distribution of its currents. When solving the external problem, the superposition method is widely used, which consists in dividing the antenna into elementary emitters and the subsequent summation of the fields.

The task of designing an antenna is to find the geometric shape and dimensions of the structure that provide its required functional properties. The solution of design (synthesis) problems of antennas is possible:

by applying the results of the analysis of specific types of antennas and the method of successive approximations, that is, by changing the parameters (stage of parametric optimization) with the subsequent comparison of the electrical characteristics obtained in this way by new versions of known antennas;

by means of direct synthesis, that is, bypassing the stage of parametric optimization. In this case, antenna design tasks are divided into two subtasks:

classical synthesis problem;

the problem of constructive synthesis.

The first consists in describing the amplitude-phase distribution of the current (or field) on the antenna radiator, which provides the specified functional properties of the antennas. The solution to this subproblem does not yet determine the antenna design, it only determines the requirements for its distribution.

The second is aimed at finding the complete geometry of the antenna for a given amplitude-phase distribution of the current (or field) on the antenna radiator. This problem is much more complicated than the first one and is structurally ambiguous; it is often solved approximately.

However, for some types of antennas, a strict theory of constructive synthesis has been developed.

Transmitting antennas

Their characteristics and parameters

The structure of the electromagnetic field (EMF) of the antenna

Each antenna can be considered as a system of elementary emitters concentrated in a certain limited volume of linear space (), its EM field as a superposition of the EM fields that make up its elementary emitters. To identify the structure of the antenna EMF, consider the structure of the EMF of a rectilinear element harmonically changing with an angular frequency , current with constant amplitude and length of this element in a linear unlimited isotropic medium with constant parameters,,.

- absolute dielectric constant of the medium;

ε is the relative dielectric constant of the medium;

Electric constant;

- absolute magnetic permeability of the medium;

Relative magnetic permeability of the medium;

Magnetic constant;

- specific electrical conductivity of the medium;

λ is the wavelength.

M - point of observation of EMF;

r - radial coordinate of point M (distance from the center of the spherical coordinate system to point M);

- azimuth coordinate of point M;

The meridional coordinate of point M.

To consider a Hertz vibrator located along the z axis, the middle of which is aligned with the center of the spherical coordinate system, the solution to Maxwell's equation has the form (1.1), where

Unit vectors;

– moment of electric current;

Orthogonal complex amplitude components in spherical coordinates ,, vector of electric field strength;

, , - orthogonal complex amplitude components in spherical coordinates ,, vector of magnetic field strength;

- wave number;

Wavelength in unlimited space.

From the expressions it follows that the EMF of a linear current element is orthogonal in space waves of electric and magnetic fields. In this case, the rate of change in the amplitude of each wave is determined by the relative distance of the point from the center of the vibrator.

There are three field areas:

For the far-field region, expressions take the form:

In the far region, the EMF has the following properties:

For air:.

In the regions of the intermediate and near fields, in addition to the spherical transverse wave, there are local reactive fields, the intensity of which increases very rapidly with decreasing r. These fields contain a certain amount of EM energy, which they periodically exchange with the antenna (with a period). These fields determine the reactive component of the antenna input impedance.

The properties of the EMF determine the functional properties of the antenna, and the properties of the near and intermediate EMF determine the stability of the functional properties and the broadband of the antennas.

The far EMF region is often called the radiation region, and the near EMF region is called the induction region.

For real antennas, the boundaries of the far, intermediate and near fields are determined taking into account the phase difference of the waves arriving at the observation point from the edges of the antenna and its center.

With a permissible phase difference in the far-field region equal to:

The far EMF area will be at;

The area of ​​the intermediate field;

Near-field region, where

Distance from the center of the antenna to the observation point;

- the maximum lateral dimension of the antenna radiating system.

Main characteristics and parameters of the transmitting antenna

Antenna properties are classified into:

Radio engineering;

Constructive;

Operational;

Economic;

The functional properties are entirely determined by the signal parameters.

Characteristics and parameters of the transmitting antenna:

Complex vector directional characteristic

The complex vector CNA is the dependence on the direction (polarization, phase) of the electric field of the waves emitted by the antenna at points equidistant from it (on the surface of a sphere of radius r).

In the general case, a complex HNA consists of three factors:

where are the spherical coordinates of the observation point of the field of the emitted antenna wave.

Amplitude HNA

Amplitude HNA is the dependence on the direction of the amplitude of the intensity of the electromagnetic wave emitted by the antenna at points equidistant from it.

Usually, the normalized amplitude HNA is considered:

,

where is the direction in which the value of the amplitude HNA is maximum.

Antenna radiation pattern (BOTTOM)

Antenna directivity diagram is a section of the amplitude HNA by planes passing through the direction or perpendicular to it.

The most commonly used section is with mutually orthogonal planes.

The radiation pattern has a lobe structure. The petals are characterized by amplitude and width.

BPD lobe width - the angle within which the lobe amplitude changes within the permissible specified limits.

Petals are:

Main petal;

Side petals;

Back lobe.

The width of the petals is determined by zeros or by the level of half the maximum power.

By field = 0.707;

By power = 0.5;

Logarithmic = -3 dB.

The normalized amplitude HNA in terms of power is related to the amplitude HNA in the field by the ratio:

Polar and rectangular coordinate systems and three types of scale are used to display the antenna pattern:

Linear (across the field);

Quadratic (in terms of power);

Logarithmic

Phase henna

Phase HNA is the dependence on the direction of the phase of a harmonic electromagnetic wave in the far-field region at points equidistant from the origin at a fixed time.

Reference:

Antenna phase center - a point in space, relative to which the value of the phase in the far field does not depend on the direction and changes abruptly to when moving from one petal of the HNA to another.

For a point source of an electromagnetic wave emitting a spherical wave, the surface of equal phases has the form of a sphere.

Polarizing HNA

An electromagnetic wave is polarized.

Polarization is the spatial orientation of the vector E, considered at any fixed point of the far field during one oscillation.

In the general case, the end of the vector E for one period of oscillation at any fixed point in space describes an ellipse, which is located in a plane perpendicular to the direction of wave propagation (polarization ellipse).

Polarization is characterized by:

ellipse parameters;

the spatial orientation of the ellipse;

the direction of rotation of the vector E.

Antenna radiation impedance

The radiation resistance of the antenna is the characteristic impedance of the space surrounding the antenna, which it has transferred to the entrance, or to any section of the waveguide feeding it, where the concept of total current has a meaning and can be defined.

Radiation resistance can be calculated using the formula:

ss ,

where I is the value of the total current at a given location of the antenna or two-wire line feeding it, which is equivalent to the feeding hollow waveguide.

Antenna input impedance

Antenna input impedance is the ratio of the complex amplitudes of harmonic voltages and currents at the antenna input terminals.

Antenna input impedance characterizes the antenna as a load on the supply line.

This parameter is used mainly for linear antennas, i.e. antennas for which the input voltages and currents have a clear physical meaning and can be measured.

For microwave antennas, the dimensions of the cross-section of their input waveguide are usually specified.

Antenna efficiency (COP)

Determines the efficiency of the antenna's transmission to the surrounding area.

Loss resistance

Reference:

With an increase in f, the antenna efficiency increases from a few percent at long wavelengths to 95-99% at microwave frequencies.

Dielectric strength and antenna height

The dielectric strength of an antenna is the ability of antennas to perform their functions without electrical breakdown of a dielectric in its structure or the environment with an increase in the power of an electromagnetic wave entering its input.

Quantitatively, the dielectric strength of the antenna is characterized by the maximum permissible power and the corresponding critical electric field strength at which the breakdown begins.

Antenna height

Antenna height is the ability of antennas to perform their functions without electrical breakdown of the surrounding atmosphere with an increase in the height of this antenna for a given transmission power.

Reference:

With increasing altitude, the dielectric strength first decreases, reaching a minimum at altitudes of 40-100 km, and then increases again.

Antenna operating frequency range

The frequency interval from f max to f min, within which none of the parameters and characteristics of the antenna goes beyond the limits specified in the technical specifications.

Typically, the range is determined by the parameter, the value of which, when the frequency is changed, goes out of range earlier than others. Most often, this parameter is the input impedance of the antenna.

Bandwidth and transmittance are quantitative estimates of the range properties of an antenna:

Frequent use of relative bandwidth

According to the parameter, antennas are divided into:

Directional Action Factor (LPC)

Directional coefficient of the antenna in for this direction is a number showing how many times the value of the Poynting vector in the considered direction at a fixed point in the far zone differs from the value of the Poynting vector at the same point if the considered antenna is replaced by an absolutely non-directional (isotropic) antenna, provided that their radiated powers are equal.

Reference:

Usually indicate maximum value Antenna directivity in the direction of its maximum radiation.

Vibrator: KND = 0.5;

Half-wave symmetrical vibrator: KND = 1.64;

Horn antenna: KND;

Reflector antenna: KND;

Spacecraft antennas: KND;

The limiting factor of the upper limit of the LPC is technological manufacturing errors and the influence of operating conditions.

The minimum values ​​of the maxima of the directivity of real antennas are always> 1since absolutely omnidirectional antennas do not exist.

The directivity is related to the field with the normalized amplitude HNA:

,

where – the maximum value of the directivity in the direction of the maximum radiation of the antenna, in which .

CPV showing The gain in power that the directional antenna provides, but does not take into account the heat loss in it.

NS NS antenna gain

Antenna gain in a given direction is a number showing the power gain from using a directional antenna, taking into account heat losses in it:

Equivalent isotropic radiated power

Equivalent isotropic radiated power is the product of the power supplied to the antenna by the antenna's maximum gain.

Antenna dissipation factor

Antenna Dissipation Factor is a number that represents the proportion of radiated power attributable to the side and back lobes.

Determines the power falling on the main lobe of the HNA

Effective antenna length

The effective length of the antenna is the length of a hypothetical rectilinear vibrator with a uniform current distribution along its entire length, which, in the direction of its maximum radiation, creates the same field strength as the antenna under consideration with the same input current.

In a medium with characteristic impedance, the effective length of the antenna is determined by the expression.

Polarization of electromagnetic waves

Polarization of electromagnetic waves (French polarization; original source: Greek polos axis, pole) is a violation of the axial symmetry of a transverse wave relative to the direction of propagation of this wave. In an unpolarized wave, oscillations of the vectors s and v of displacement and velocity in the case of elastic waves or vectors E and H of the strengths of electric and magnetic fields in the case of electromagnetic waves at each point in space along all possible directions in a plane perpendicular to the direction of wave propagation, quickly and randomly replace each other so that none of these directions of vibration is predominant. A transverse wave will be called polarized if at each point in space the direction of oscillations remains unchanged or changes over time according to a certain law. A plane-polarized (linearly polarized) wave is called a wave with a constant direction of oscillations, respectively, of the vectors s or E. If the ends of these vectors describe circles or ellipses over time, then the wave is called circular or elliptically polarized. A polarized wave can arise: due to the absence of axial symmetry in the emitter exciting the wave; when waves are reflected and refracted at the interface between two media (see Brewster's law); when a wave propagates in an anisotropic medium (see Birefringence).
(see Big Encyclopedic Polytechnic Dictionary)
In practice: if the signal from the telecentre goes in horizontal polarization, then the antenna vibrators should be located parallel to the ground plane, if the signal is transmitted in vertical polarization, then the antenna vibrators should be located perpendicular to the ground plane, if the signals are transmitted in two polarizations, then two summarize antennas and signals from them. In the zone of reliable reception, you can place one antenna at an angle of 45 degrees to the ground plane.
Satellite television signal is transmitted to Earth in linear and circular polarization. To receive such signals, different converters are used: for example, for Continent TV, a linear converter, and for Tricolor TV, a circular converter. The shape and size of the dish has no effect on polarization.

An important parameter of antennas is the input impedance: (the input impedance of the antenna), which characterizes it as a load for the transmitter or feeder. Antenna input impedance is the ratio of the voltage between the connection point (excitation point) of the antenna to the feeder, to the current at these points. If the antenna is powered by a waveguide, the input impedance is determined by reflections in the waveguide path. The antenna input impedance consists of the sum of the antenna radiation resistance and the loss resistance: Z = R (out) + R (sweat). R (rad) - in the general case, the value is complex. At resonance, the reactive component of the input impedance should be zero. At frequencies above the resonant impedance, the impedance is inductive, and at frequencies below the resonant, it is capacitive, which causes a loss of power at the boundaries of the antenna's working band. R (sweat) - antenna loss resistance depends on many factors, for example, its proximity to the Earth's surface or conductive surfaces, ohmic losses in antenna elements and wires, insulation losses. The input impedance of the antenna must be matched with the characteristic impedance of the feed path (or with the output impedance of the transmitter) so as to provide in the latter a mode close to the traveling wave mode.
For television antennas, the input impedance is: log-periodic antenna - 75 Ohm, for wave channel - 300 Ohm. For wave channel antennas, when using a television cable with a characteristic impedance of 75 ohms, a matching device, an RF transformer is required.

Standing Wave Ratio (KSV)

The standing wave ratio characterizes the degree of antenna-feeder matching, as well as the transmitter-to-feeder output matching. In practice, a portion of the transmitted energy is always reflected and returned to the transmitter. Reflected energy causes the transmitter to overheat and may damage it.

VSWR is calculated as follows:
KSV = 1 / KBV = (U pad + U ref) / (U pad - U ref), where U pad and U ref are the amplitudes of the incident and reflected electromagnetic waves.
The amplitudes of the incident (U pad) and reflected (U ref) waves in the KBV line are related by the ratio: KBV = (U pad + U ref) / (U pad - U ref)
Ideally, SWR = 1, values ​​up to 1.5 are considered acceptable.

Directional pattern (DP)

The radiation pattern is one of the most obvious characteristics of the antenna's receiving properties. The construction of radiation patterns is performed in polar or in rectangular (Cartesian) coordinates . Consider, for example, built in polar coordinates the directional diagram of the "wave channel" type antenna in the horizontal plane (Fig. 1). The coordinate grid consists of two systems of lines. One system of lines is concentric circles centered at the origin. The circle of the largest radius corresponds to the maximum EMF, the value of which is conventionally assumed to be equal to one, and the remaining circles correspond to intermediate values ​​of the EMF from one to zero. Another system of lines that form a coordinate grid is a bundle of straight lines that divide a central angle of 360 ° into equal parts. In our example, this angle is divided into 36 parts of 10 ° each.

Suppose that the radio wave comes from the direction shown in Fig. 1 arrow (angle 10 °). From the directional diagram, it can be seen that this direction of arrival of the radio wave corresponds to the maximum EMF at the antenna terminals. When receiving radio waves coming from any other direction, the EMF at the antenna terminals will be less. For example, if the radio waves arrive at angles of 30 and 330 ° (i.e., at an angle of 30 ° to the antenna axis from the direction of the directors), then the EMF value will be 0.7 maximum, at angles 40 and 320 ° - 0.5 maximum and etc.

The radiation pattern (Fig. 1) shows three characteristic areas - 1, 2 and 3. Area 1, which corresponds to the highest received signal level, is called the main , or the main lobe of the radiation pattern. Regions 2 and 3, located on the side of the antenna reflector, are called the back and side lobes of the radiation pattern. . The presence of back and side lobes indicates that the antenna receives radio waves not only from the front (from the directors' side), but also from the back (from the reflector side), which reduces the noise immunity of reception. In this regard, when tuning the antenna, they tend to reduce the number and level of the back and side lobes.
The described radiation pattern, which characterizes the dependence of the EMF at the antenna terminals on the direction of arrival of the radio wave, is often called the “field” radiation pattern , since the EMF is proportional to the strength of the electromagnetic field at the receiving point. By squaring the EMF corresponding to each direction of arrival of the radio wave, it is possible to obtain the power directivity diagram ( dotted line in fig. 2).
For a numerical assessment of the directional properties of the antenna, the concepts of the opening angle of the main lobe of the directional pattern and the level of the back and side lobes are used. The opening angle of the main lobe of the radiation pattern is the angle within which the EMF at the antenna terminals falls to a level of 0.7 of the maximum. The opening angle can also be determined using the power directivity diagram, by its fall to the level of 0.5 of the maximum (the opening angle for "half" power). In both "cases, the numerical value of the opening angle is obtained, naturally, the same.
The level of the back and side lobes of the voltage radiation pattern is defined as the ratio of the EMF at the antenna terminals when received from the side of the maximum of the rear or side lobe to the EMF from the side of the maximum of the main lobe. When the antenna has multiple trailing and side lobes of different sizes, the level of the largest lobe is indicated.

Directional Action Factor (LPC)

Directional factor: (directivity) of the transmitting antenna - the ratio of the square of the field strength created by the antenna in the direction of the main lobe to the square of the field strength created by the non-directional or directional reference antenna (half-wave dipole - dipole, the directional factor of which with respect to the hypothetical non-directional antenna is 1 , 64 or 2.15 dB) with the same input power. (KND) is a dimensionless quantity, it can be expressed in decibels (dB, dBi, dBd). The narrower the main lobe (MD) and the lower the level of the side lobes, the higher the directivity.
The real gain of the antenna in terms of power relative to a hypothetical isotropic emitter or half-wave vibrator is characterized by the power gain KU (Power), which is related to the (KND) ratio:
KU (Power) = KND - efficiency (antenna efficiency)

Gain (KU)

Antenna gain (GF) is the ratio of the power at the input of the reference antenna to the power supplied to the input of the antenna under consideration, provided that both antennas create in a given direction on equal distance equal values ​​of the field strength during the emission of power, and during the reception - the ratio of the powers allocated to the matched loads of the antennas.
KU is a dimensionless quantity, it can be expressed in decibels (dB, dBi, dBd).
Antenna gain is characterized by a power (voltage) gain, which is released in the matched load connected to the output terminals of the antenna in question, compared to an "isotropic" (that is, having a circular pattern) antenna or, for example, a half-wave vibrator. In this case, it is necessary to take into account the directional properties of the antenna and the loss in it (efficiency). For television receiving antennas (KU) it is approximately equal to the directional action factor (directivity) of the antenna, because the efficiency of such antennas is in the range of 0.93 ... 0.96. The gain of wideband antennas is frequency dependent and uneven across the entire frequency band. In the passport, the maximum value (KU) is often indicated on the antenna.

Coefficient of performance (COP)

In the transmission mode, (efficiency) is the ratio of the power emitted by the antenna to the power supplied to it, since there are losses in the output stage of the transmitter, in the feeder and the antenna itself, the antenna efficiency is always less than 1. In receiving television antennas, the efficiency is within 0 , 93 ... 0.96.

The design, manufacture and use of antennas for long (LW), medium (MW), and short (KB) wavelengths are significantly less problematic than antennas for VHF, especially television. The fact is that in the DV, SV, KB bands, transmitters, as a rule, have a high power, the propagation of radio waves in these bands is associated with high values ​​of diffraction and refraction in the atmosphere, and the receiving devices have high sensitivity.

When transmitting and receiving a signal in the VHF range and in particular television signal ensuring the necessary values ​​of these parameters causes a number of difficulties, namely: the achievement of the power of television transmitters, such as broadcasting ones, has turned out to be impossible so far; the phenomena of diffraction and refraction in the VHF range are insignificant; the sensitivity of a television receiver is limited by its own noise level and is, due to the need to receive a broadband signal, about 5 μV. Therefore, to get on the TV screen high level the image level of the input signal must be at least 100 µV. However, due to the low power of the transmitter and the worse conditions for the propagation of radio waves, the strength of the electromagnetic field at the receiving point is low. Hence, one of the main requirements for a television antenna arises: for a given field strength at the receiving point, the antenna must provide the necessary signal voltage for the normal operation of the television receiver.

The receiving antenna is a single wire or a system of wires designed to convert the energy of electromagnetic waves into energy of high frequency currents. The parameters of the antennas during operation for reception and transmission are identical, therefore, the principle of reciprocity of antenna devices can be applied, which makes it possible to determine some characteristics and parameters of the antennas in the transmission mode, and others in the reception mode.

Radio waves, falling on surrounding objects, induce high-frequency electric currents in them. The latter create an electromagnetic field, and the electromagnetic wave is reflected. The antenna receives both direct and reflected radio waves, which distort the image on the TV screen.

Experimental studies have shown that when using vertical polarization, significantly more reflected waves come to the receiving site than when using horizontal polarization. This is due to the fact that in the surrounding space, especially in cities, there are many vertical, well-reflecting obstacles (buildings, poles, pipes, magnets). When choosing the type of polarization, the properties of the antennas are also taken into account. Structurally, horizontal antennas are simpler than vertical ones. Almost all of them have directionality in the horizontal plane, which weakens the reception of interference and reflected waves due to spatial selectivity.

Receiving television antennas must meet the following basic requirements:

Have a simple and easy-to-use design;

High spatial selectivity;

Pass a wide frequency band;

Provide a high ratio of the signal level to the level of interference during reception;

Have a weak dependence of the input impedance and gain on frequency.

Antenna input impedance

An antenna is a signal source that is characterized by an electromotive force (EMF) and an internal resistance called the antenna input impedance. The input impedance is determined by the ratio of the direction at the antenna terminals to the current at the feeder input. The value of the antenna input impedance must be known in order to correctly match the antenna with the cable and the TV: only under this condition the highest power is supplied to the TV input. With proper matching, the input impedance of the antenna should be equal to the input impedance of the cable, which, in turn, should be equal to the input impedance of the TV.

Antenna input impedance has active and reactive components. The input impedance of the tuned antenna is purely active. It depends on the type of antenna and its design features... For example, the input impedance of a linear half-wave vibrator is 75 ohms, and that of a loop vibrator is about 300 ohms.

Matching the antenna with the feeder cable

Antenna-to-cable matching is characterized by traveling wave ratio (TWR). In the absence of a perfect match between the antenna and the cable, the incident wave (input voltage) is reflected, for example, from the end of the cable or another point where its property changes dramatically. In this case, the incident and reflected waves propagate along the cable in opposite directions. At those points where the phases of both waves coincide, the total voltage is maximum (antinode), and at the points where the phases are opposite, it is minimal (node).

The traveling wave coefficient is determined by the ratio:

In the ideal case, KBV = 1 (when there is a traveling wave mode, that is, a signal of the maximum possible power is transmitted to the input of the TV, since there are no reflected waves in the cable). This is possible by matching the input impedances of the antenna, cable and TV. In the worst case (when U min = 0) KBV = 0 (there is a standing wave mode, that is, the amplitudes of the incident and reflected waves are equal, and the energy is not transmitted along the cable).

The standing wave ratio is determined by the ratio:

Directional and antenna gain

The receiving omnidirectional antenna receives signals from all directions. The directional receiving antenna has spatial selectivity. This is important, because with a low level of field directivity at the receiving site, such an antenna increases the received signal level and attenuates external interference coming from other directions.

The directional gain of a receiving antenna is a number indicating how many times the power supplied to the TV input when receiving on a directional antenna is greater than the power that can be obtained when receiving on an omnidirectional antenna (at the same field strength).

Antenna's directivity properties are characterized by a directivity pattern. The directional diagram of the receiving antenna is a graphical representation of the dependence of the signal voltage at the input of the TV on the angle of rotation of the antenna in the corresponding plane. This diagram characterizes the dependence of the EMF induced in the antenna by the electromagnetic field on the direction of arrival of the signal. It is built in a polar or rectangular coordinate system. On rice. 12 the antenna radiation patterns of the "wave channel" type are presented.

Rice. 1. The radiation pattern of the antenna in the polar coordinate system

Antenna radiation patterns are most often multi-lobe. The petal corresponding to the direction of arrival of the wave at which the maximum EMF is induced in the antenna is called the main one. In most cases, the radiation pattern also has reverse (rear) and side lobes. For the convenience of comparing different antennas with each other, their directional patterns are normalized, that is, they are plotted in relative values, taking the largest EMF as one (or one hundred percent).

The main parameters of the radiation pattern are the width (opening angle) of the main lobe in the horizontal and vertical planes. The width of the main lobe is used to judge the directional properties of the antenna. The smaller this width, the greater the directivity.

Rice. 2. Antenna radiation pattern in a rectangular coordinate system

The level of the side and back lobes characterizes the noise immunity of the antenna. It is determined using the antenna protection factor (SPC), which is understood as the ratio of the power emitted by the antenna at the matched load when receiving from the rear or lateral direction to the power at the same load when receiving from the main direction.

The coefficient of protective action is often expressed in logarithmic units - decibels:

The directional properties of the antenna are also characterized by the directional action coefficient (DIR) - a number that shows how many times the signal power entering the input of the TV when received at a given directional antenna is greater than the power that could be obtained when receiving on an omnidirectional or directional reference antenna. A half-wave dipole (dipole) is most often used as a reference antenna, the directivity of which with respect to a hypothetical omnidirectional antenna is 1.64 (or 2.15 dB). The LPC characterizes the maximum possible power gain that an antenna can give due to its directional properties, assuming that there are no losses in it at all. In fact, any antenna has losses and the power gain it gives is always less than the maximum possible. The real gain of the antenna in power relative to a hypothetical isotropic radiator or half-wave vibrator is characterized by the power gain K p, which is related to the LPC by the ratio:

where η - coefficient of performance (efficiency) of antennas.

Antenna efficiency characterizes the power loss in the antenna and is the ratio of the radiation power to the sum of the radiation powers and losses, that is, to the total power supplied to the antenna from the transmitter:

where P u- radiation power, P n- power losses.

Antenna bandwidth

The bandwidth of a receiving television antenna is a frequency spectrum within which all the main values ​​of its electrical characteristics are maintained. The frequency response of a tuned antenna is similar to a resonance curve oscillatory circuit... Therefore, by analogy with the loop bandwidth, the antenna bandwidth can also be determined.

At the resonant (fixed) frequency, the antenna has a certain value of the input impedance, which is consistent with the load impedance. This frequency is usually taken as the average frequency television channel where the antenna reactance is zero. At frequencies below the resonant, it is capacitive, and at frequencies above the resonant, it is inductive.

Thus, a change in frequency leads both to a change in the active component and to the appearance of a reactive component of the input resistance. As a result, the power supplied to the load is reduced.

This is especially noticeable at the extreme frequencies farthest from resonant frequency... The power can be reduced by no more than two times. Based on this bandwidth 2Af such a frequency spectrum near the resonant frequency is considered, within which the power supplied to the load will decrease by no more than two times.

To provide good quality The antenna must transmit the entire frequency spectrum of the television signal, which for one channel is 8 MHz. The image quality is still good enough if the antenna has a bandwidth of at least 6 MHz. Further narrowing of the frequency band leads to a deterioration in the quality of the image and to the loss of its clarity. Most effective method bandwidth expansion - a decrease in the equivalent wave impedance of the vibrator by increasing its transverse dimensions. In this way, the linear capacitance increases and the linear inductance of the vibrator decreases. Among other things, the antenna bandwidth is limited by the bandwidth of the drop feeder.

antenna input impedance. It is believed that it is a series-connected reactance and resistance. But there is no real resistor, capacitor or inductor in the antenna or feeder. All this is only the result of calculating the equivalent resistances of the antenna circuit. Let a certain "black box" be used as a load, the input connector of which is supplied with RF voltage. On this connector, you can actually measure the instantaneous voltage u 'and the current i', as well as the phase difference between them j. The input resistance is the calculated active and reactance, connecting to which the given HF voltage we get exactly the same u ’, i’ and j.
It is known that such an equivalent can have both serial (serial, Zs = Rs + jXs) and parallel (parallel, Zp = Rp || + jXp) connection of active and reactance resistances. Each series connection of active (Rs) and reactive (Xs) resistances corresponds to a parallel connection of active (Rp) and reactance (Xp) resistances. In general, Rs # Rp and Xs # Xp. Here are the formulas by which you can recalculate the numerical values ​​from one compound to another.

For example, let's convert the serial connection Zs = 40 + j30W to parallel Zp.

More often, the equivalent of series connection is used, but the equivalent of parallel connection has the same practical significance. Zs is called series impedance, R is resistance, X is reactance, and Zp is parallel impedance. In parallel connection, administration is often used, but this is conductivity, and visibility when using it is greatly reduced. Usually the term "impedance" indicates that we are talking about a series connection of equivalent resistance and reactance.

88) The powers supplied to the antenna and radiated by the antenna.

The power is divided into two parts:

1) emitted

2) losses on active resistance (in the ground, in the surrounding metal conductors, guys, buildings, etc.)

- the radiated power, as for any linear circuit, is proportional to the square of the effective value of the current in the antenna.

- coefficient of proportionality.

Radiation resistance can be defined as the coefficient that connects antennas with at a given antenna point.

(antenna shape, geometric dimensions, l)

- useful power

Power loss:

- Equivalent loss resistance related to current I

– full power(supplied to the antenna)

where - active resistance of the antenna at the feed point

To assess the efficiency of the antenna, the concept of antenna efficiency is introduced , to increase it is necessary to decrease.

89) Symmetrical electric vibrator in free space.

Approximate laws of current and charge distribution over the vibrator.

Rice. 15. Symmetrical vibrator

Symmetrical vibrato - two arms of the same size and shape, between which the generator is switched on.

Before the development of a rigorous theory of a symmetrical vibrator (late 30s - early 40s), an approximate method was used to calculate the vibrator field. It is based on the assumption of a sinusoidal current distribution over the vibrator (the law of standing waves) associated with some external analogy between a symmetrical vibrator and a 2-wire line open at the end.

LECTURE 9.

• 
  ^ Isotropic emitter
• 
  Symmetrical vibrator
• 
  Main characteristics of antennas. Amplitude characteristic of directivity of antennas
• 
  Radiation resistance
• 
  Antenna impedance
• 
  Input impedance
• 
  Loss resistance

^

ISOTROPIC RADIATOR.

An isotropic emitter is understood as a device that uniformly and equally radiates electromagnetic energy in all directions.

However, in practice, non-directional emitters do not exist. Each transmitting antenna, even the simplest one, emits energy unevenly and there is always a direction in which the maximum energy is radiated.

The simplest or elementary emitter is an electromagnetic electric vibrator, which consists of a very short wire compared to the wavelength, streamlined electric shock, the amplitude and phase of which are the same at any point in the wire. A practical model of an elementary vibrator is the Hertz dipole. The structure of the radiation field of the Hertz dipole has a maximum at a point lying on a straight line perpendicular to the dipole. Along the dipole field = 0.
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SYMMETRIC VIBRATOR.

Consists of two conductors of the same length, between which a supply line is switched on - a feeder, which connects the antenna to the transmitter.

The most frequencies used is a symmetrical vibrator with a length of l in half , called a half-wave vibrator in Fig. 37a.

Due to the reflection of current and voltage at the ends of the antenna wires, a standing wave of current and voltage is established along the wires.

Along the half-wave vibrator, the half of the current and voltage wave is set, along the wavelength vibrator - the current and voltage wave Fig. 37b. However, in any case, a current node and a voltage antinode are installed at the ends.
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MAIN CHARACTERISTICS OF ANTENNAS.

AMPLITUDE CHARACTERISTIC OF ANTENNA DIRECTION.

It is customary to determine the directional properties of antennas by the directional amplitude characteristic, i.e. dependence of the intensity of the field emitted by the antenna E (, ) at the observation point at a constant distance. The graphic image of the directional amplitude characteristic is called the directional diagram, which is depicted as a surface described by the radius vector outgoing from the origin, the length of which in each direction is proportional to the function F (, ) .

The radiation pattern is built in both polar (Fig. 38a) and rectangular (Fig. 38b) coordinate systems.

The direction of maximum radiation of the antennas is called the main direction. And the petal corresponding to it is the main one. The rest of the petals are lateral. The directions in which the antenna does not receive or radiate are called pattern zeros.

The main lobe is characterized by a width of half the power  0.5 and a width of zeros  0. The width  0.5 is determined from the RP at the level of 0.707, it is taken on the basis that the power at the level of 0.5 and the field strength at the level of 0.707 are related by the ratio

R 0,5 / R swing = E 2 0,707 / E 2 swing = 0,5 .

Directional coefficient of directivity of directivity characterizes the ability of the antenna to concentrate the radiated electromagnetic field in any direction. It is the ratio of the power flux density emitted by the antenna in a given direction to the power flux density averaged over all directions. In other words, when determining the directivity, the antenna is compared with an imaginary, absolutely non-directional or isotropic antenna emitting the same power as the considered one.

For aperture antennas

TO nd = 4 TO isp S a /  2 ,

Where: TO isp is the utilization factor of the instrumentation emitting surface;

S a - antenna aperture area.

Most RRL antennas and satellite systems transmission width of the pattern at half power in the vertical plane is approximately equal to the width of the diagram in the horizontal plane.

To take into account the efficiency of a real antenna, the concept of the gain of the antenna gain is introduced, which is determined by the ratio

G =  a TO nd ,

where:  a = R  / R 0 - antenna efficiency;

R  is the radiated power of the antenna;

R 0 is the power supplied to the antenna.

Antenna gain indicates how many times the power supplied to the antenna should be reduced in comparison with the power supplied to an isotropic radiator with an efficiency equal to 1 so that the field strength at the receiving point remains unchanged.

In the range of decimeter and centimeter waves  a  1 , therefore

G = K nd.

The coefficient of protective action of the CPC is introduced to characterize the degree of attenuation of the antenna signals received from the side directions, and is calculated by the formula TO bld = G swing / G bb, where G swing and G bp - antenna gains in the direction of the main lobe of the antenna pattern and in the side direction.
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RADIATION RESISTANCE.

Antenna radiation impedance R rad - an indicator that has the dimension of resistance and connects the radiated power P rad with current I A flowing through any section of the antenna

R out = R out / I A 2 .

Since currents and voltages are unevenly distributed along the length of the antenna, to round off the value R izl, in most cases, the radiated power is referred to the square of the maximum current amplitude (at the antinode) or to the square of the current at the input terminals of the antenna.

The quantity R Radius depends on the relationship between antenna size and wavelength, antenna shape, and other factors.

So, an increase in the length of a solitary symmetrical vibrator up to l = , leads to an increase in the radiation resistance. However, further it falls, then rises again.

In general R izl has a complex character.

For example, for a thin half-wave vibrator R out = 73,1 Ohm, a NS out = 42,5 Ohm.

An increase in the thickness of the vibrator leads to a decrease in the magnitude of the wave resistance.
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ANTENNA WAVE RESISTANCE.

Antenna impedance Z OA is one of the important parameters. Under consideration wave impedance methods of the theory of long lines.

For single cylindrical conductor length l , to which the antenna in the form of a symmetrical dipole can be assigned, the calculation formula has the form

,

where: r n is the radius of the conductor.

An increase in the thickness of the conductor leads to a decrease in the characteristic impedance.
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INPUT RESISTANCE.

Antenna input impedance - An indicator representing the ratio of the voltage at the antenna terminals to the current flowing through them. In general, this resistance is complex.

Z Abh = R Abh + iХ Abh

where: R Avx is the active component of the input resistance;

NS Avx is the reactive component of the input resistance.
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LOSS RESISTANCE.

Loss resistance is defined as:

R NS = R n + R and + R 3 ,

where: R n - resistance of losses for heating wires;

R and is the loss resistance in the antenna insulators;

R 3 - resistance to losses in the ground and in grounding systems.