Antenna active and reactive impedance components. Antenna parameters. Directional antennas and radiation polarization

Heating

Polarization of electromagnetic waves

Polarization of electromagnetic waves (French polarisation; original source: Greek polos axis, pole) - violation axial symmetry transverse wave relative to the direction of propagation of this wave. In an unpolarized wave, oscillations of the displacement and velocity vectors s and v in the case elastic waves or the vectors E and H of the electric and magnetic field strengths in the case of electromagnetic waves at every point in space in all possible directions in a plane perpendicular to the direction of propagation of the wave, quickly and randomly replace each other, so that none of these directions of oscillation is predominant. A transverse wave will be called polarized if at each point in space the direction of oscillation remains unchanged or changes over time according to a certain law. Plane-polarized (linearly polarized) will be called a wave with a constant direction of oscillation, respectively, of vectors s or E. If the ends of these vectors describe circles or ellipses over time, then the wave will be called circularly or elliptically polarized. A polarized wave can arise: due to the lack 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 television center comes 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 should be used antennas and signals from them are summed up. In the area of ​​reliable reception, you can place one antenna at an angle of 45 degrees to the ground plane.
Satellite television signals are 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 plate does not have any effect on polarization.

An important parameter of antennas is the input impedance: (antenna input impedance), which characterizes it as a load for a transmitting device or feeder. The antenna input impedance is the ratio of the voltage between the connection point (excitation point) of the antenna to the feeder and the current at these points. If the antenna is fed by a waveguide, then the input impedance is determined by the reflections that occur in the waveguide path. The input impedance of the antenna consists of the sum of the radiation resistance of the antenna and the loss resistance: Z = R(emission) + R(pot). R(izl) - in general case the value is complex. At resonance, the reactive component of the input impedance must be zero. At frequencies above the resonant impedance it is inductive in nature, and at frequencies below the resonant it is capacitive in nature, which causes a loss of power at the boundaries of the antenna's operating band. R (sweat) - the loss resistance of the antenna depends on many factors, for example, its proximity to the Earth’s surface or conductive surfaces, ohmic losses in the antenna elements and wires, and insulation losses. The input impedance of the antenna must be matched with the characteristic impedance of the feeder path (or with the output impedance of the transmitter) so as to ensure in the latter a mode close to the traveling wave mode.
Television antennas have an input impedance: a log-periodic antenna is 75 Ohms, and a wave channel is 300 Ohms. 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 (SWR)

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

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

Radiation Pattern (DP)

The radiation pattern is one of the most visual characteristics of the receiving properties of an antenna. Directional patterns are constructed in polar or rectangular (Cartesian) coordinates . Let us consider, for example, the radiation pattern of an antenna of the “wave channel” type constructed in polar coordinates in horizontal plane(Fig. 1). The coordinate grid consists of two systems of lines. One system of lines represents 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 unity, 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 bunch of straight lines that divide a central 360° angle into equal parts. In our example, this angle is divided into 36 parts of 10° each.

Let us assume that the radio wave comes from the direction shown in Fig. 1 arrow (angle 10°). From the radiation pattern it is clear 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 radio waves arrive at angles of 30 and 330° (i.e., at an angle of 30° to the antenna axis from the directors), then the EMF value will be equal to 0.7 maximum, at angles of 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 level of the received signal, is called the main , or the main lobe of the radiation pattern. Regions 2 and 3, located on the reflector side of the antenna, 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’s side), which reduces the reception’s noise immunity. In this regard, when tuning the antenna, they strive 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, we can obtain the power radiation pattern (dashed line in Fig. 2).
To numerically evaluate the directional properties of an antenna, the concepts of the opening angle of the main lobe of the radiation pattern and the level of the rear 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 drops to a level of 0.7 from the maximum. The opening angle can also be determined using the power directional pattern, by its decline to a level of 0.5 from the maximum (opening angle at “half” power). In both cases, the numerical value of the opening angle is 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 receiving from the side of the maximum of the back or side lobe to the EMF from the side of the maximum of the main lobe. When an antenna has several back and side lobes of different sizes, the level of the largest lobe is indicated.

Directional coefficient (DC)

Directional coefficient: (DC) of a 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 an omnidirectional or directional reference antenna (half-wave vibrator - dipole, the directional coefficient of which in relation to a hypothetical omnidirectional antenna is 1 .64 or 2.15 dB) with the same input power. (KND) is a dimensionless quantity and can be expressed in decibels (dB, dBi, dBd). The narrower the main lobe (LM) and the lower the level of the side lobes, the greater the directivity.
The real power gain of the antenna relative to a hypothetical isotropic emitter or half-wave vibrator is characterized by the power gain KU(Power), which is related to the ratio:
KU(Power) = KND - efficiency (antenna efficiency)

Gain

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 in question, provided that both antennas create equal values ​​of field strength in a given direction at the same distance when emitting power, and when receiving - the ratio of powers, antennas allocated to matched loads.
KU is a dimensionless quantity and can be expressed in decibels (dB, dBi, dBd).
Antenna gain is characterized by a gain in power (voltage), which is released in a 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 losses in it (efficiency). For television receiving antennas (KU) it is approximately equal to the directivity coefficient (DAC) of the antenna, because The efficiency of such antennas is in the range of 0.93...0.96. The gain of broadband antennas depends on frequency and is uneven across the entire frequency band. The antenna data sheet often indicates the maximum value (KV).

Efficiency factor (efficiency)

In 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.

Antennas are devices that match the artificial channeling system of electromagnetic waves (EMW) with the surrounding natural environment of their propagation.

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

Reference:

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

There are receiving and transmitting antennas.

Transmitting antennas

Block diagram

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

2 – a matching device that ensures traveling wave mode in the supply waveguide;

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

4 – radiating system (emitter), provides specified polarization and directional radiation of electromagnetic waves.

Receiving antennas

Block diagram

1 – antenna output, to which the waveguide connecting the antenna to the receiver is connected;

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 electromagnetic waves entering the antenna from the natural environment surrounding it.

Reference:

    Elements of the structure of the transmitting and receiving antennas, designated by the same numbers, may have identical designs, as a result of which, in isolation from the system in which the antennas operate, it is impossible to distinguish the transmitting antenna from the receiving antenna and vice versa.

    There are transmitting and receiving antennas.

Antenna classification

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

    operating wave range;

    commonality of design;

    robot principle;

    appointment.

Classes can be divided into subclasses, etc.

According to their purpose, all antennas are divided into two large classes:

    transmitting;

    receptions.

These two classes include subtypes:

    standing wave antennas;

    traveling wave antennas;

    aperture antennas;

    antennas with signal processing;

    active antenna arrays;

    scanning antenna arrays.

Main tasks of antenna theory

There are two tasks:

    the task 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 based on the conditions: the required electromagnetic waves must satisfy Maxwell's equations, boundary conditions at the interface and Sommerfeld radiation conditions.

In such harsh conditions for posing problems, analysis is only possible for some special cases (for example, for a symmetrical electric vibrator).

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

Internal task;

External task.

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 an external problem, the superposition method is widely used, which consists of dividing the antenna into elementary radiators and subsequent summation of the fields.

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

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

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

    classical synthesis problem;

    the task of constructive synthesis.

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

The second is aimed at finding the complete geometry of the antenna based on a given amplitude-phase distribution of the current (or field) at the antenna emitter. 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 rigorous theory of constructive synthesis has been developed.

Transmitting antennas

Their characteristics and parameters

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 EMF antenna, consider the structure of the EMF element of a rectilinear element that changes harmoniously with 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 – relative dielectric constant of the medium;

Electrical constant;

– absolute magnetic permeability of the medium;

Relative magnetic permeability of the medium;

Magnetic constant;

– specific electrical conductivity of the medium;

λ – wavelength.

M – EMF observation point;

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

– azimuthal coordinate of point M;

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 electric current;

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

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

- wave number;

Wavelength in infinite space.

From the expressions it follows that the EMF of a linear current element represents waves of electric and magnetic field strength orthogonal in space. 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 areas of the field:

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

In the far region, EMF has the following properties:

For air: .

In the regions of intermediate and near fields, in addition to the spherical transverse wave, there are local reactive fields, the intensity of which increases very quickly with decreasing r. These fields contain a certain supply 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 emission region, and the near EMF region is often called the induction region.

For real antennas, the boundaries of the far, intermediate and near field regions 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 an allowable phase difference in the far-field region equal to:

The far-field EMF region will be at ;

Intermediate field area;

Near field region where

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

- the maximum transverse size of the radiating antenna system.

Main characteristics and parameters of the transmitting antenna

Antenna properties are divided into:

    Radio engineering;

    Constructive;

    Operational;

    Economic;

Functional properties are entirely determined by signal parameters.

Characteristics and parameters of the transmitting antenna:

    Complex vector directional characteristic

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

In general, a complex XNA consists of three factors:

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

    Amplitude Henna

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

Normalized amplitude CNA is usually considered:

,

where is the direction in which the amplitude CNA value is maximum.

    Antenna radiation pattern (APP)

The antenna radiation pattern is a section of the amplitude XNA by planes passing through the direction or perpendicular to it.

The most commonly used section is by mutually orthogonal planes.

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

The width of the bottom lobe is the angle within which the amplitude of the lobe changes within the permissible specified limits.

Petals are:

    Main petal;

    Side petals;

    Back petal.

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;

    On a logarithmic scale = -3 dB.

The normalized amplitude CNA in terms of power is related to the amplitude CNA in the field by the relation:

To image the bottom, polar and rectangular coordinate systems and three types of scale are used:

    Linear (across the field);

    Quadratic (power);

    Logarithmic

Phase Henna

Phase XNA is a 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 point in time.

Reference:

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

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

    Polarizing HNA

An electromagnetic wave is characterized by polarization.

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

In the general case, the end of the vector E during 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;

    spatial orientation of the ellipse;

    direction of rotation of vector E.

    Antenna radiation resistance

The radiation resistance of an antenna is the wave resistance of the space surrounding the antenna, transferred by it to the input, or to any section of the waveguide feeding it, where the concept of total current has 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 the two-wire line feeding it, which is equivalent to the feeding hollow waveguide.

    Antenna input impedance

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

The antenna input impedance characterizes the antenna as a load for the supply line.

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

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

    Antenna efficiency (efficiency)

Determines the efficiency of transmission by the antenna to the surrounding space.

Loss resistance

Reference:

As f increases, the antenna efficiency increases from a few percent at long waves to 95-99% at microwave frequencies.

    Electrical strength and antenna height

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

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

    Antenna height

Antenna height is the ability of antennas to perform their functions without electrical breakdown of the surrounding atmosphere when the height of this antenna increases at a given transmit power.

Reference:

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

    Antenna operating frequency range

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 whose value, when the frequency changes, goes out of the permissible limits before others. Most often, this parameter turns out to be the input impedance of the antenna.

Quantitative estimates of the range properties of an antenna are the bandwidth and transmittance:

Often use relative bandwidth

Antennas are divided into:

    Directional coefficient (DC)

The directional coefficient of an antenna in a given direction is a number showing how many times the value of the Poynting vector in the direction under consideration at a fixed point in the far zone differs from the value of the Poynting vector at the same point if we replace the antenna in question with an absolutely omnidirectional (isotropic) antenna, subject to equality their radiated powers.

Reference:

Typically, the maximum antenna efficiency value is indicated in the direction of its maximum radiation.

Vibrator: KND=0.5;

Half-wave symmetrical vibrator: KND=1.64;

Horn antenna: KND;

Mirror antenna: KND;

Spacecraft antennas: KND;

The limiter for the upper limit of the efficiency factor is technological manufacturing errors and the influence of operating conditions.

The minimum values ​​of the maximum efficiency of real antennas are always >1, because There are no completely omnidirectional antennas.

The directivity factor is related in field to the normalized amplitude XNA:

,

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

KND show This is the gain in power that the use of a directional antenna provides, but does not take into account the thermal losses in it.

    Co. uh antenna gain

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

    Equivalent isotropically radiated power

Equivalent isotropically radiated power is the product of the power supplied to the antenna and the maximum value of its gain.

    Antenna dispersion factor

An antenna's dissipation factor is a number indicating the proportion of radiated power attributable to the side and back lobes.

Determines the power attributable to the main lobe of the XNA

    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 value of field strength as the antenna in question with the same value of current at the input.

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

LECTURE 9.


  • ^ Isotropic emitter

  • Symmetrical vibrator

  • Main characteristics of antennas. Amplitude characteristic of antenna directivity

  • Radiation resistance

  • Antenna characteristic impedance

  • Input impedance

  • Loss resistance
^

ISOTROPIC EMITTER.
An isotropic emitter means a device that emits uniformly and equally. electromagnetic energy in all directions.

However, in practice, omnidirectional 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 emitted.

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

SYMMETRICAL VIBRATOR.


It consists of two conductors of the same length, between which a power line is connected - a feeder, connecting the antenna to the transmitter.

At the most frequencies, a symmetrical vibrator with a length of l and half  is used, called a half-wave vibrator (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, a half-wave of current and voltage is established, along the vibrator the length of the wave - a wave of current and voltage, Fig. 37b. However, in any case, a current node and a voltage antinode are installed at the ends
^

MAIN CHARACTERISTICS OF ANTENNAS.

AMPLITUDE CHARACTERISTICS OF ANTENNA DIRECTIVENESS.

The directional properties of antennas are usually determined by the amplitude directivity characteristic, i.e. dependence of the intensity of the field emitted by the antenna E (,) at the observation point at a constant distance. A graphical representation of the amplitude directional characteristic is called a radiation pattern, which is depicted as a surface described by a radius vector emanating from the origin, the length of which in each direction is proportional to the function F (, ) .

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

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

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

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

The directivity coefficient of directivity characterizes the ability of the antenna to concentrate the radiated electromagnetic field in any direction. It represents the ratio of the power flux density emitted by an antenna in a given direction to the power flux density averaged over all directions. In other words, when determining the efficiency, the antenna is compared with an imaginary, completely omnidirectional or isotropic antenna emitting the same power as the one under consideration.

For aperture antennas

TO nd = 4 TO isp S a /  2 ,

Where: TO isp – coefficient of utilization of the radiating surface of the instrumentation;

S a is the antenna opening area.

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

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

G=  a TO nd ,

Where: A = R /R 0 - antenna efficiency;

R  - power radiated by the antenna;

R 0 – power supplied to the antenna.

The antenna gain shows how many times the power supplied to the antenna should be reduced compared to the power supplied to an isotropic emitter with an efficiency of 1 so that the field strength at the receiving point remains unchanged.

In the range of decimeter and centimeter waves  a 1 , That's why

G = K n.d.

The protective action coefficient KZD is introduced to characterize the degree of attenuation by the antenna of signals received from side directions and is calculated by the formula TO building = G swing /G pob, where G swing and G ab – antenna gains in the direction of the main lobe of the radiation pattern and in the secondary direction.
^

RADIATION RESISTANCE.


Antenna radiation resistance R izl - an indicator that has the dimension of resistance and connects the emitted power P irs with current I A, flowing through any section of the antenna

R izl = R izl / 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 related to the square of the maximum current amplitude (at the antinode) or to the quadrature of the current at the antenna input terminals.

Magnitude R The radius depends on the relationship between the dimensions of the antenna and the wavelength, the shape of the antenna and other factors.

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

In general R Ill is complex in nature.

For example, for a thin half-wave vibrator R izl = 73,1 Om, ah X izl = 42,5 Ohm.

An increase in the thickness of the vibrator leads to a decrease in the magnitude of the wave resistance.
^

WAVE RESISTANCE OF AN ANTENNA.


Antenna characteristic impedance Z OA is one of the important parameters. Wave resistance is considered using the methods of long line theory.

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

,

Where: r n is the radius of the conductor.

Increasing the thickness of the conductor leads to a decrease in wave resistance.
^

INPUT RESISTANCE.


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

Z Avx = R Avx + iX Avx

Where: R Авх – active component of the input resistance;

X Avh is the reactive component of the input resistance.
^

LOSS RESISTANCE.


Loss resistance is defined as:

R n = R n + R And + R 3 ,

Where: R n - resistance of losses due to heating of wires;

R and - loss resistance in the antenna insulators;

R 3 - loss resistance in the ground and in grounding systems.

The high frequency measurement bridge is a conventional Wheatstone bridge and can be used to determine the degree of matching of the antenna to the transmission line. This scheme is known by many names (for example, "antennascope", etc.), but it is always based on circuit diagram, shown in Fig. 14-15.

The bridge circuit carries high frequency currents, so all resistors used in it must be purely active resistance for the excitation frequency. Resistors R 1 and R 2 are selected exactly equal to each other (with an accuracy of 1% or even more), and the resistance itself does not matter much. Under the assumptions made, the measuring bridge is in equilibrium (zero reading of the measuring device) with the following relationships between the resistors: R 1 = R 2 ; R 1: R 2 =1:1; R 3 = = R 4 ; R3:R4 = 1:1.

If, instead of resistor R 4, a test sample is included, the resistance of which needs to be determined, and a calibrated variable resistance is used as R 3, then the zero reading of the bridge unbalance meter will be achieved at a variable resistance value equal to active resistance tested sample. In this way, the radiation resistance or input impedance of the antenna can be directly measured. It should be remembered that the antenna input impedance is purely active only when the antenna is tuned, so the measurement frequency must always correspond to the resonant frequency of the antenna. In addition, the bridge circuit can be used to measure the characteristic impedance of transmission lines and their shortening factors.

In Fig. 14-16 shows a diagram of a high-frequency measuring bridge designed for antenna measurements, proposed by the American radio amateur W 2AEF (the so-called “antennascope”).

Resistors R1 and R2 are usually chosen equal to 150-250 ohms, and their absolute value does not play a special role, it is only important that the resistance of resistors R1 and R2, as well as the capacitances of capacitors C1 and C2, are equal to each other. As a variable resistance, only non-inductive volumetric variable resistors should be used and in no case wirewound potentiometers. The variable resistance is usually 500 ohms, and if the measuring bridge is used for measurements only on transmission lines made of coaxial cables, then 100 ohms, which allows more accurate measurements. The variable resistance is calibrated, and when the bridge is balanced, it should be equal to the resistance of the test sample (antenna, transmission line). The additional resistance R Ш depends on the internal resistance of the measuring device and the required sensitivity of the measuring circuit. Magnetoelectric milliammeters with a scale of 0.2 can be used as a measuring device; 0.1 or 0.05 ma. The additional resistance should be selected as high-resistance as possible, so that connecting the measuring device does not cause a significant imbalance of the bridge. Any germanium diode can be used as a rectifying element.

Bridge circuit conductors should be kept as short as possible to reduce their own inductance and capacitance; When designing a device, symmetry in the arrangement of its parts should be observed. The device is enclosed in a casing divided into three separate compartments, in which, as shown in Fig. 14-16, individual elements of the device circuit are placed. One of the points of the bridge is grounded, and therefore the bridge is asymmetrical with respect to the ground. Therefore, the bridge is most suitable for measurements on unbalanced (coaxial) transmission lines. If it is necessary to use the bridge for measurements on balanced transmission lines and antennas, it must be carefully isolated from the ground using an insulating stand. The antennoscope can be used both in the range of short and ultrashort waves, and the limit of its applicability in the VHF range mainly depends on the design and individual circuit elements of the device.

It is quite sufficient to use a heterodyne resonance meter as a measuring generator that excites the measuring bridge. It should be borne in mind that the high-frequency power supplied to the measuring bridge should not exceed 1 W, and a power of 0.2 W is quite sufficient for normal operation of the measuring bridge. The input of high-frequency energy is carried out using a coupling coil having 1-3 turns, the degree of coupling of which with the coil of the heterodyne resonance meter circuit is adjusted so that when the test sample is turned off, the measuring device gives a full deviation. It should be taken into account that if the coupling is too strong, the frequency calibration of the heterodyne resonance meter is slightly shifted. To avoid errors, it is recommended to listen to the tone of the measuring frequency using a precisely calibrated receiver.

The functionality of the measuring bridge is checked by connecting a non-inductive resistor having a precisely known resistance to the measuring socket. The variable resistance at which the measuring circuit is balanced must be exactly equal (if the measuring bridge is properly designed) to the resistance being tested. The same operation is repeated for several resistances at different measuring frequencies. In this case, the frequency range of the device is determined. Due to the fact that the circuit elements of the measuring bridge in the VHF range are already complex, the balance of the bridge becomes inaccurate, and if in the 2 m range it can still be achieved by carefully constructing the bridge, then in the 70 cm range the considered measuring bridge is completely inapplicable.

After checking the functionality of the measuring bridge, it can be used for practical measurements.

In Fig. 14-17 show the antenna design proposed by W 2AEF.

Determining Antenna Input Impedance

The measuring socket of the measuring bridge is directly connected to the antenna power terminals. If the resonant frequency of the antenna was previously measured using a heterodyne resonance meter, then the bridge is powered by a high-frequency voltage of this frequency. By changing the variable resistance, they achieve a zero reading on the measuring device; in this case, the read resistance is equal to the input resistance of the antenna. If the resonant frequency of the antenna is not known in advance, then the frequency feeding the measuring bridge is changed until an unambiguous balance of the measuring bridge is obtained. In this case, the frequency indicated on the scale of the measuring generator is equal to the resonant frequency of the antenna, and the resistance obtained on the scale of variable resistance is equal to the input impedance of the antenna. By changing the parameters of the matching circuit, it is possible (without changing the excitation frequency of the high-frequency measuring bridge) to obtain the specified input impedance of the antenna, monitoring it with an antenoscope.

If it is inconvenient to measure directly at the antenna feed points, then between the measuring bridge you can connect a line having an electrical length R/2 or a length multiple of this length (2 λ/2, 3 λ/2, 4 λ/ 2, etc.) and having any characteristic impedance. As is known, such a line transforms the resistance connected to its input in a ratio of 1: 1, and therefore its inclusion does not affect the accuracy of measuring the input resistance of the antenna using a high-frequency measuring bridge.

Determination of the shortening factor of a high-frequency transmission line

The exact length λ/2 of the line segment can also be determined using an antennascope.

A sufficiently long, freely suspended section of line is short-circuited at one end and connected to the measuring socket of the bridge at the other end. The variable resistance is set to zero. Then slowly change the frequency of the heterodyne resonance meter, starting at low frequencies and moving to higher frequencies, until the balance of the bridge is achieved. For this frequency the electrical length is exactly λ/2. After this, it is easy to determine the line shortening factor. For example, for a piece of coaxial cable 3.30 m long at a measurement frequency of 30 MHz (10 m), the first bridge balance is achieved; hence λ/2 is equal to 5.00 m. We determine the shortening coefficient: $$k=\frac(geometric length)(electrical length)=\frac(3.30)(5.00)=0.66.$$

Since the balance of the bridge occurs not only with an electrical line length equal to λ/2, but also with lengths that are multiples of it, the second balance of the bridge should be found, which should be at a frequency of 60 MHz. The line length for this frequency is 1λ. It is useful to remember that the shortening factor of coaxial cables is approximately 0.65, ribbon cables are 0.82, and two-wire air insulated lines are approximately 0.95. Since measuring the shortening factor using an antennascope is not difficult, all transformer circuits should be designed using the method for measuring the shortening factor described above.

The antenna scope can also be used to check the dimensional accuracy of the λ/2 line. To do this, a resistor with a resistance of less than 500 ohms is connected to one end of the line, and the other end of the line is connected to the measuring socket of the bridge; in this case, the variable resistance (in case the line has an electrical length exactly equal to λ/2) is equal to the resistance connected to the other end of the line.

Using an antennascope, the exact electrical length λ/4 of the line can also be determined. To do this, the free end of the line is not closed, and by changing the frequency of the heterodyne resonance meter in the same way as described above, the most low frequency, at which (at zero position of the variable resistance) the first balance of the bridge circuit is achieved. For this frequency the electrical line length is exactly λ/4. After this, the transforming properties of the λ/4 line can be determined and its characteristic impedance can be calculated. For example, a resistor with a resistance of 100 ohms is connected to the end of a quarter-wave line. By changing the variable resistance, the bridge is balanced with a resistance of Z M = 36 ohms. After substituting into the formula $Z_(tr)=\sqrt(Z_(M)\cdot(Z))$ we get: $Z_(tr)=\sqrt(36\cdot(100))=\sqrt(3600)=60 om$. Thus, as we have seen, the antennascope, despite its simplicity, allows you to solve almost all problems associated with matching the transmission line with the antenna.

Antenna- a device that converts oscillations of electric current into an electromagnetic field wave (radio wave) and vice versa.

Antennas are reversible devices, that is, just as an antenna works for transmission, it will also work for reception; if it works effectively for reception, it will also work well for transmission.

Feeder- cable connecting the radio station to the antenna.
Cables come in different impedances and designs.
Since in civilian radio stations the output/input impedance is 50 Ohms and the output is unbalanced, coaxial cables with a characteristic impedance of 50 Ohms are suitable for us as a feeder, for example: RK 50-3-18 or RG 8 or RG 58.
There is no need to confuse wave impedance and ohmic impedance. If you measure the cable resistance with a tester, the tester will show 1 ohm, although the wave impedance of this cable may be 75 ohms.
The characteristic impedance of a coaxial cable depends on the ratio of the diameters of the inner conductor and the outer conductor (a cable with a characteristic impedance of 50 Ohms has a thicker central core than a 75 Ohm cable of the same external diameter).

SWR- standing wave coefficient, that is, the ratio of the power that goes along the cable to the antenna and the power that returns along the cable, reflecting from the antenna due to the fact that its resistance is not equal to the cable resistance.
Yes, high-frequency voltage does not travel through wires like direct current; it can be reflected from the load if the load or cable is of the wrong characteristic impedance.
SWR shows the quality of energy transmission from the radio station to the antenna and back; the lower the SWR, the better the match between the radio station and the feeder and antenna. SWR cannot be less than 1.
SWR does not indicate the efficiency of the antenna and at what frequency it operates more efficiently. For example, the SWR will be 1 if a 50 Ohm resistor is connected to the end of the cable, but no one will hear you at the resistor and you will not hear anyone at it.

How does the antenna work?

Alternating current, as is known, changes its polarity with a certain frequency. If we are talking about 27 MHz, then 27 million times per second its polarity (+/-) changes places. Accordingly, 27 million times per second, electrons in the cable run from left to right, then from right to left. Considering that electrons run at the speed of light 300 million meters per second, then for a frequency of 27 megahertz they only manage to run 11 meters (300/27) before the current polarity changes, and then return back.
Wavelength is the distance that electrons travel before they are pulled back by the changing polarity of the source.
If we connect a piece of wire to the output of the radio station, the other end of which is simply hanging in the air, then electrons will run in it, the running electrons create a magnetic field around the conductor, and at its end an electrostatic potential, which will change with the frequency at which the radio station operates , that is, the wire will create a radio wave.
The minimum distance that electrons must travel to effectively convert alternating current into a radio wave and radio waves into current is 1/2 the wavelength.
Since any current (voltage) source has two terminals, the minimum effective antenna consists of two pieces of wire 1/4 wavelength long (1/2 divided by 2), with one piece of wire connected to one terminal of the source (output radio station), another in to another output.
One of the conductors is called radiating and is connected to the central core of the cable, the other is a “counterweight” and is connected to the cable braid.
* If you place 2 pieces of wire each 1/4 wavelength long, one above the other, the resistance of such an antenna will be approximately 75 Ohms, in addition, it will be symmetrical, that is, connecting it directly with a coaxial (not symmetrical) cable is not a good idea.

Wait, how do shortened antennas work then (for example, 2 meters at 27 MHz) and antennas consisting only of a pin on a car?
For a pin on a car, the pin is the first piece of wire (the “emitter”), and the body of the car is the second wire (the “counterweight”).
In shortened antennas, part of the wire is twisted into a coil, that is, for electrons the length of the pin is equal to 1/4 of the wavelength (2 meters 75 cm at 27 MHz), and for the owner of the pin it is only 2 meters, the rest is in the coil, which is hidden from the weather at the base of the antenna .

What happens if you connect very short or very long wires to a radio station as an antenna?
As mentioned above, the wave impedance of the radio station’s output/input is 50 Ohms; accordingly, the antenna, which is a load for it, must also have a resistance of 50 Ohms.
Wires shorter or longer than 1/4 wavelength will have a different characteristic impedance. If the wires are shorter, then the electrons will have time to reach the end of the wire and want to run further before they are pulled back, accordingly they will bury themselves at the end of the wire, they will understand that there is a break there, that is, there is a large, infinite resistance and the resistance of the entire antenna will be greater, the more the shorter the wire. A wire that is too long will also not work correctly, its resistance will also be higher than necessary.
It is impossible to make an electrically short antenna effective; it will always lose 1/4 of the electrical length; an electrically long antenna requires resistance matching.
* The difference between “electrically short” and “physically short” is that you can twist a wire of sufficient length into a coil, but physically the coil will not be so long. Such an antenna will be quite effective, but on a small number of channels and in any case will lose to a pin 1/4 wavelength long.
It is also important to understand that quite a lot also depends on the angle at which the antenna conductors, the emitter and the counterweight are located to each other - its directivity (the direction of its radiation) and its wave impedance.

There is also such a phenomenon as the antenna shortening coefficient, this phenomenon is due to the fact that the conductors are thick, and the end of the conductor has a capacitance to the surrounding space. The thicker the antenna conductor and the higher the frequency at which the antenna must operate, the greater the shortening. Also, the thicker the conductor from which the antenna is made, the more broadband it is (the more channels it covers).

Directional antennas and radiation polarization

Antennas are:
+ With horizontal polarization - the antenna conductors are located horizontally;
+ With vertical polarization - the conductors are arranged vertically.
If you try to receive signals transmitted by an antenna with horizontal polarization on an antenna with vertical polarization, there will be a loss of 2 times (3 dB) compared to reception on an antenna of the same polarization as the transmitting one.

In addition, antennas can be:
+ Directional - when the emission and reception of waves goes in one or more directions.
+ Non-directional (with a circular radiation pattern) - when radio waves are emitted and received evenly from all directions.

Example: a vertical pin has a circular radiation pattern in the horizontal plane, that is, it equally emits and receives radio waves from sources around it.

What is antenna gain?

If we are talking specifically about antenna amplification, and not about an amplifier connected to the antenna and requiring power wires, then antenna amplification is its ability to concentrate radio waves in a certain plane or direction, to where the correspondents desired for communication are located.
For example, two vertically located pins of 1/4 wavelength (vertical dipole) radiate evenly in a circle, but this is if you look at it from above, and if from the side, it turns out that part of the energy is radiated into the ground, and part into space. The dipole gain is 0 dBd. There are no useful signals for us in the ground and in space, accordingly, by changing the configuration of the dipole (by lengthening one part of it to 5/8 of the wavelength), it is possible to ensure that the radiation is concentrated in the horizon, and little radiation will be emitted into space and into the ground, the gain of such an antenna will be approximately 6 dBd.

If you are interested in learning in detail how antennas and feeders work, see complete formulas, read the book: K. Rothhammel Antennas.

Let's remember the main thing:

Wavelength = 300 / communication channel frequency

Minimum effective antenna length = wavelength / 2

The thicker the conductors from which the antenna is made, the greater the contribution the shortening factor makes to its length.

SWR indicates the quality of energy transmission from the radio to the antenna, but does not indicate the efficiency of the antenna.

Now for examples:
300 / 27.175 = 11 meters 3 centimeters wavelength.
The entire antenna for effective operation must have a length of 5 meters 51 centimeters, respectively, the pin will have a length of 2 meters 76 centimeters.
Taking into account K_shortening, for a pin made from a tube with a diameter of 20 mm, the length of the pin will be approximately 2 meters 65 centimeters.

What antennas are usually used on the civil band?

Antenna 1/4 GP ("gepeshka" or "quadruple")

A pin on a mortise or magnetic base, inside of which an extension coil is installed, adding up to 1/4 of its electrical length. The counterweight is the car body, which is connected either directly (for embedded antennas) or through the capacitor capacitance formed by the magnet base and the surface of the body.

On high-frequency bands, such as LPD and PMR, gaps or 5/8 are usually used, even in a car and in a wearable version; in the basic version, collinear antennas are used (antenna systems of several 1/2 or 5/8 antennas electrically and mechanically interconnected , which makes it possible to achieve a K_gain of the antenna of 10 dbi or more, that is, to compress the radiation into a thin horizontal pancake).

Did you like the article? Share with friends:
You may also be interested