Suppression of side lobes of drl and prl diagrams. Phase radiation pattern. The concept of the phase center of the antenna Requirements for electrical parameters

The antenna, regardless of its design, has the property of reversibility (it can work both for reception and emission). Often in radio relay paths the same antenna can be connected simultaneously to the receiver and transmitter. This allows a signal to be emitted and received in the same direction at different frequencies.

Almost all parameters of the receiving antenna correspond to the parameters of the transmitting antenna, but sometimes have a slightly different physical meaning.

Despite the fact that the receiving and transmitting antennas have the principle of duality, in terms of design they can differ significantly. This is due to the fact that the transmitting antenna must pass significant powers through itself in order to transmit an electromagnetic signal over large (maximum possible) distances. If the antenna works for reception, then it interacts with fields of very low intensity. The type of current-transmitting antenna structure often determines its final dimensions.

Perhaps the main characteristic of any antenna is its radiation pattern. It implies many auxiliary parameters and such important energy characteristics as gain and directional coefficient.

Directional pattern

Radiation pattern (DP) is the dependence of the field strength created by the antenna for a sufficiently long distance, from observation angles in space. In volume, the directional antenna diagram may look as shown in Figure 1.

Picture 1

What is shown in the figure above is also called the spatial pattern, which is the surface of the volume and can have several maxima. The main maximum, highlighted in red in the figure, is called the main lobe of the diagram and corresponds to the direction of the main radiation (or reception). Accordingly, the first minimum or (less often) zero values of the field strength around the main lobe determine its boundary. All other maximum field values are called side lobes.

In practice, there are various antennas that may have several directions of maximum radiation, or may not have side lobes at all.

For convenience of depiction (and technical application) of DPs, they are usually considered in two perpendicular planes. As a rule, these are the planes of the electric vector E and the magnetic vector H (which are perpendicular to each other in most environments), Figure 2.

Figure 2

In some cases, patterns are considered in the vertical and horizontal planes relative to the plane of the Earth. Planar diagrams are depicted using polar or Cartesian (rectangular) coordinate systems. In polar coordinates, the diagram is more visual, and when overlaid on a map, you can get an idea of the coverage area of the radio station’s antenna, Figure 3.

Figure 3

Representation of the radiation pattern in a rectangular coordinate system is more convenient for engineering calculations; such a construction is more often used to study the structure of the pattern itself. For this purpose, the diagrams are built normalized, with the main maximum reduced to unity. The figure below shows a typical normalized radiation pattern of a mirror antenna.

Figure 4

In the case where the intensity of lateral radiation is quite small and it is difficult to measure lateral radiation on a linear scale, a logarithmic scale is used. As you know, decibels make small values large and large values small, so the same diagram on a logarithmic scale looks like the one below:

Figure 5

From the radiation pattern alone you can extract a fairly large number of characteristics that are important for practice. Let's take a closer look at the diagram shown above.

One of the most important parameters is the width of the main lobe at zero radiation θ 0 and the width of the main lobe at half power θ 0.5. Half the power corresponds to the 3 dB level, or 0.707 field strength level.

Figure 6

From Figure 6 it can be seen that the width of the main lobe at zero radiation is θ 0 = 5.18 degrees, and the width at half power level is θ 0.5 = 2.15 degrees.

The diagrams are also evaluated by the intensity of the lateral and backward radiation (the power of the side and rear lobes), from which two more follow: important parameters antennas - this is the protective coefficient and the level of side lobes.

The protective action factor is the ratio of the field strength emitted by the antenna in the main direction to the field strength emitted in the opposite direction. If we consider the orientation of the main lobe of the diagram in the direction of 180 degrees, then the reverse one is 0 degrees. Any other radiation directions are possible. Let us find the protective action coefficient of the diagram under consideration. For clarity, let’s depict it in the polar coordinate system (Figure 7):

Figure 7

On the diagram, markers m1, m2 depict radiation levels in the reverse and forward directions, respectively. The protective coefficient is defined as:

In relative units. Same value in dB:

The side lobe level (SLL) is usually indicated in dB, thereby showing how weak the level of side radiation is compared to the level of the main lobe, Figure 8.

Figure 8

These are two important parameters of any antenna system, which directly follow from the definition of the radiation pattern. KND and KU are often confused with each other. Let's move on to consider them.

Directional coefficient

Directional coefficient (DC) is the ratio of the square of the field strength created in the main direction (E 0 2) to the average value of the square of the field strength in all directions (E cf 2). As is clear from the definition, the directivity characteristic characterizes the directional properties of the antenna. The efficiency does not take into account losses, since it is determined by the radiated power. From the above, you can specify the formula for calculating the efficiency factor:

D=E 0 2 /E avg 2

If the antenna works for reception, then the efficiency shows how many times the signal-to-noise ratio in terms of power will improve when replacing a directional antenna with an omnidirectional one, if interference comes evenly from all directions.

For a transmitting antenna, the directivity factor shows how many times the radiation power must be reduced if the omnidirectional antenna is replaced by a directional one, while maintaining the same field strengths in the main direction.

The efficiency of an absolutely omnidirectional antenna is obviously equal to unity. Physically, the spatial radiation pattern of such an antenna looks like an ideal sphere:

Figure 9

Such an antenna radiates equally well in all directions, but is not feasible in practice. So it's a kind of mathematical abstraction.

Gain

As mentioned above, the efficiency factor does not take into account losses in the antenna. The parameter that characterizes the directional properties of the antenna and takes into account losses in it is called gain.

Gain factor (GC) G is the ratio of the squared field strength created by the antenna in the main direction (E 0 2) to the average value of the squared field strength (E oe 2) created by the reference antenna, with equal powers supplied to the antennas. We also note that when determining the gain, the efficiency of the reference and measured antennas is taken into account.

The concept of a reference antenna is very important in understanding gain, and in different frequency ranges use different types reference antennas. In the long/medium wave range, a quarter-wave vertical monopole vibrator is taken as the standard (Figure 10).

Figure 10

For such a reference vibrator D e = 3.28, therefore the gain of a long-wave/medium-wave antenna is determined through the gain as follows: G = D * ŋ/3.28, where ŋ is the antenna efficiency.

In the short wave range, a symmetrical half-wave vibrator is taken as a reference antenna, for which De = 1.64, then the gain is:

G=D*ŋ/1.64

In the microwave range (and this is almost all modern Wi-Fi, LTE and other antennas), an isotropic emitter giving D e = 1 and having a spatial diagram shown in Figure 9 is taken as a reference emitter.

The gain is a determining parameter of transmitting antennas, since it shows how many times the power supplied to the directional antenna must be reduced compared to the reference one so that the field strength in the main direction remains unchanged.

KND and KU are mainly expressed in decibels: 10lgD, 10lgG.

Conclusion

Thus, we examined some of the field characteristics of the antenna, resulting from the radiation pattern and energy characteristics (DC and gain). The antenna gain is always less than the directional coefficient, since the gain takes into account losses in the antenna. Losses can arise due to the reflection of power back into the feed line of the feed, the flow of currents behind the walls (for example, a horn), shading of the diagram by structural parts of the antenna, etc. In real antenna systems, the difference between the gain and gain can be 1.5-2 dB.

Level of side lobes of the radiation pattern

Side lobe level (SLL) antenna radiation pattern (DP) - the relative (normalized to the maximum RP) level of antenna radiation in the direction of the side lobes. Typically, UBL is expressed in decibels.

An example of an antenna radiation pattern and parameters: width, directivity, UBL, backward radiation suppression coefficient

The pattern of a real (finite size) antenna is an oscillating function in which the direction of the main (maximum) radiation and the main lobe of the pattern corresponding to this direction are identified, as well as the directions of other local maximums of the pattern and the corresponding so-called side lobes of the pattern.

Usually, UBL is understood as the relative level of the largest side lobe of the pattern. For directional antennas, as a rule, the largest side lobe is the first (adjacent to the main) side lobe.

Also used average lateral radiation level(the pattern is averaged in the sector of lateral radiation angles), normalized to the maximum pattern.

As a rule, to assess the level of radiation in the “backward” direction (in the direction opposite to the main lobe of the pattern), a separate parameter is used, and this radiation is not taken into account when estimating the UBL.

Reasons for the decline in UBL

In the receiving mode, an antenna with a low UBL is “more noise-resistant”, since it better selects the desired signal space against the background of noise and interference, the sources of which are located in the directions of the side lobes
An antenna with a low UBL provides the system with greater electromagnetic compatibility with other radio electronics and high-frequency devices
An antenna with a low UBL provides the system with greater stealth
In the antenna of the automatic target tracking system, erroneous tracking by side lobes is possible
A decrease in the UBL (at a fixed width of the main lobe of the pattern) leads to an increase in the level of radiation in the direction of the main lobe of the pattern (to an increase in the directivity): antenna radiation in a direction other than the main one is a waste of energy. However, as a rule, with fixed dimensions of the antenna, a decrease in the UBL leads to a decrease in the coefficient of performance, an expansion of the main lobe of the pattern and a decrease in the efficiency.

The price to pay for a lower UBL is the expansion of the main lobe of the radiation pattern (with fixed antenna dimensions), as well as, as a rule, a more complex design of the distribution system and lower efficiency (in phased array).

Ways to reduce UBL

The main way to reduce the UBL when designing an antenna is to choose a smoother (declining towards the edges of the antenna) spatial distribution of the current amplitude. A measure of this “smoothness” is the surface utilization factor (SUF) of the antenna.

Reducing the level of individual side lobes is also possible by introducing emitters with a specially selected amplitude and phase of the exciting current - compensation emitters in the phased array, as well as by smoothly changing the length of the wall of the radiating aperture (in aperture antennas).

An uneven (different from linear law) spatial distribution of the current phase across the antenna (“phase errors”) leads to an increase in the UBL.

Main lobe width and side lobe level

The width of the pattern (main lobe) determines the degree of concentration of the emitted electromagnetic energy. DN width is the angle between two directions within the main lobe in which the amplitude of the electromagnetic field strength is 0.707 levels from the maximum value (or 0.5 levels from the maximum power density value). The width of the bottom line is indicated as follows:

2i is the width of the pattern in terms of power at the level of 0.5;

2i - width of the pattern in terms of tension at the level of 0.707.

The index E or H denotes the width of the pattern in the corresponding plane: 2i, 2i. A level of 0.5 in power corresponds to a level of 0.707 in field strength or a level of 3 dB on a logarithmic scale:

It is convenient to experimentally determine the width of the pattern using a graph, for example, as shown in Figure 11.

Figure 11

The level of the side lobes of the pattern determines the degree of spurious radiation of the electromagnetic field by the antenna. It affects the quality of electromagnetic compatibility with nearby radio-electronic systems.

The relative sidelobe level is the ratio of the field strength amplitude in the direction of the maximum of the first side lobe to the field strength amplitude in the direction of the maximum of the main lobe (Figure 12):

Figure 12

This level is expressed in absolute units, or in decibels:

Directional coefficient and gain of the transmitting antenna

Directional coefficient (DC) quantitatively characterizes the directional properties of a real antenna in comparison with a reference omnidirectional (isotropic) antenna with a spherical pattern:

KND is a number showing how many times the power flux density P (u, q) of a real (directional) antenna is greater than the power flux density P (u, q) of a reference (non-directional) antenna for the same direction and at the same distance, provided that the radiation powers of the antennas are the same:

Taking (25) into account, we can obtain:

The gain factor (GC) of an antenna is a parameter that takes into account not only the focusing properties of the antenna, but also its ability to convert one type of energy into another.

KU- this is a number showing how many times the power flux density P (u, c) of a real (directional) antenna is greater than the power flux density PE (u, c) of a reference (non-directional) antenna for the same direction and at the same distance, provided that the powers supplied to the antennas are the same.

The gain can be expressed in terms of efficiency:

where is the antenna efficiency. In practice, the antenna gain is used in the direction of maximum radiation.

Phase radiation pattern. The concept of the antenna phase center

Phase diagram focus is the dependence of the phase of the electromagnetic field emitted by the antenna on the angular coordinates.

Since in the far zone of the antenna the field vectors E and H are in phase, the phase pattern is equally related to the electrical and magnetic components of the EMF emitted by the antenna. The phase pattern is designated as follows: Ш = Ш (u, ц) at r = const.

If W (u, q) = const at r = const, then this means that the antenna forms the phase front of the wave in the form of a sphere. The center of this sphere, where the origin of the coordinate system is located, is called the phase center of the antenna (PCA). It should be noted that not all antennas have a phase center.

For antennas that have a phase center and a multi-lobe amplitude pattern with clear zeros between them, the field phase in adjacent lobes differs by p (180°). The relationship between the amplitude and phase radiation patterns of the same antenna is illustrated in Figure 13.

Figure 13 - Amplitude and phase patterns

The direction of propagation of electromagnetic waves and the position of its phase front at each point in space are mutually perpendicular.

The level of the back and side lobes of the voltage radiation pattern γυ is defined as the ratio of the EMF at the antenna terminals during reception - 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 usually indicated. The level of the back and side lobes can also be determined by power (γ P) by squaring the level of the back and side lobes by voltage. In the radiation pattern shown in Fig. 16, the back and side lobes have the same level, equal to 0.13 (13%) in EMF or 0.017 (1.7%) in power. Back and side lobes of directional receivers television antennas are usually in the range of 0.1....25 (voltage).

In the literature, when describing the directional properties of receiving television antennas, the level of the back and side lobes is often indicated, equal to the arithmetic mean of the levels of the lobes at the middle and extreme frequencies television channel. Let us assume that the level of the lobes (according to the EMF) of the antenna pattern of the 3rd channel (f = 76 ... 84 MHz) is: at frequencies 75 MHz - 0.18; 80 MHz - 0.1; 84 MHz - 0.23. The average level of the petals will be equal to (0.18+0.1+0.23)/3, i.e. 0.17. The noise immunity of an antenna can be characterized by the average level of the lobes only if in the frequency band of the television channel there are no sharp “spikes” in the level of the lobes that significantly exceed the average level.

An important note must be made regarding the noise immunity of a vertically polarized antenna. Let us turn to the radiation pattern shown in Fig. 16. In this diagram, typical of horizontally polarized antennas in the horizontal plane, the main lobe is separated from the back and side lobes by the directions of zero reception. Vertical polarization antennas (for example, “wave channel” antennas with vertical vibrators) do not have zero reception directions in the horizontal plane. Therefore, the back and side lobes in this case are not clearly defined and noise immunity is defined in practice as the ratio of the signal level received from the forward direction to the signal level received from the rear direction.

Gain. The more directional the antenna, i.e., the smaller the opening angle of the main lobe and the lower the level of the rear and side lobes of the radiation pattern, the greater the EMF at the antenna terminals.

Let's imagine that a symmetrical half-wave vibrator is placed at a certain point in the electromagnetic field, oriented towards maximum reception, that is, located so that its longitudinal axis is perpendicular to the direction of arrival of the radio wave. A certain voltage Ui develops at a matched load connected to the vibrator, depending on the field strength at the receiving point. Let's put it next! at the same point in the field, instead of a half-wave vibrator, an antenna with greater directivity oriented towards maximum reception, for example, an antenna of the “wave channel” type, the directional pattern of which is shown in Fig. 16. We will assume that this antenna has the same load as the half-wave vibrator, and is also matched with it. Since the “wave channel” antenna is more directional than a half-wave vibrator, the voltage across its load U2 will be greater. The voltage ratio U 2 /’Ui is the voltage gain Ki of a four-element antenna or, as it is otherwise called, the “field”.

Thus, the voltage or “field” gain of an antenna can be defined as the ratio of the voltage developed by the antenna at a matched load to the voltage developed at the same load by a half-wave vibrator matched to it. Both antennas are considered to be located at the same point in the electromagnetic field and oriented towards maximum reception. The concept of power gain Kp is also often used, which is equal to the square of the voltage gain (K P = Ki 2).

In determining the gain, two points must be emphasized. Firstly, in order for antennas of different designs to be compared with each other, each of them is compared with the same antenna - a half-wave vibrator, which is considered a reference antenna. Secondly, to obtain in practice a gain in voltage or power, determined by the gain, it is necessary to orient the antenna towards the maximum of the received signal, i.e. so that the maximum of the main lobe of the radiation pattern is oriented towards the arrival of the radio wave. The gain depends on the type and design of the antenna. Let us turn to an antenna of the “wave channel” type for clarification. The gain of this antenna increases with the number of directors. The four-element antenna (reflector, active vibrator and two directors) has a voltage gain of 2; seven-element (reflector, active vibrator and five directors) - 2.7. This means that if instead of half-wave

vibrator use a four-element antenna), then the voltage at the input of the television receiver will increase by 2 times (power by 4 times), and a seven-element antenna by 2.7 times (power by 7.3 times).

The value of the antenna gain is indicated in the literature either in relation to a half-wave vibrator, or in relation to the so-called isotropic emitter. An isotropic radiator is an imaginary antenna that completely lacks directional properties, and the spatial radiation pattern has the corresponding shape of a -sphere. Isotropic emitters do not exist in nature, and such an emitter is simply a convenient standard with which the directional properties of various antennas can be compared. The calculated voltage gain of the half-wave vibrator relative to the isotropic emitter is 1.28 (2.15 dB). Therefore, if the voltage gain of any antenna relative to an isotropic emitter is known, then divide it by 1.28. we obtain the gain of this antenna relative to the half-wave vibrator. When the gain relative to an isotropic driver is specified in decibels, then to determine the gain relative to a half-wave vibrator, subtract 2.15 dB. For example, the voltage gain of the antenna relative to an isotropic emitter is 2.5 (8 dB). Then the gain of the same antenna relative to the half-wave vibrator will be 2.5/1.28, i.e. 1.95^ and in decibels 8-2.15 = 5.85 dB.

Naturally, the real gain in signal level at the TV input, given by one or another antenna, does not depend on which reference antenna - half-wave vibrator or isotropic emitter - the gain is specified in relation to. In this book, gain values are given in relation to a half-wave vibrator.

In the literature, the directional properties of antennas are often assessed by the directivity coefficient, which represents the gain in signal power in the load, provided that the antenna has no losses. The directional coefficient is related to the power gain Kr by the relation

If you measure the voltage at the receiver input, you can use the same formula to determine the field strength at the receiving location.

The width of the pattern (main lobe) determines the degree of concentration of the emitted electromagnetic energy.

The width of the pattern is the angle between two directions and within the main lobe, in which the amplitude of the electromagnetic field strength is a level of 0.707 from the maximum value (or a level of 0.5 from the maximum power density value).

The width of the pattern is designated as follows: 2θ 0.5 is the width of the pattern in terms of power at the level of 0.5; 2θ 0.707 - width of the pattern according to the intensity at the level of 0.707.

The index E or H shown above means the width of the pattern in the corresponding plane: , . A level of 0.5 in power corresponds to a level of 0.707 in field strength or a level of 3 dB on a logarithmic scale:

The beam width of the same antenna, represented by field strength, power or logarithmic scale and measured at the corresponding levels, will be the same:

Experimentally, the width of the pattern can be easily found from the graph of the pattern depicted in one or another coordinate system, for example, as shown in the figure.

The level of the side lobes of the pattern determines the degree of spurious radiation of the electromagnetic field by the antenna. It affects the secrecy of the operation of a radio-technical device and the quality of electromagnetic compatibility with nearby radio-electronic systems.

Relative sidelobe level is the ratio of the field strength amplitude in the direction of the side lobe maximum to the field strength amplitude in the direction of the main lobe maximum:

In practice, this level is expressed in absolute units, or in decibels. The level of the first side lobe is of greatest interest. Sometimes they operate with the average level of side lobes.

4. Directional coefficient and gain of the transmitting antenna.

The directional coefficient quantitatively characterizes the directional properties of real antennas in comparison with a reference antenna, which is a completely omnidirectional (isotropic) emitter with a spherical pattern:

The efficiency factor is a number showing how many times the power flux density P(θ,φ) of a real (directional) antenna is greater than the power flux density

PE (θ,φ) of the reference (omnidirectional) antenna for the same direction and at the same distance, provided that the radiation powers of the antennas are the same:

Taking into account (1) we can obtain:

where D 0 is the directivity in the direction of maximum radiation.

In practice, when talking about antenna efficiency, we mean a value that is completely determined by the antenna radiation pattern:

In engineering calculations, an approximate empirical formula is used that relates the directivity factor to the width of the antenna pattern in the main planes:

Since in practice it is difficult to determine the radiation power of an antenna (and even more so to fulfill the condition of equality of the radiation powers of the reference and real antennas), the concept of antenna gain is introduced, which takes into account not only the focusing properties of the antenna, but also its ability to convert one type of energy into another .

This is expressed in the fact that in a definition similar to the efficiency factor, the condition changes, and it is obvious that the efficiency of the reference antenna is equal to unity:

where P A is the power supplied to the antenna.

Then the directional coefficient is expressed in terms of the directional coefficient as follows:

where η A is the antenna efficiency.

In practice, G 0 is used - the antenna gain in the direction of maximum radiation.

5. Phase radiation pattern. The concept of the phase center of the antenna.

The phase radiation pattern is the dependence of the phase of the electromagnetic field emitted by the antenna on the angular coordinates. Since in the far zone of the antenna the field vectors E and H are in phase, the phase pattern is equally related to the electrical and magnetic components of the EMF emitted by the antenna. FDN is designated as follows:

Ψ = Ψ (θ,φ) for r = const.

If Ψ (θ,φ) at r = const, then this means that the antenna forms the phase front of the wave in the form of a sphere. The center of this sphere, where the origin of the coordinate system is located, is called the phase center of the antenna (PCA). Not all antennas have a phase center.

For antennas that have a phase center and a multi-lobe amplitude pattern with clear zeros between them, the field phase in adjacent lobes differs by (180 0). The relationship between the amplitude and phase radiation patterns of the same antenna is illustrated by the following figure.

Since the direction of propagation of electromagnetic waves and the position of its phase front are mutually perpendicular at each point in space, by measuring the position of the phase front of the wave, it is possible to indirectly determine the direction to the radiation source (direction finding by phase methods).