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What determines the level of the side lobes. Phase pattern

The level of the rear and side lobes of the voltage pattern γυ is defined as the ratio of the EMF at the antenna terminals when receiving - from the side of the maximum rear or side lobe to the EMF from the side of the maximum of the main lobe. When the 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 from the power (γ P) by squaring the level of the back and side lobes from the 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. Rear and side lobes of directional receivers television antennas are usually in the range of 0.1 ... .25 (in voltage).

In the literature, when describing the directional properties of receiving television antennas, the level of the back and side lobes is often indicated, which is equal to the arithmetic mean of the levels of the lobes at the middle and extreme frequencies of the television channel. Let's assume that the level of petals (according to EMF) of the antenna pattern of the 3rd channel (f = 76 ... 84 MHz) is: at frequencies of 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 an average lobe level only if there are no sharp “outliers” of the lobe level in the frequency band of the television channel that are significantly higher than the average level.

It is necessary to make an important remark regarding the noise immunity of a vertically polarized antenna. Let us turn to the radiation pattern shown in Fig. 16. In this diagram, which is characteristic of horizontally polarized antennas in the horizontal plane, the main lobe is separated from the back and side lobes by the zero reception directions. Antennas of vertical polarization (for example, "wave channel" antennas with a vertical arrangement of vibrators) do not have zero reception directions in the horizontal plane. Therefore, the back and side lobes in this case are not uniquely defined and the noise immunity is determined 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.

Imagine that a symmetrical half-wave vibrator is placed at a certain point of the electromagnetic field, oriented to the maximum reception, i.e., located so that its longitudinal axis is perpendicular to the direction of arrival of the radio wave. At the matched load connected to the vibrator, a certain voltage Ui develops, depending on the field strength at the receiving point. Let's put it next! to the same point of the field, instead of a half-wave vibrator, an antenna oriented towards the reception maximum with a greater directivity, for example, an antenna of the “wave channel” type, the radiation 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 consistent with it. Since the "wave channel" antenna is more directional than the half-wave vibrator, the voltage at its load U2 will be greater. The voltage ratio U 2 / 'Ui is the voltage gain Ki of the four-element antenna, or, as it is otherwise called, the "field".

Thus, the antenna voltage gain or “field” gain can be defined as the ratio of the voltage developed by the antenna on a matched load to the voltage developed on the same load by a half-wave vibrator matched to it. Both antennas are considered to be located at the same point of the electromagnetic field and focused on 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. First, 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, in order to obtain in practice a gain in voltage or power, determined by the gain, it is necessary to orient the antenna to 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. For clarification, let us turn to the antenna of the “wave channel” type. The gain of this antenna increases with the number of directors. A 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 a half-wave

vibrator to 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 the 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 radiator. An isotropic radiator is such an imaginary antenna that completely lacks directional properties, and the spatial radiation pattern accordingly * has the form of a -sphere. In nature, isotropic radiators do not exist, and such a radiator is just a convenient standard against which to compare the directional properties of various antennas. The calculated value of the voltage gain of the half-wave vibrator relative to the isotropic radiator is 1.28 (2.15 dB). Therefore, if the voltage gain of any antenna relative to an isotropic radiator is known, then dividing it by 1.28. we obtain the gain of this antenna relative to the half-wave vibrator. When the gain relative to an isotropic radiator is specified in decibels, then 2.15 dB must be subtracted to determine the gain relative to a half-wave vibrator. For example, the voltage gain of an antenna relative to an isotropic radiator 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 radiator - the gain is indicated in relation to. In this book, the values of the gain are given in relation to the half-wave vibrator.

In the literature, the directional properties of antennas often measure the directivity factor of the SOI, which is the gain in signal power at the load, provided that the antenna has no loss. The directional factor is related to the power gain Kp by the relation

If you measure the voltage at the input of the receiver, then you can use the same formula to determine the field strength at the place of reception.

Reducing the level of side lobes of reflector antennas by positioning metal strips in the aperture

Akiki D, Biaineh V., Nassar E., Harmoush A,

University of Notre Dame, Tripoli, Lebanon

Introduction

In a world of increasing mobility, there is an increasing need for people to interact and access information, regardless of the location of the information or the individual. From these considerations, it is impossible to deny that telecommunications, namely, the transmission of signals over a distance, is an urgent need. The requirement for wireless communication systems to be perfect and ubiquitous means that ever more efficient systems need to be developed. When improving a system, the main initial step is to improve the antennas, which are the main element of current and future systems. wireless communication. At this stage, by improving the quality of the antenna parameters we will understand the decrease in the level of its side lobes of its radiation pattern. Lowering the level of the side lobes, of course, should not affect the main lobe of the diagram. Reducing the sidelobe level is desirable because for antennas used as receiving antennas, sidelobes make the system more vulnerable to spurious signals. In transmitting antennas, side lobes reduce the security of information, since the signal can be received by an unwanted receiving side. The main difficulty is that the higher the level of side lobes, the higher the probability of interference in the direction of the side lobe with the highest level. In addition, an increase in the level of the side lobes means that signal power is wasted uselessly. Much research has been done (see, for example, ), but the purpose of this article is to review the "strip positioning" method, which has proven to be simple, efficient, and low cost. Any parabolic antenna

can be designed or even modified using this method (Fig. 1) to reduce interference between antennas.

However, the conductive strips must be very precisely positioned in order to achieve side-lobe reduction. In this article, the "strip positioning" method is tested by experiment.

Task description

The task is formulated as follows. For a particular parabolic antenna (Fig. 1), it is required to reduce the level of the first side lobe. The antenna pattern is nothing but the Fourier transform of the antenna aperture excitation function.

On fig. 2 shows two diagrams of a parabolic antenna - without stripes (solid line) and with stripes (line depicted by *), illustrating the fact that when using stripes, the level of the first side lobe decreases, however, the level of the main lobe also decreases, and the level also changes the rest of the petals. This shows that the position of the stripes is very critical. It is necessary to arrange the strips in such a way that the main lobe width at half power or the antenna gain does not change noticeably. The level of the rear lobe should also not change noticeably. The increase in the level of the remaining lobes is not so significant, since the level of these lobes is usually much easier to reduce than the level of the first side lobes. However, this increase should be moderate. Let us also remember that Fig. 2 is illustrative.

For the reasons stated, when using the "strip positioning" method, the following must be borne in mind: the strips must be metallic in order to fully reflect the electric field. In this case, the position of the strips can be clearly defined. Currently for measuring the level of side lobes

Rice. 2. Antenna pattern without bands (solid)

and with stripes

Rice. 3. Theoretical normalized radiation pattern in dB

two methods are used - theoretical and experimental. Both methods complement each other, but since our proofs are based on a comparison of the experimental diagrams of antennas without breaks and with stripes, in this case we will use the experimental method.

A. Theoretical method. This method consists of:

Finding the theoretical radiation pattern (DN) of the antenna under test,

Measurements of the side lobes of this pattern.

The RP can be taken from the technical documentation of the antenna, or can be calculated, for example, using the Ma1!ab program or using any other suitable program using known field relations.

The R2P-23-YXA reflective parabolic antenna was used as the antenna under test. The theoretical value of the RP was obtained using the formula for a uniformly driven circular aperture:

]ka2E0e ikg Jl (ka 8Іpv)

Measurements and calculations were performed in the E-plane. On fig. 3 shows the normalized radiation pattern in the polar coordinate system.

B. Experimental method. Two antennas must be used in the experimental method:

The receiving antenna under test,

transmitting antenna.

The antenna pattern under test is determined by rotating it and fixing the field level with the required accuracy. To improve accuracy, it is preferable to perform readings in decibels.

B. Adjustment of the level of the side lobes. By definition, the first side lobes are those closest to the main lobe. To fix their position, it is necessary to measure the angle in degrees or radians between the direction of the main radiation and the direction of maximum radiation of the first left or right lobe. The directions of the left and right side lobes should be the same due to the symmetry of the pattern, but this may not be the case in the experimental pattern. Next, it is also necessary to determine the width of the side lobes. It can be defined as the difference between the zeros of the pattern to the left and right of the sidelobe. Symmetry should also be expected here, but only theoretically. On fig. 5 shows the experimental data for determining the parameters of the side lobe.

As a result of a series of measurements, the position of the strips for the P2P-23-YXA antenna was determined, which are determined by the distance (1.20-1.36)^ from the antenna symmetry axis to the strip.

After determining the parameters of the sidelobe, the position of the stripes is determined. The corresponding calculations are performed for both theoretical and experimental RPs using the same method described below and illustrated in Fig. 6.

The constant d - the distance from the axis of symmetry of the parabolic antenna to the strip located on the surface of the aperture of the parabolic mirror, is determined by the following relationship:

"d"<Ф = ъ,

where d is the experimentally measured distance from the symmetry point on the mirror surface to the strip (Fig. 5); 0 - the angle between the direction of the main radiation and the direction of the side lobe maximum found experimentally.

The range of C values is found by the relation: c! \u003d O / dv

for values of 0 corresponding to the beginning and end of the sidelobe (corresponding to the zeros of the pattern).

After determining the range C, this range is divided into a number of values, from which the optimal value is experimentally selected

Rice. 4. Experimental setup

Rice. Fig. 5. Experimental determination of the parameters of the side lobes. 6. Strip positioning method

results

Several strip positions were tested. When moving the strips away from the main lobe, but within the range C found, the results improved. On fig. 7 shows two RPs without stripes and with stripes, showing a clear reduction in the level of side lobes.

In table. 1 shows the comparative parameters of the RP in terms of the level of the side lobes, directivity and width of the main lobe.

Conclusion

Reducing the level of side lobes when using strips - by 23 dB (the level of side lobes of an antenna without strips -

12.43 dB). The width of the main lobe remains almost unchanged. The considered method is very flexible, since it can be applied to any antenna.

However, a certain difficulty is the influence of multipath distortions associated with the influence of the ground and surrounding objects on the RP, which leads to a change in the level of side lobes up to 22 dB.

The considered method is simple, inexpensive and can be performed within a short time. In the following, we will try to add additional strips in different positions and explore absorption strips. In addition, work will be carried out on the theoretical analysis of the problem using the method of the geometric theory of diffraction.

Far field radiation pattern of the antenna P2F- 23-NXA linear magnitude - polar plot

Rice. 7. Antenna pattern P2F-23-NXA without stripes and with stripes

Comparative Antenna Parameters

side lobe level

Theoretical RP (Ma11ab program) RP according to technical documentation 18 dB 15 dB

Measured RP without bands 12.43 dB

Measured pattern with stripes With multipath Without multipath

Main lobe width in degrees D D, dB

Theoretical DN (program Ma-Lab) 16,161.45 22.07

DN according to technical documentation 16,161.45 22.07

Measured DN without strips 14 210.475 23.23

Measured DN with stripes 14 210.475 23.23

Literature

1. Balanis. C Antenna Theory. 3rd Ed. Wiley 2005.

2. IEEE standard test procedures for antennas IEEE Std. 149 - 1965.

3. http://www.thefreedictionary.com/lobe

4. Searle AD., Humphrey AT. Low side lobe reflector antenna design. Antennas and Propagation, Tenth International Conference on (Conf. Publ. No. 436) Volume 1, 14-17 April 1997 Page(s):17 - 20 vol.1. Retrieved on January 26, 2008 from IEEE databases.

5. Schrank H. Low sidelobe reflector antennas. Antennas and Propagation Society Newsletter, IEEE Volume 27, Issue 2, April 1985 Page(s):5 - 16. Retrieved on January 26, 2008 from IEEE databases.

6. Satoh T. shizuo Endo, Matsunaka N., Betsudan Si, Katagi T, Ebisui T. Sidelobe level reduction by improvement of strut shape. Antennas and Propagation, IEEE Transactions on Volume 32, Issue 7, Jul 1984 Page(s):698 - 705. Retrieved on January 26, 2008 from IEEE databases.

7. D. C. Jenn and W. V. T. Rusch. "Low sidelobe reflector design using resistive surfaces," in IEEE Antennas Propagat., Soc./ URSI Int. Symp. Dig., vol. I, May

1990, p. 152. Retrieved on January 26, 2008 from IEEE databases.

8. D. C. Jenn and W. V. T. Rusch. "Low sidelobe reflector synthesis and design using resistive surfaces," IEEE Trans. Antennas Propagat., vol. 39, p. 1372, Sep.

1991. Retrieved on January 26, 2008 from IEEE databases.

9. Monk AD., and Cjamlcoals PJ.B. Adaptive null formation with a reconfigurable reflector antenna, IEEE Proc. H, 1995, 142, (3), pp. 220-224. Retrieved on January 26, 2008 from IEEE databases.

10. Lam P., Shung-Wu Lee, Lang K, Chang D. Sidelobe reduction of a parabolic reflector with auxiliary reflectors. Antennas and Propagation, IEEE Transactions on . Volume 35, Issue 12, Dec 1987 Page(s):1367-1374. Retrieved on January 26, 2008 from IEEE databases.

Sidelobe level

Sidelobe level (SBL) Directional diagram (RP) of the antenna - relative (normalized to the maximum RP) radiation level of the antenna in the direction of the side lobes. As a rule, RBL is expressed in decibels.

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

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

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

Also used average level of lateral radiation(RP is averaged over the sector of lateral radiation angles) normalized to the RP maximum.

As a rule, a separate parameter is used to estimate the level of radiation in the "backward" direction (in the direction opposite to the main lobe of the RP), and this radiation is not taken into account when estimating the LBL.

Reasons for the decrease in UBL

In the receive mode, an antenna with a low NBL is “more noise-resistant”, since it is better at selecting the useful signal space against the background of noise and interference, the sources of which are located in the directions of the side lobes
Antenna with low NBL provides the system with greater electromagnetic compatibility with other electronic equipment and high-frequency devices
Antenna with low UBL provides the system with greater stealth
In the antenna of the target auto-tracking system, erroneous side-lobe tracking is possible
A decrease in UBL (with a fixed width of the main lobe of the AP) leads to an increase in the radiation level in the direction of the main lobe of the AP (to an increase in the directivity factor): the radiation of the antenna in a direction other than the main one is a waste of energy. However, as a rule, with fixed antenna dimensions, a decrease in UBL leads to a decrease in instrumentation, expansion of the main lobe of the RP, and a decrease in directivity factor.

The payoff for a lower UBL is the expansion of the main lobe of the DN (with a fixed antenna size), 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 LBL when designing an antenna is to choose a smoother (decreasing towards the edges of the antenna) spatial distribution of the current amplitude. A measure of this "smoothness" is the surface utilization factor (SUI) 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 - compensating emitters in the phased array, as well as by smoothly changing the length of the wall of the emitting aperture (in aperture antennas).

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

Main lobe width and side lobe level

The width of the DN (main lobe) determines the degree of concentration of the emitted electromagnetic energy. DN width- this is the angle between two directions within the main lobe, in which the amplitude of the electromagnetic field strength is the level of 0.707 of the maximum value (or the level of 0.5 of the maximum value in terms of power density). The width of the DN is indicated as follows:

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

2i - the width of the DN by tension at the level of 0.707.

The index E or H denotes the width of the DN 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 from the graph, for example, as shown in Figure 11.

Figure 11

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

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

Figure 12

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

Directionality and transmitting antenna gain

The directivity coefficient (DFA) quantitatively characterizes the directional properties of a real antenna compared to a reference non-directional (isotropic) one with a spherical pattern:

KND 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 P (u, c) 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 into account (25), we can get:

The gain factor (GA) 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 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 that the powers supplied to the antennas are the same.

The gain can be expressed in terms of the CND:

where is the efficiency of the antenna. In practice, they use - the gain of the antenna in the direction of maximum radiation.

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

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

Since the field vectors E and H are in-phase in the far zone of the antenna, the phase DN is equally related to the electrical and magnetic components of the EMF emitted by the antenna. The phase RP is denoted as follows: W = W (u, q) 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, in which the origin of the coordinate system is located, is called the antenna phase center (PCA). It should be noted that not all antennas have a phase center.

For antennas with 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.