What determines the level of the side lobes. Ways to Reduce Side Lobe Level in Emitter Systems

Let the current distribution along the length of the antenna be constant:

Real antennas (for example, slotted waveguide) or printed antenna arrays often have this current distribution. Let's calculate the radiation pattern of such an antenna:

Now let's construct a normalized DN:

(4.1.)

Rice. 4.3 Linear antenna pattern with uniform current distribution

In this radiation pattern, the following areas can be distinguished:

1) The main lobe is the area of the radiation pattern where the field is maximum.

2) Lateral petals.

The following figure shows the polar pattern in which
has a more visual form (Figure 4.4).

Rice. 4.4 The radiation pattern of a linear antenna with a uniform current distribution in a polar coordinate system

A quantitative estimate of the antenna directivity is considered to be the width of the antenna's main lobe, which is determined either by the level of -3 dB from the maximum or by zero points. Determine the width of the main lobe at the level of the zeros. Here, we can roughly assume that for highly directional antennas:
... The condition for the equality of the system factor to zero can be approximately written as follows:

Considering that
, the last condition can be rewritten as follows:

For large values of the electrical length of the antenna (for small values of the half-width of the main antenna lobe), taking into account the fact that the sine of a small argument is approximately equal to the value of the argument, the last relation can be rewritten as:

Whence we finally get the ratio between the width of the main lobe and the size of the antenna in fractions of the wavelength:

An important conclusion follows from the last relation: for an in-phase linear antenna at a fixed wavelength, an increase in the antenna length leads to a narrowing of the radiation pattern.

Let us estimate the level of side lobes in this antenna. From relation (4.1), we can obtain the condition for the angular position of the first (maximum) side lobe:

(-13 dB)

It turns out that in this case the level of the side lobes does not depend on the antenna length and frequency, but is determined only by the form of the amplitude distribution of the current. To reduce the UBL, one should abandon the accepted form of the amplitude distribution (from a uniform distribution), and go to a distribution that falls to the edges of the antenna.

5. Linear antenna array

5.1. Deriving an expression for dn lar

Expression 4.2. allows you to easily switch from the field of a linear continuous antenna system to the field of a discrete antenna array. To do this, it is sufficient to set the current distribution under the integral sign in the form of a lattice function (a set of delta functions) with weights corresponding to the amplitudes of excitation of the elements and the corresponding coordinates. In this case, the result is the antenna array radiation pattern as a discrete Fourier transform. Master students are given the opportunity to implement this approach on their own as an exercise.

6. Synthesis of afr for a given day.

6.1. Historical overview, features of antenna synthesis problems.

Often, in order to ensure the correct operation of radio engineering systems, special requirements are imposed on the antenna devices that are their integral part. Therefore, designing antennas with specified characteristics is one of the most important tasks.

Basically, the requirements are imposed on the directional pattern (BP) of the antenna device and are of a very diverse nature: a specific shape of the antenna pattern main lobe (for example, the form of a sector and cosecant), a certain level of side lobes, a dip in a given direction or in a given range of angles may be required. The section of antenna theory devoted to solving these problems is called the theory of antenna synthesis.

In most cases, the exact solution to the synthesis problem has not been found, and we can talk about approximate methods. Such problems have been studied for a long time and many methods and techniques have been found. Certain requirements are also imposed on the methods for solving problems of antenna synthesis: to speed; stability, i.e. low sensitivity to minor changes in parameters (frequency, antenna sizes, etc.); practical feasibility. The simplest methods are considered: partial diagrams and the Fourier integral. The first method is based on the analogy of the Fourier transform and the relationship between the amplitude-phase distribution and the MD, the second is based on the expansion of the MD series in basis functions (partial MDs). Often, the solutions obtained by these methods are difficult to apply in practice (antennas have poor instrumentation, a difficult-to-implement amplitude-phase distribution (AFD), the solution is unstable). In and considered methods to take into account the restrictions on PRA and avoid the so-called. "Superdirectional effect".

Separately, it is worth highlighting the problems of mixed synthesis, the most important of which is the problem of phase synthesis, that is, finding the phase distribution at a given amplitude, leading to the required DP. The relevance of the problems of phase synthesis is explained by the large use of phased antenna arrays (PAR). Methods for solving such problems are described in, and.

Side-lobe level

Side lobe level (LBL) antenna radiation pattern (BP) - the relative (normalized to the BP maximum) 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, coefficient of suppression of backward radiation

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

Usually, UBL is understood as the relative level of the largest side lobe of the DN... Directional antennas usually have the largest side lobe (adjacent to the main).

Also use average side emission(BP is averaged in the sector of lateral emission angles), normalized to the BP 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 beam of the pattern), and this radiation is not taken into account when assessing the UBL.

Reasons for the decrease in UBL

In the receive mode, an antenna with a low UBL is "more noise-immune", since it performs better selection in the space of the useful signal against the background of noise and interference, the sources of which are located in the directions of the side lobes
Antenna with a low UBL provides the system with greater electromagnetic compatibility with other radio electronic means and high-frequency devices
Low UBL antenna provides the system with more stealth
In the antenna of the automatic target tracking system, erroneous tracking along the side lobes is possible
A decrease in UBL (with a fixed width of the main lobe of the pattern) leads to an increase in the radiation level in the direction of the main lobe of the pattern (to an increase in directivity): the radiation of the antenna in a direction other than the main one is empty energy loss. However, as a rule, with fixed dimensions of the antenna, a decrease in the LBL leads to a decrease in the instrumentation, an expansion of the main lobe of the AP, and a decrease in the directivity.

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

Ways to reduce UBL

The main way to reduce the UBL when designing an antenna is to choose a smoother (falling to the edges of the antenna) spatial distribution of the current amplitude. A measure of this "smoothness" is the surface utilization factor (UUF) of the antenna.

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

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

UBL reduction targets

In the receive mode, an antenna with a low UBL is "more noise-immune", since it performs better selection in the space of the useful signal against the background of noise and interference, the sources of which are located in the directions of the side lobes
Antenna with a low UBL provides the system with greater electromagnetic compatibility with other radio electronic means and high-frequency devices
Low UBL antenna provides the system with more stealth
In the antenna of the automatic target tracking system, erroneous tracking along the side lobes is possible
A decrease in the UBL (with a fixed width of the main lobe of the pattern) leads to an increase in the radiation level in the direction of the main lobe of the pattern (to an increase in directivity): the radiation of the antenna in a direction other than the main one is an empty loss of energy. However, as a rule, with a fixed antenna dimensions, a decrease in the UBL leads to a decrease in the instrumentation, an expansion of the main lobe of the AP and a decrease in the directivity.

Ways to reduce UBL

Since the antenna pattern in the far zone and the amplitude-phase distribution (APD) of the currents along the antenna are related to each other by the Fourier transform, the UBL as a secondary parameter of the pattern is determined by the APR law. The main way lowering UBL when designing an antenna is the choice of a smoother (falling to the edges of the antenna) spatial distribution of the current amplitude. A measure of this "smoothness" is the surface utilization factor (UUF) of the antenna.

Markov G. T., Sazonov D. M. Antennas. - M.: Energiya, 1975 .-- S. 528.

Voskresensky D.I. Microwave devices and antennas. Design of phased antenna arrays .. - M.: Radiotekhnika, 2012.

The difference in energy levels of the main and side lobes is used to suppress the request from the side lobes.

1.2.1. The suppression of the request from the side lobes of the directional pattern of the dispatcher SSR is carried out using the so-called three-pulse system (see Fig. 2 *).

Rice. 2 Suppression of the request from the side lobes of the DRL using a three-pulse system

To the two impulses of the inquiry code P1 and P3 emitted by the directional radar antenna, a third impulse P2 (suppression pulse), emitted by a separate omnidirectional antenna (suppression antenna), is added. The suppression pulse is 2 μs behind the first pulse of the request code. The energy level of the jamming antenna radiation is selected in such a way that the level of the jamming signal at the receiving points is obviously higher than the level of signals emitted by the side lobes and less than the level of signals emitted by the main lobe.

The transponder compares the amplitudes of the code pulses P1, РЗ and the suppression pulse P2. When the interrogation code is received in the direction of the side lobe, when the suppression signal level is equal to or exceeds the level of the interrogation code signals, no response is made. The answer is made only when the level Р1, РЗ is more than the level Р2 by 9 dB or more.

1.2.2. The suppression of the request from the side lobes of the directional pattern of the landing radars is performed in the BPS unit, which implements the suppression method with a floating threshold (see Fig. 3).

Fig. 3 Receiving a packet of response signals
when using a floating threshold suppression system

This method consists in the fact that in the BTS, with the help of an inertial tracking system, the level of signals received from the main lobe of the directional pattern is stored in the form of a voltage. The part of this voltage corresponding to a predetermined level exceeding the level of the sidelobe signals is set as a threshold at the amplifier output, and in the next irradiation, a response is made only when the request signals exceed this threshold. This voltage is corrected in subsequent exposures.

1.3. Response signal structure

The response signal containing any word of information consists of a coordinate code, a key code and an information code (see Fig. 4a *).

Fig. 4 Structure of the response code

The coordinate code is two-pulse, its structure is different for each word of information (see Fig. 4b, c *).

The key code is three-pulse, its structure is different for each word of information (see Fig. 4b, c *).

The information code contains 40 pulses that make up 20 bits of the binary code. Each discharge (see Fig. 4a, d) contains two pulses spaced 160 μs apart. The interval between pulses of one discharge is filled with pulses of other discharges. Each bit carries binary information: the character "1" or the character "0". In the SO-69 transponder, the method of active pause is used to transmit two symbols, the symbol “0” is transmitted by a pulse delayed by 4 μs relative to the moment in time at which the pulse denoting the symbol “1” would be transmitted. The two possible pulse positions for each digit (“1” or “0”) are indicated by crosses. The time interval between two characters “1” (or “0”) following each other is taken to be 8 µs. Therefore, the interval between the following characters “1” and “0” will be 12 µs, and if the character “0” is followed by the character “1”, then the interval between pulses will be 4 µs.

The first bit transmits one pulse, which denotes one if it is delayed by 4 μs, and zero if it is delayed by 8 μs. The second bit also transmits one pulse, which means 2 if it is delayed by 4 μs relative to the previous bit, zero if it is delayed by 8 μs. The third bit conveys 4 and 0, also depending on their position, the 4th bit conveys 8 and 0.

So, for example, the digit 6 is transmitted as the number 0110 in binary notation, that is, as the sum of 0 + 2 + 4 + 0 (see Figure 1)

Information transmitted in 160 μs is transmitted a second time in the next 160 μs, which significantly increases the noise immunity of information transmission.

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

Akiki D, Biayneh V., Nassar E., Kharmush A,

University of Notre Dame, Tripoli, Lebanon

Introduction

In a world of increasing mobility, there is a growing need for people to communicate and access information, regardless of where the information is located or the individual. From these considerations, it cannot be denied that telecommunications, namely the transmission of signals over a distance, is an absolute must. The requirements for wireless communication systems for their perfection and ubiquity lead to the fact that more and more efficient systems need to be developed. When improving the system, the main starting step is to improve the antennas, which are the main building blocks of current and future wireless communication systems. At this stage, by improving the quality of the antenna parameters, we mean a decrease in the level of its side lobes of its directional pattern. A decrease in the level of side lobes, of course, should not affect the main lobe of the diagram. Lowering the side lobe level is desirable because for antennas used as receive antennas, the side lobes make the system more vulnerable to unwanted 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 the 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 sidelobe level means that signal power is wasted unnecessarily. A lot of research has been done (see, for example), but the purpose of this article is to consider the "strip positioning" method, which has proven to be simple, effective 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 a reduction in the level of the side lobes. In this article, the "strip positioning" method is tested by experiment.

Description of the task

The problem is formulated as follows. For a particular parabolic antenna (Fig. 1), it is required to lower the level of the first side-lobe. Antenna radiation pattern is nothing more than the Fourier transform of the excitation function of the antenna aperture.

In fig. 2 shows two diagrams of a parabolic antenna - without stripes (solid line) and with stripes (the line shown by *), illustrating the fact that when using strips, 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 position the strips so that the half-power main lobe width or antenna gain does not change appreciably. The level of the back lobe should also not change noticeably. The increase in the level of the remaining petals is not so significant, since the level of these petals is usually much easier to lower 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 stated reasons, 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 stripes can be clearly identified. Currently to measure the level of side lobes

Rice. 2. Antenna radiation pattern without stripes (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 evidence is based on a comparison of experimental antenna diagrams without breakages and with stripes, in this case we will use the experimental method.

A. Theoretical method. This method consists of:

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

Measurements of the side lobes of this DN.

The antenna pattern 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 relationships.

A P2P-23-YKHA reflector parabolic antenna was used as a test antenna. The theoretical value of the DP was obtained using the formula for a round aperture with uniform excitation:

] ka2E0e іkg Jl (ka 8Ipv)

Measurements and calculations were performed in the E-plane. In fig. 3 shows the normalized polar pattern.

B. Experimental method. In the experimental method, two antennas should be used:

Receiving antenna under test,

Transmitting antenna.

The antenna pattern of the antenna under test is determined by rotating it and fixing the field level with the required accuracy. For improved accuracy, it is preferable to read in decibels.

B. Adjusts 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, you also need to determine the width of the side petals. It can be defined as the difference between the DN zeros to the left and right of the side lobe. Symmetry should also be expected here, but only in theory. In 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-NKhA antenna was determined, which are determined by the distance (1.20-1.36) ^ from the axis of symmetry of the antenna to the strip.

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

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

The range of values for C is found by the ratio: s! = O / dv

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

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

Rice. 4. Experimental setup

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

results

Several positions of the strips have been tested. When moving the stripes away from the main lobe, but within the found C range, the results improved. In fig. 7 shows two BPs without stripes and with stripes, showing a clear decrease in the level of side lobes.

Table 1 shows the comparative parameters of the antenna pattern in terms of the level of side lobes, directivity and width of the main lobe.

Conclusion

Reduction of the side lobe level when using strips - by 23 dB (the level of the side lobes of the antenna without stripes -

12.43 dB). In this case, the width of the main lobe remains almost unchanged. This method is very flexible as 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 pattern, which leads to a change in the level of the side lobes up to 22 dB.

This method is simple, inexpensive, and can be completed in a short time. In what follows, we will try to add additional stripes at different positions and explore absorption stripes. In addition, work will be performed 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. DN antenna P2F-23-NXA without stripes and with stripes

Antenna Comparative Parameters

Side lobe level

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

Measured AP without stripes 12.43 dB

Measured DN with stripes With multipath Without multipath

Main lobe width in degrees D D, dB

Theoretical DN (Ma ^ ab program) 16 161.45 22.07

DN according to technical documentation 16 161.45 22.07

Measured DN without stripes 14 210.475 23.23

Measured MD with stripes 14 210.475 23.23

Literature

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

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