Suppression of side lobes of drl and prl diagrams. Suppression of side lobes of drl and prl diagrams Objectives of reducing the UBL
Ensuring a sufficiently low level of side lobes in the pattern, as noted earlier, is one of the most important requirements for modern antennas.
When analyzing linear systems of continuously located emitters, the dependence of the level of side lobes on the AR law in the system was noticed.
In principle, it is possible to select an AR law in the system in which there are no side lobes in the pattern.
Indeed, let there be an in-phase lattice of two isotropic
emitters located at a distance d= - from each other (Fig. 4.36).
We will consider the excitation amplitudes of the emitters to be identical (uniform AR). In accordance with formula (4.73) DN of a two-element lattice
When 0 changes from ± - the value of sin0 changes from 0 to ±1, and the value of D0) - from 2 to 0. The DN has only one (main) lobe (Fig. 4.36). There are no side petals.
Consider a linear lattice consisting of two elements, each of which represents the lattice discussed above. We still consider the new array to be in phase, the distance between the elements X
d = -(Fig. 4.37, A).
Rice. 4.36. In-phase array of two isotropic emitters
Rice. 4.37.
The AR law in a lattice takes the form 1; 2; 1 (Fig. 4.37, b).
In accordance with the multiplication rule, the array pattern has no side lobes (Fig. 4.37, V):
The next step is an in-phase linear system consisting of two
previous ones, displaced in a straight line by a distance - (Fig. 4.38, A). We get a four-element lattice with AR 1; 3; 3; 1 (Fig. 4.38, b). The pattern of this array also does not have side lobes (Fig. 4.38, c).
Continuing according to the planned algorithm to increase the number of emitters in the system, for the pattern of a common-mode array consisting of eight elements, we obtain the formula
Rice. 4.38.
AR in such a lattice will be written accordingly in the following form: 1; 7; 21; 35; 35; 21; 7; 1. The written numbers are coefficients in the series expansion of Newton’s binomial (1 + x) 7, therefore the corresponding AR is called binomial.
If present in a linear discrete system n emitters, the binomial AR is determined by the coefficients in the expansion of the Newton binomial (1 + x) n ~ 1, and the DN of the system is the expression
As we see from expression (4.93), the pattern has no side lobes.
Thus, by using a binomial AR in an in-phase discrete system, it is possible to achieve complete elimination of side lobes. However, this is achieved at the cost of a significant expansion (compared to a uniform AR) of the main lobe and a decrease in the efficiency of the system. In addition, difficulties arise in practically ensuring in-phase excitation of emitters and sufficiently accurate binomial AR in the system.
A system with binomial AR is very sensitive to changes in AFR. Small distortions in the ADF law cause the appearance of side lobes in the pattern.
For these reasons, binomial AR is practically not used in antennas.
AR, which produces the so-called optimal DP, turns out to be more practical and expedient. By optimal we mean such a DN, in which, for a given width of the main lobe, the level of the side lobes is minimal, or for a given level of the side lobes, the width of the main lobe is minimal. The AR corresponding to the optimal AP can also be called optimal.
For a discrete in-phase system of isotropic emitters, located
laid at a distance A> - from each other, optimal is
Dolph - Chebyshevsky AR. However, in a number of cases (with a certain number of emitters and a certain level of side lobes) this AR is characterized by sharp “bursts” at the edges of the system (Fig. 4.39, A) and difficult to implement. In these cases, they move to the so-called quasi-optimal AR with a smooth decay to the edges of the system (Fig. 4.39, b).
Rice. 4.39. Amplitude distributions: A- Dolph - Chebyshevskoe;
b - quasi-optimal
With a quasi-optimal AR, compared to the optimal level, the level of the side lobes increases slightly. However, implementing a quasi-optimal AR is much simpler.
The problem of finding an optimal and, accordingly, quasi-optimal AR has also been solved for systems of continuously located emitters. For such systems, the quasi-optimal AR is, for example, the Taylor distribution.
Relative (normalized to the maximum radiation pattern) level of antenna radiation in the direction of the side lobes. As a rule, UBL is expressed in decibels; less commonly, UBL is determined "by power" or "across the field".
An example of an antenna radiation pattern and radiation pattern parameters: width, directivity, UBL, relative level of rear radiation
The pattern of a real (finite size) antenna is an oscillating function in which a global maximum is identified, which is the center main petal DP, as well as other local maxima of DP and the corresponding so-called side lobes DN. Term side should be understood as side, and not literally (petal directed “sideways”). The DN petals are numbered in order, starting with the main one, which is assigned the number zero. The diffraction (interference) lobe of the pattern that appears in a sparse antenna array is not considered lateral. The minima of the pattern that separate the lobes of the pattern are called zeros(the level of radiation in the directions of the nulls of the pattern can be arbitrarily small, but in reality radiation is always present). The lateral radiation region is divided into subregions: near side lobe region(adjacent to the main lobe of the pattern), intermediate area And posterior lateral lobe region(the entire rear hemisphere).
- UBL is understood as relative level of the largest side lobe of the pattern. As a rule, the largest in size is the first (adjacent to the main) side lobe.
For antennas with high directivity they also use average lateral radiation level(the pattern normalized to its maximum is averaged in the sector of lateral radiation angles) and level of far side lobes(relative level of the largest sidelobe in the region of the rear sidelobes).
For longitudinal radiation antennas, to estimate the radiation level in the “backward” direction (in the direction opposite to the direction of the main lobe of the radiation pattern), the parameter relative rear radiation level(from English front/back, F/B- forward/backward ratio), and this radiation is not taken into account when assessing the UBL. Also, to estimate the level of radiation in the “sideways” direction (in the direction perpendicular to the main lobe of the pattern), the parameter relative lateral radiation(from English front/side, F/S- front/side ratio).
UBL, as well as the width of the main lobe of the radiation pattern, are parameters that determine the resolution and noise immunity of radio engineering systems. Therefore, in the technical specifications for the development of antennas, these parameters are given great importance. The beam width and UBL are controlled both when the antenna is put into operation and during operation.
UBL reduction goals
- 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 antenna dimensions, 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
Since the antenna pattern in the far zone and the amplitude-phase distribution (APD) of currents along the antenna are interconnected by the Fourier transform, the UBL as a secondary parameter of the pattern is determined by the APD law. The main way To reduce the UBL when designing an antenna is to select a smoother (falling 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 side lobes of mirror antennas by positioning metal strips in the aperture
Akiki D, Biayneh V., Nassar E., Harmush A,
University of Notre Dame, Tripoli, Lebanon
Introduction
In a world of increasing mobility, there is a growing need for people to connect and access information, regardless of where the information is located or the individual. From these considerations, it is impossible to deny that telecommunications, namely the transmission of signals over distances, is an urgent need. The demands for wireless communication systems to be so perfect and ubiquitous mean that increasingly more efficient systems need to be developed. When improving a system, a key initial step is to improve the antennas, which are the core element of current and future wireless communication systems. At this stage, by improving the quality of the antenna parameters we will understand a decrease in the level of its side lobes of its radiation pattern. Reducing the level of side lobes, naturally, should not affect the main lobe of the diagram. Reducing the sidelobe level is desirable because for antennas used as receives, the sidelobes make the system more vulnerable to stray signals. In transmitting antennas, side lobes reduce information security, since the signal may be received by an unwanted receiving party. The main difficulty is that the higher the sidelobe level, the higher the probability of interference in the direction of the sidelobe with the highest level. In addition, increasing the level of side lobes means that signal power is dissipated unnecessarily. 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, effective and low cost. Any parabolic antenna
can be developed or even modified using this method (Fig. 1) to reduce interference between antennas.
However, the conductive strips must be very precisely positioned to achieve sidelobe reduction. In this paper, the "strip positioning" method is tested through experiment.
Description of the task
The problem is formulated as follows. For a particular parabolic antenna (Fig. 1), it is necessary to reduce the level of the first side lobe. The antenna radiation pattern is nothing more than the Fourier transform of the antenna aperture excitation function.
In Fig. Figure 2 shows two diagrams of a parabolic antenna - without stripes (solid line) and with stripes (line shown with *), illustrating the fact that when stripes are used, the level of the first side lobe decreases, but at the same time the level of the main lobe also decreases, and the level also changes the remaining petals. This shows that the position of the stripes is very critical. It is necessary to position the strips in such a way that the width of the main lobe 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 petals is not so significant, since the level of these petals 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 above reasons, when using the "strip positioning" method, the following must be kept in mind: the strips must be metal in order to fully reflect the electric field. In this case, the position of the stripes can be clearly determined. Currently, side lobe level measurements
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 diagrams of antennas without breakdowns and with stripes, in this case we will use the experimental method.
A. Theoretical method. This method consists of:
Finding the theoretical radiation pattern (RP) of the antenna under test,
Measurements of the side lobes of this pattern.
The 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 relationships for the field.
The P2P-23-YHA mirror parabolic antenna was used as the antenna under test. The theoretical value of the DP was obtained using the formula for a circular aperture with uniform excitation:
]ka2E0e іkg Jl (ka 8Іпв)
Measurements and calculations were performed in the E-plane. In Fig. Figure 3 shows the normalized radiation pattern in the polar coordinate system.
B. Experimental method. In the experimental method two antennas must be used:
The receiving antenna under test,
Transmitting antenna.
The pattern of the antenna 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. Adjusting the level of side lobes. By definition, the first side petals are those closest to the main petal. 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 the 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 in an experimental pattern this may not be the case. Next, you also need to determine the width of the side lobes. It can be defined as the difference between the pattern zeros to the left and right of the side lobe. Here we should also expect symmetry, but only theoretically. In Fig. Figure 5 shows experimental data on determining the side lobe parameters.
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 axis of symmetry of the antenna to the strip.
After determining the side lobe parameters, the position of the stripes is determined. The corresponding calculations are performed for both theoretical and experimental patterns 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 surface of the mirror 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 C values is found by the relationship: c! = O/dv
for values of 0 corresponding to the beginning and end of the side lobe (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. 5. Experimental determination of side lobe parameters Fig. 6. Strip positioning method
Results
Several positions of the strips were tested. When moving the strips away from the main lobe, but within the found range C, the results improved. In Fig. Figure 7 shows two patterns without stripes and with stripes, demonstrating a clear decrease in the level of side lobes.
In table Table 1 shows comparative parameters of the pattern in terms of the level of the side lobes, directivity and width of the main lobe.
Conclusion
Reduction in the level of side lobes when using stripes - by 23 dB (level of side lobes of an antenna without stripes -
12.43 dB). The width of the main petal remains almost unchanged. The method discussed 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 earth and surrounding objects on the pattern, which leads to a change in the level of the side lobes up to 22 dB.
The method discussed is simple, inexpensive and can be completed in a short time. In the following we will try to add additional stripes in different positions and examine the absorption stripes. In addition, work will be carried out on the theoretical analysis of the problem using the method of geometric diffraction theory.
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 comparison parameters
Side lobe level
Theoretical pattern (program Ma11a) pattern according to technical documentation 18 dB 15 dB
Measured pattern without stripes 12.43 dB
Measured pattern with stripes With multipath Without multipath
Main lobe width in degrees D D, dB
Theoretical DN (program Ma^ab) 16,161.45 22.07
DN for technical documentation 16,161.45 22.07
Measured pattern without stripes 14,210.475 23.23
Measured pattern 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 sidelobe 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, Sept.
1991. Retrieved on January 26, 2008 from IEEE databases.
9. Monk A.D., and Cjamlcoals P.J.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.
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 through 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).
- The side lobe level (SLL) of the antenna radiation pattern is the relative (normalized to the maximum radiation pattern) level of antenna radiation in the direction of the side lobes. As a rule, UBL is expressed in decibels; less often, UBL is defined “by power” or “by field”.
The pattern of a real (finite size) antenna is an oscillating function in which a global maximum is identified, which is the center of the main lobe of the pattern, as well as other local maximums of the pattern and the corresponding so-called side lobes of the pattern. The term lateral should be understood as sideways, and not literally (petal directed “sideways”). The DN petals are numbered in order, starting with the main one, which is assigned the number zero. The diffraction (interference) lobe of a pattern that appears in a sparse antenna array is not considered lateral. The minima of the pattern separating the lobes of the pattern are called zeros (the level of radiation in the directions of the pattern zeros can be arbitrarily small, but in reality radiation is always present). The lateral radiation region is divided into subregions: the region of the near side lobes (adjacent to the main lobe of the pattern), the intermediate region and the region of the rear side lobes (the entire rear hemisphere).
By UBL we mean the relative level of the largest side lobe of the pattern. As a rule, the largest side lobe is the first (adjacent to the main) side lobe. For antennas with high directivity, the average level of side radiation is also used (the pattern normalized to its maximum is averaged in the sector of side radiation angles) and the level of far side lobes (the relative level of the largest side lobe petal in the area of the rear side petals).
For longitudinal radiation antennas, to assess the radiation level in the “backward” direction (in the direction opposite to the direction of the main lobe of the radiation pattern), the relative rear radiation level parameter is used (from the English front/back, F/B - forward/back ratio), and when estimating UBL does not take this radiation into account. Also, to assess the level of radiation in the “sideways” direction (in the direction perpendicular to the main lobe of the pattern), the relative side radiation parameter (from the English front/side, F/S - forward/side ratio) is used.
UBL, as well as the width of the main lobe of the radiation pattern, are parameters that determine the resolution and noise immunity of radio engineering systems. Therefore, in the technical specifications for the development of antennas, these parameters are given great importance. The beam width and UBL are controlled both when the antenna is put into operation and during operation.
Related concepts
A photonic crystal is a solid structure with a periodically changing dielectric constant or inhomogeneity, the period of which is comparable to the wavelength of light.
A fiber Bragg grating (FBG) is a distributed Bragg reflector (a type of diffraction grating) formed in the light-carrying core of an optical fiber. FBGs have a narrow reflection spectrum and are used in fiber lasers, fiber-optic sensors, to stabilize and change the wavelength of lasers and laser diodes, etc.