Aerospace and Electronic Systems Magazine August 2016 - 33

Shifrin to employ additional limitations on currents, Q-factor, etc., as one would in deterministic synthesis problems. Statistical Antenna Measurement Theory (jointly with Vladimir A. Usin) One of the key SAT aspects was statistical antenna measurements theory (SAMT) we developed, as applied to the near-field methods of antenna measurements. These methods are now widely employed in the antenna practice since a conventional method of antenna parameters' measurements in the far-field region often appears unsuitable due to difficulties with realization of the far-field region, influence of ground, and local objects. Issues concerning estimation of the influence of random errors on the accuracy of antenna characteristics' restoration occupy an important place in near-field measurements and are addressed by the SAMT. Problems being solved with methods of this theory can be divided into the direct and inverse ones. The direct problems are aimed at defining the possibilities of the equipment employed in taking measurements at a given accuracy of the RP restoration. A goal of the inverse problem is to define how the measurer should be built in order to provide the RP restoration with a desired accuracy. The most effective in the near field is the amplitude-phasemetric method (APM), at which the antenna characteristics in the far-field region are defined by measuring APD in the antenna aperture (or near it) with the following recount of them to the far-field region. Procedures of solving direct and inverse problems with this method are considered in detail for a case of measurements along the linear aperture. It is noted that the results of these problem solutions essentially depend on a chosen criterion of proximity of the restored and true RPs. Also, under consideration were peculiarities of the APM at other options of its realization: measurements on a plane, in the Fresnel region, on the cylindrical surface embracing an antenna, when measuring the quadrature components of the field in the aperture. The results we obtained during the SAMT development are presented in detail in monograph [22] and in work [23]. The theory of antenna measurements we developed was widely used in many national organizations, especially in the All-Union Research Institute of Radio Measurements (ARIRM), Yerevan, Armenia. This institute used to be a head organization in the Governmental Antenna Measurements Standard system of the USSR. They used SAMT results when developing prototypes of antenna measurers, as well as in a great deal of standards, instructions, nomograms, and other documents they issued. Those results have also been used when developing requirements for an expensive antenna standard. For many years, I was a deputy Head of the governmental commission, through which all the ARIRM work had to be accepted, and one of the chairmen of conferences on antenna measurements, which were regularly held in Yerevan. On Minimal (Limiting) SRL. As is known, the decrease in SRL is usually achieved by controlling the sources' amplitude distribution in an antenna. As the nominal SRL decreases, the background being created by random errors becomes more and more important. The latter restricts the minimum AUGUST 2016 Figure 9. Dependences of vst (curve 1) and mean directivity (curve 2) on the nominal sidelobe level for a Dolph-Chebyshev array antenna. attainable SRL. However, if to define the SRL correctly, by the probability criterion, i.e., from the condition that the entire RP at the given sector y1 -y2, with a high degree of probability, FRP will not exceed the acceptable level v(y) (we designate it vst(y)), then the picture of the minimal SRL will be essentially different. Figure 9 presents the results of corresponding calculations for an 8-element Dolph-Chebyshev array given the standard deviation of phase errors s0=16° [24]. In this case, it is natural to choose v(ψ)=сonst as all the sidelobes in the Dolph-Chebyshev array are identical. Curve 1 in Figure 9 shows the dependence of vst on nominal side lobe level Fs. This curve is obtained at FRP = 0.99. The sector of angles ψ1 -ψ2, for which vst is defined, covers all the sidelobes, the number of which for the 8-element array is equal to six. As seen from Figure 9, the statistical SRL decreases slower than the nominal one, reaching, at Fs ≈ -35 dB, minimum vst.min that approximately equals -12.4 dB. In practice, the situation is even worse. At the domain of minimum, vst varies insignificantly. It is reasonable therefore to stop at the beginning of this domain, since with the decrease in Fs, the directivity of the Dolph-Chebyshev − array decreases (curve 2 in Figure 9). In this figure, D is the mean directivity; D0 is the directivity at the absence of errors, given the uniform amplitude distribution. The joint consideration of curves 1 and 2 allows choosing the reasonable Fs and conforming to it vst, which should be accepted as a reasonable limiting value. In the considered example, it is possible to take Fs,rs ≈ −(22 − 23) dB, and vst.rs.lim ≈ -11 dB. It is noteworthy that a correctly found value of minimal SRL is usually worse by 7 − 8 dB (when the number of elements N varies from 8 to 40) than the level that corresponds to the mean RP [18]. The latter for the above example constitutes ≈ −19dB. Besides the aforementioned, we considered a number of other interesting SAT issues as well. Among them are a theory of random arrays for the far-field and Fresnel regions [25], statistics of the field polarization characteristics [26], the influence of random inhomogeneities of antenna radomes and covers on characteristics of antenna systems located under them [27], statistics of acoustic antennas for systems of acoustic and radioacoustic sounding of the atmosphere [28], etc. The latter is of interest since the requirements for the SRL of these antennas are extremely high due to acoustic pollution of the environment, especially in the direction of the horizon. IEEE A&E SYSTEMS MAGAZINE 33

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine August 2016