Aerospace and Electronic Systems Magazine August 2016 - 29

Shifrin Figure 6. Figure 7. (the first one I know of in the world literature), in monograph [7] and in [8]. The investigations of LDTP are of great practical importance, as the work of tropospheric radio stations and a number of other radio lines is based on this phenomenon. However, we also were interested in another thing-a link between LDTP and the SAT. The matter is that the LDTP line can be considered the equipment, created by nature itself, which realizes one version of time antenna statistics. It is essential that depending on a "specific weight" of two aforementioned LDTP mechanisms, field fluctuations in the receiving antenna aperture can vary within wide limits. For example, at the first mechanism (Figure1), when the direct signal does not fit the antenna, the field in the aperture is considerably random and, correspondingly, the statistical effects are boldly expressed. We experimentally observed the decrease in the directivity (loss) by 6 dB to 7 dB, and the mean RP widening 2.3 to 2.5 times. At the second mechanism, the field in the aperture is more "calm." In the book, examples of a number of statistical characteristics of the receiving antenna in the LDTP field are given: mean RP, loss of directivity, mean RP widening, MMD fluctuations, the field mean intensity distribution and correlation in the focal plane, etc. It is shown that application of approaches and results of SAT allows (at least qualitatively) not only confirming an order of these effects, but also their interrelations. Especially noteworthy are the complicated experiments on receiving "instantaneous" RPs (IRP)-RPs picked up for the time the field near the receiving antenna can be considered "frozen." Figures 6 (a) and (b) show two series of many IRPs we picked up, whereas Figure 6(c) presents the RP in free space (the undisturbed antenna RP).1 The width of this RP was 0.9°. Figure 6(a) corresponds to the situation when the field in the aperture is relatively "calm," i.e., the "coherent" mechanism of LDTP prevails. As a rule, this took place at night, in winter. The IRPs are comparatively less distorted compared with the undisturbed RP. Their separate realizations picked up with the interval of 20 s noticeably correlate with each other. The IRPs in Figure 6(b) are of another character, highly variable and not correlated to IRPs in 6(c). These IRPs take place when a main role in the field formation near the aperture is played by the mechanism of Figure 1-the mechanism of incoherent scattering. The fluctuations of the field in the aperture are large and vary rapidly. Correspondingly, the IRPs are strongly distorted and even when picking them up with the interval of 10 s, they are absolutely uncorrelated with each other. We picked up many hundreds of IRPs. They contain rich information about the field in the antenna aperture at a given time of day and season of the year. This, in its turn, enables one to elucidate a mechanism of this field formation. Of significant interest also was a simultaneous study of values of the field level at two points (symmetric with respect to a focus) of the focal plane and the degree of their correlation, which were carried out with the help of a specially designed adjustable primary radiator. Similar investigations constitute grounds for solving the issue about whether it is reasonable to employ the angular diversity reception system in the LDTP line. Such a system is usually realized by placing in the focal plane, two radiators symmetrically shifted with respect to the focus. As the SAT considerations show, at a given statistics of incident waves, with the increase in the antenna dimension, the pattern of field distribution in the focal plane smooths strongly whereas decorrelation of the field at symmetric points strengthens. Both these factors enhance the reasonability of using the angular diversity reception systems. This conforms to the data available in the literature that such systems are usually employed at large antenna dimensions (L ≫ 30..50λ) [9]. A number of interesting issues are considered in appendices to the book. Let us dwell on two of them. The first issue concerns limitations imposed by inhomogeneities of the atmosphere on large antennas' directivities [10]. Statements on this issue in the literature are contradictory. In some works, it is written that there are no limitations, in other ones that such limitations exist. Results of our investigations are illustrated by Figure 7. Here ρ is the correlation radius of the refraction coefficient' fluctuations for the troposphere. It is equal, in the order of magnitude, to the middle size of tropospheric inhomogeneities. As the figure shows, theoretically, at any value of variance α of the fluctuations in the incident wave, there is no limiting length of the antenna. At large enough antenna dimensions (L ≫ ρ), a value of the mean directivity is defined by the formula where D0 is the theoretical directivity. The reasonability of building an- Instantaneous RPs. 1 RP is the upper envelope on the oscillogram AUGUST 2016 Mean directivity of the linear antenna versus its dimension. IEEE A&E SYSTEMS MAGAZINE 29

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