is usually used (i.e. the variation of the radiated signal with azimuth angle for a fixed antenna), is similar to the interferometer pattern. If the ARA were perfectly retrodirective, the dashed curve would be the envelope of the interferometer pattern over the entire scan, but, as we can see, this is true only within about +5 degrees of broadside. The main reason for the increasing departure from perfect retrodirectivity for scan angles greater than 5 degrees is the increase in the multipath contribution to the signals received by both the ARA horns and the diplexed pilot horn at large scan angles. The effect of multipath is probably aggravated by amplitude to phase conversion in the ARA electronics. This conclusion is qualitatively supported by the fact that retrodirectivity dramatically improved when: 1) the set-up was reoriented to minimize reflections from nearby buildings, and 2) absorbing panels and material were placed in front of and on reflecting surfaces as shown in Figure 13. The central phasing concept is incorporated into the ARA breadboard (Figure 12) by connecting the remote element to its PCC by a coaxial line. The line stretcher included in this coaxial line enables us to make an experimental check of that concept. The line stretcher is simply a section of coaxial line whose length can be changed mechanically. Hence it is completely bilateral and (at IF) nondispersive. Therefore, apart from the mismatch errors discussed in Subsection IIC(d) above, stretching the line should not affect ARA performance. The results of changing line length are shown in Figure 15. The ARA patterns for the indicated line length changes were recorded in top to bottom time sequence starting with the zero line length change ([] = 0 in.) pattern as a datum. The near optimum retrodirectivity of the A£ = 0 pattern was obtained by trimming the reference channel output phase as explained above. The other two [] patterns (between 2 in. and 3 in. and between 3 in. and 4 in.) were run as checks on thermal drift.
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