to perturb the phase of the received signal. This, as in the transmission line case, would be harmless if [], for then reciprocity would assure exactly the same phase perturbation of the retrodirected signal received back at the pilot source no matter how complicated the multipath situation. But since [], reciprocity fails and phase errors result. Except in the very simplest cases, multipath effects are too difficult to calculate. Their presence can be inferred, however, by an irregular variation of pointing error with scan angle. We will see such evidence of multipath below in data obtained for a breadboard ARA. Multipath due to scattering from the solar panels may be a significant problem for the SPS ARA. III. EXPERIMENTAL RESULTS Figure 12 is the block diagram of a two element X-band ARA breadboard which was built and tested by the author at the Jet Propulsion Laboratory. The purpose of the breadboard is to demonstrate exact phase conjugation and central phasing, both of which, as we saw above, would be necessary features of very large, ultra- precise ARA's. The PCC in Figure 12 is just that of Figure 6 for n = 4, so that the frequency translation ratio is R = 2. Fast MECL flip-flops are used for the X 1/2 and X 1/4 circuits. Since this array has only two elements, both of which must transmit a conjugate signal, one of the elements must serve as reference element both for the other element (the "remote horn” in Figure 12) and for itself (" ref erence horn"). But, according to (5), the conjugate of the reference phase is [] for R = 2, so that one needs only a doubler, not a complete PCC, to conjugate []. Because of their dissimilarity, the internal phase delays in the doubler and the remote PCC are unequal. A phase shifter at the doubler output is used to equalize their overall delays thereby eliminating the pointing
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