We see from Figure 15 that [] 2 in. does not appreciably affect the retro- directivity of the ARA while extending A£ to 3 in. almost completely destroys it. The similarity of the three A£ = 0 patterns shows that thermal drift is not a factor in these measurements. The fact that [] in. = 1.43 wavelengths at [] = 8.434 GHz shows that central phasing is highly effective in spite of the poorly matched rudimentary VHF diplexers used in this breadboard. By means of the expression (derived in Subsection IIC(d) above) for the phase error produced by such mismatches, it can be shown that these results are consistent with VSWR ~ 1.5 for both diplexers at both frequencies, in rough agreement with the measured VSWR values. The spikes in the patterns in Figures 14 and 15 look like cycle slips, probable in auxiliary equipment (signal generator "lock boxes," etc.) rather than in loops in the ARA itself. In any case, they do not affect the validity of the data. IV. CONCLUSION Centrally phased ARA’s can be used to point the downlink beam of spaceborne antennas which are too large and floppy to be pointed by methods which require structural rigidity (such as mechanical steering or conventional phased array techniques). Antennas for solar power satellites, multiple beam communication satellites, and deep space probes are possible ARA applications. The retrodirec- tivity of an ARA is the result of phase conjugation of the pilot signal received by each element of the array. The same principle enables an ARA to simultaneously function as a phased receiving array. Precision pointing as well as inputoutput isolation is provided by exact, frequency translating phase conjugation circuits (PCC’s). Pointing errors in ARA’s are caused by doppler, aberration, charged particle media, and multipath effects. A two element ARA breadboard,
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