The adaptive scheme requires that a reference phase be distributed to all of the subarrays as a standard against which the incoming signal from the ground pilot transmitter can be compared. Any errors in the reference will induce equal errors in the phasing of the outgoing power phase front so that it must be accurate and must penalize the antenna as little as possible as regards weight and reliability. Four all-rf schemes for achieving this are listed and compared in Figure 7-5, and a description of the front side space distribution scheme and transmission line approach are given in Figure 7-6. The spacefed frontside, spacefed rearside, transmission line, and subarray- to-subarray transfer schemes are leading candidates. The latter was proposed by Jet Propulsion Laboratory as a technique to eliminate cable weight and potential cable errors; an error buildup will be developed as the reference signal propagates from the center to the edges of the antenna. On the other hand the transmission line approach requires that the cable be calibrated against thermal path length changes using a two-way path measurement technique. The spacefed frontside approach does not appear to offer a weight advantage while it does produce rf blockage and adds complexity in the antenna mechanical design. The spacefed rearside approach has multipath problems due to structure geometry. Figure 7-5. Reference Distribution Systems
RkJQdWJsaXNoZXIy MTU5NjU0Mg==