+1.2 x 10^-5 But this length change would be produced in an aluminum line by a temperature change of only 0.5 degrees C, or by a mechanical stress of only 120 psi. The results for other good conductors are similar. Since we would expect temperature and stress changes far greater than these in this huge structure, it is clear that the required dimensional stability cannot be met with materials commonly used for transmission lines. While it might be possible to solve this problem with uncommon materials, we can avoid it altogether by locating all PCC’s at the reference source rather than at their respective elements. This method of providing the phase reference, which Margerum calls "central phasing" [8], is illustrated in Figure 2, which, for the sake of clarity, shows only two elements of the ARA. The phase reference for this ARA is the pilot signal received by the 0-th, or reference, element. The pilot signal received by the kth element is transmitted to its associated PCC located at the reference element via the transmission line and diplexer, 2PLX. The PCC conjugates the entire phase delay, i.e. the sum of the space delay, [], and the transmission line delay, [] , where [] is the phase velocity of the line, and transmits that conjugate signal back down the same transmission line to the kth element, which retransmits it. Its phase at that point is [] which is exactly what it would be were the PCC located at the kth element rather than at the reference element. Thus the length of the transmission line is immaterial provided only that: 1) the line is dispersionless, and 2) its length is constant with time. The importance of central phasing lies in the fact that it liberates the ARA from the rigid structure which would otherwise be needed in order to realize accurate retrodirectivity. The elements need not be arranged in any particular geometrical pattern, and may, in fact, move about with respect to one another
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