VLBI collimation-tower technique for time-delay studies of a large ground-station communications antenna

A need for an accurate but inexpensive method for measuring time delays of large ground antennas for very long baseline interferometry (VLBI) applications motivated the development of the collimation-tower technique described. Supporting analytical work that was performed primarily to verify time-delay measurement results obtained for a large antenna when the transmitter was at a collimation distance of 1/25 of the usual far-field criterion is discussed. Comparisons of theoretical and experimental results are given.