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The case of a parabolic reflector and a point feed is considered, taking into account the question regarding the location in which the feed should be placed for an achievement of maximum directivity. Based on the tracing of geometrical rays, the obvious answer is obtained that the feed should be placed at the focal point. In the present paper, it is shown that this answer is not always correct. There are situations in which the maximum directivity is achieved when the feed is axially displaced toward the reflector or away from it. This 'focal shift' phenomenon is a result of three competing factors which affect the directivity of a reflector. The factors are related to phase synchronism over the reflector aperture, aperture illumination efficiency, and spillover loss. For achieving the maximum directivity, it is necessary to find the best compromise among the three factors.

A need for an accurate but inexpensive method for measuring and evaluating time delays of large ground antennas for VLBI applications motivated the development of the collimation tower technique. Supporting analytical work which 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 also given.

A need for an accurate but inexpensive method for measuring and evaluating time delays of large ground antennas for VLBI applications motivated the development of the collimation tower technique. Supporting analytical work which 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 also given.

A technique is presented in which the time an RF signal is delayed in propagating through a Cassegrain antenna is determined. The technique utilizes the group delay time and the envelope delay time as found from the antenna transfer function. The calculations show that a focused antenna is basically a nondispersive device whose delay time may be found from an optics formula. Small subreflector displacements result in significant delay changes requiring calibrations for many applications.