MMA Memo #183

A Suggested Receiver Layout for the MMA Antenna

John Lugten [1], Jack Welch [2]

September 15, 1997

Keywords: antenna optics, receivers

This memo suggests an optical arrangement for the Millimeter Array antennas in which stationary receivers are located near the Cassegrain focal plane, with each feed slightly off the antenna optical axis. This is one of the four schemes proposed in MMA Memo 163. No movable optical elements are required; switching frequency bands is accomplished by offsetting the antenna pointing slightly. We propose enlarging the subreflector diameter to 28 inches, which decreases the telescope effective focal length to 1156.9 inches. With all feeds located less than 6 inches off axis, the resulting coma produces a negligible loss of gain and negligible distortion of the primary beam (see also MMA Memo 175, in which Bill Shillue shows that for off axis distances of greater than 4 inches gain losses due to astigmatism are larger than coma losses; for a 6 inch off axis distance, the total loss is still less than 1\%). The off axis illumination results in a rearward spillover loss of up to 0.65 K and a loss of forward gain of up to 0.21\%. We suggest the possibility of placing the receivers in two dewars rather than a single dewar in order to possibly improve maintainability and reliability.

[1] NRAO
[2] BIMA

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Last modified: 09 December, 1999