MMA Memo #175

Gain Degradation in a Symmetrical Cassegrain Antenna Due to Laterally Offset Feeds

Bill Shillue [1]

June 18, 1997

Keywords: antenna, aberration, astigmatism, cassegrain, coma, efficiency, feeds, optics

The maximimum distance off-axis that a feed can be placed in a symmetrical Cassegrain antenna without significant degradation in efficiency is investigated. The results place limits on the size and position of receivers at the secondary focus. For example, for the proposed MMA geometry, if we use a one percent reduction in gain as a figure-of-merit, then the maximum off-axis distance is about 165 mm or 6.5 in. for a 350 micron wavelength. For longer wavelengths the allowable distance is much greater.

[1] NRAO

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Last modified: 09 December, 1999