Elevation angle dependence of the SMA antenna focus position
2006
We report the measurement results and compensation of the antenna elevation angle dependences of the Submillimeter
Array (SMA) antenna characteristics. Without optimizing the subreflector (focus) positions as a
function of the antenna elevation angle, antenna beam patterns show lopsided sidelobes, and antenna efficiencies
show degradations. The sidelobe level increases and the antenna efficiencies decrease about 1% and a few %,
respectively, for every 10° change in the elevation angle at the measured frequency of 237 GHz. We therefore
obtained the optimized subreflector positions for X (azimuth), Y (elevation), and Z (radio optics) focus axes at
various elevation angles for all the eight SMA antennas. The X axis position does not depend on the elevation
angle. The Y and Z axes positions depend on the elevation angles, and are well fitted with a simple function for
each axis with including a gravity term (cosine and sine of elevation, respectively). In the optimized subreflector
positions, the antenna beam patterns show low level symmetric sidelobe of at most a few%, and the antenna
efficiencies stay constant at any antenna elevation angles. Using one set of fitted functions for all antennas,
the SMA is now operating with real-time focusing, and showing constant antenna characteristics at any given
elevation angle.
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