The amplitude of standing waves are calculated in various subreflector geometries (subreflector with absorber, subreflector with aperture to accommodate calibration loads). The ability of scattering cones to reduce the standing waves are compared for several types of cones (straight cone, inverse straight cone within the aperture, curved cone) and cone sizes. At 3 mm, the peak-to-peak ripple due to standing waves is around 0.7% for a subreflector with absorber, which can be reduced by a factor of 9 (~0.08%) by fitting a straight cone on the blockage zone. The ripple is on the other hand increased to 1.4% when the subreflector presents an aperture in the blockage zone. It can further be reduced to 0.8% by fitting an inverted cone inside of the aperture. Using a curved cone instead of a straight one can only moderately improve the standing wave level. We recommend implementing a straight cone covering somewhat more than the blockage zone. Further optimization of this parameter requires more elaborate software and should be carried out jointly with the Receiver and Antenna IPTs.
View a pdf version of ALMA Memo #457.
Last modified: 2003-06-09
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