[asac][almanews] ALMA Memo 457 Released

[Stacy Oliver]

ALMA MEMO #457

Standing Waves for ALMA : simulations of various tangent cones on the subreflector

A. Bacmann (ESO), S. Guilloteau (IRAM/ESO)

2003-05-24

Keywords: standing waves, subreflector, cone

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.
http://www.alma.nrao.edu/memos/html-memos/alma457/memo457.pdf

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