archive: Re: SETI Theoretical question

Re: SETI Theoretical question

Dr. H. Paul Shuch ( (no email) )
Sat, 19 Sep 1998 09:40:25 -0400

At 11:07 AM 9/18/98, R.J. Fear wrote:
>Hi list,
>
>Here's my question: Is it possible to apply sustained wiggle to the angle
>of the feed of a dish arrangement and widen the pickup area? A guess
>looking at the formuli is that the gain may drop in proportion to the
>deviation of the feed from center.
>
>R.J. Fear
>

The technique is called nutating the feed. It is used in radar systems to
scan a small area of sky repeatedly. The idea is it's easier and cheaper
to move the feed around than it is to move the whole dish. The beamwidth
of the dish doesn't change (nor does the gain, appreciably). Only the spot
on the sky moves around.
Nutating feeds are easiest with Cassegrain antennas (that is, those using
both primary and secondary reflectors). The convex secondary reflector can
be mounted just slightly off-center, and then rotated along its axis of
(a)symmetry. Since it's only a reflector that has to be motorized, no
flexible coax or waveguide is required (that is, the LNA or PA remains
stationary). If you ever see a big dish with its secondary reflector
spinning around continuously (those of you in the Bay Area can see one just
off the Bayshore, at the Sunnyvale Air Force Station between Lockheed and
Moffit Field), it's probably a nutating feed.
I see no application of this technique to SETI, but it was an interesting
question!

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H. Paul Shuch, Ph.D. Executive Director, The SETI League, Inc.
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