Re: beamwidth vs diameter

[T3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Tue, 10 Jun 1997 12:12:25 -0500
>From: T3D Bob Wier <wier@xxxxxxxxxxxxxxx>
>Subject: beamwidth vs diameter

>This question has come up in a satellite tv list, but the optical principals
>are the same, so I thought I'd ask here...

>...The question has come up - suppose you set up say four 18" dbs dishes
>and install c-band electronics taking the combined signal from all
>of them additively. Would that work?

>My first impression was that the main limitation would be beamwidth
>since it would be so broad at c-band freqs that you'd be getting
>multiple satellites - the reduced gain could be made up by using
>higher amplification factors  but the broader beamwidth produces
>lower s/n so that  would be the main limitation.

>But then, I remembered that multiple small optical (and I presume radio,
>as in the VLA) units can be used to get effectively a very narrow beamwidth,
>although the actual physical surface area is very small compared to
>effective surface area (ie, diameter) of the virtual mirror/dish.

>I can't seem to reconcile these two factors which seem to be
>operating in opposite directions. 

>Any thoughts?

The signals are combined *interferometrically*, to get greater effective
aperture (and directionality) than you would get with a single small
dish. Note that the overall gain is not necessarily as great as it would
be with a single large dish at the "effective aperture". Another nifty
trick is phased arrays, where there are multiple antennas, not all the
same distance from the source.

As you say, similar tricks can be used in optics. The Multiple Mirror
Telescope (MMT) was built with six parabolic mirrors (and the signals
were optically combined. (I think they later replaced those six smaller
mirrors with one large mirror, to improve performance). The Keck observatory
in Hawaii was designed for optical combination of signals from two identical
large telescopes (via and underground tunnel) for extremely high resolution
in one dimension.

The folks at JPL understand this stuff much better than I do.

John R


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