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Re: "Precision" Antenna Pointing

I just got around to reading this thread now since I finally have S-band
capability using a 60-cm dish ( G3RUH special) and 2.5 turn helix.  I just
put together my S-band receive this weekend and am double checking things
like focal length.  By calculation the focal length of my dish is 18.7-cm.
What part of the feed section should be at that distance, the reflector
plate, front, middle, or back of the helix?  I currently have the reflector
plate set at the calculated focal length.

Kevin Smith

----- Original Message -----
From: "Tom Clark (W3IWI)" <tac@clark.net>
To: <howard@hanlincrest.co.uk>
Cc: "Louis A. Mamakos" <louie@TransSys.COM>; "Clifford Buttschardt"
<cbuttsch@slonet.org>; "Dave Guimont" <dguimon1@san.rr.com>; "jeff w
griffin" <kb2wqm@juno.com>; <amsat-bb@AMSAT.Org>
Sent: Saturday, January 13, 2001 15:30 PM
Subject: Re: [amsat-bb] "Precision" Antenna Pointing


> On the surface of the dish, glue a few small mirrors. Reflective mylar
> would be better/easier than glass [In the bygone days of yore with 7/9
> track computer tapes, small (~1/4" x 3/4") adhesive-backed mylar
> were used as end-of-tape sensors. If you find some they are worth gold.
> Another alternative is adhesive-backed aluminum foil tape is an
> alternative.]
> Each mirror will reflect the sun (or on a clear night the moon, or a
> distant streetlight) will reflect the target "transmitter" onto the feed
> point as a spot of light. If you move a piece of cardboard in 3 dimensions
> in the vicinity of the focal point, you will find that all the dots will
> converge at the focal point. If the spots of light don't converge into a
> single point but instead look like a comet, then either the dish axis
> doesn't point towards the target or it is warped. You can  tell what part
> of the dish is off axis/warped by shadowing the various mirrors one at a
> time to see which is causing the asymmetric spot.

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