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Re: "Precision" Antenna Pointing





Howard, G6LVB wrote:

> I tried to send the following note to amsat-bb yesterday but I
> don't think it made it...
>
> Two solutions: place a small non-flammable screen at the
> focal point of a dish on the az/el system and calibrate it by
> pointing the system at the sun (eg Nova will do this). If the
> dish is at least marginally reflective to light, the sun's image
> should appear in the middle if you're correctly calibrated.

Howard-- here is an even easier idea that will also verify that you have
the focus correct.

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 reflectors
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.

By the way -- I almost missed seeing you message. I normally sort mail by
date/time stamp and you message, received Jan.10th had an origination time
stamp reading
>      Date:  Tue, 1 Jan 1980 2:01 +0000
Perhaps you computer's clock has a residual Y2K problem?

73, Tom

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