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RE: "Precision" Antenna Pointing

Hi Tom

Yes, a neat idea using mirrors to check out focal point. I'm finding out
some really clever tricks on the BB relating to the topic of calibrating the
position of antennas. Now if the weather would just warm up...

Sorry about the e-mail getting lost... I was away in Edinburgh last week,
and I sent the mail on my Nokia Communicator phone which I'd just had to do
a quick reconfiguration on since a cold reset... hence the send date of 1
Jan 1980.

73 Howard G6LVB

-----Original Message-----
From: Tom Clark (W3IWI) [mailto:tac@clark.net]
Sent: 13 January 2001 20:31
To: howard@hanlincrest.co.uk
Cc: Louis A. Mamakos; Clifford Buttschardt; Dave Guimont; jeff w
griffin; amsat-bb@AMSAT.Org
Subject: Re: [amsat-bb] "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

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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