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Re: "Precision" Antenna Pointing (Was Ant. Comparisons)


The optical boresighting method is good, especially on small angular
objects like Venus.  However, your actual RF beam may not point exactly in
the same direction.  One way to determine this is by observing sun noise
and comparing your bearings against where your tracking software says the
sun is.  Now you can make small adjustments to your feed position to bring
the RF and optical pointing into agreement.

Many eme'rs also place a video camera on their system so they may visually
track the moon.  Of course that won't help track a small satellite.  ISS?

Another plus of tracking the sun is you can measure the RF performance of
your dish by comparing measurements of the amount of sun noise vs cold sky.
 This can provide you with dish gain or system noise figure.  At 10 GHz
there is even enough thermal noise from the moon for measurement.

I have written a program to calculate these parameters from solar
measurements and is available on the SETI-League web site under
software/spreadsheets.  The program is called G-T.


>From: "Steve Diggs, W4EPI" <w4epi@mediaone.net>
>The end of this thread centered around tools to verify the pointing accuracy
>of beam antennas. Alan NE1H and I have been using a method that is both
>simple, easy to carry out, and produces impressive results. Bottom line: use
>the natural objects in the sky, your antennas, and your tracking software as
>a cross-check for pointing accuracy.
>We first heard of this idea last summer when a similar approach centered
>around using shadows at noon. Not so easy to carry out. However, the Moon,
>Venus, Saturn, and Jupiter are easy targets to use. The idea is to find the
>natural object in the late afternoon sky. Some daylight is beneficial, so
>that you can clearly see the alignment of your antennas. Set your tracking
>software (we use Nova for antenna control, mostly) to track the respective
>object. Go out and find the point on the ground where you can bore sight the
>beams towards the sky object and verify the pointing accuracy. Dial in the
>azimuth and elevation error into the offset values in Nova's rotator setup,
>then double check against multiple objects on multiple occasions. Using this
>method you can adjust your pointing accuracy with a high degree of
>confidence. The only weaknesses we can find in this approach is that rotator
>deadband and parallax error in sighting tend to reduce your ability to
>refine the pointing. The deadband acts to decrease the actual number of
>correction commands needed to follow an object across the sky, therefore
>reducing wear and tear on the rotor. Bad news is that says that your antenna
>is always going to lead or lag the object being tracked. Parallax error is
>evident when attempting to line up the shaft of the beam with the object in
>the sky. The beams don't point exactly at the object in the sky, rather they
>are on the same plane as the object being tracked with perfect, so you can
>only get so close.
>All in all, this method has worked extremely well for us. You have to find a
>natural object that is best for you, considering  its normal path across the
>sky, and the objects in your yard, general area, etc. that restrict your
>ability to bore sight your beams. Venus turned out best for me, as it
>appears in the western sky at least 1/2 hour before sunset, and I can get
>back at least 50 yards from the tower and bore sight at it, even at
>elevations as low as 20-25 degrees. I then double check against the Moon.
>This is one skill someone with AZ/EL control and a high-gain S-Band antenna
>is going to need to know how to do <very> well to make your system a top
>performer. If you can make your system accurately track a natural object, it
>will track an artificial satellite just as well.
>If anyone has a better approach, I would like to hear about it.
>Steve Diggs, W4EPI
>Area Coordinator - Atlanta, GA
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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