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



I have a four foot dish that I use for 1.7 GHz LEO weather satellite
HRPT data collection.  I wrote a program that moves the antenna over
an area which is plus or minus 20 degrees from the current solar
position in two dimensions.  I record the noise level at each one
degree point and then do a 3D plot of the antenna pattern.  The
peak of the plot shows exactly how much pointing error occurs.
This method would work nicely for ham use and tells a lot about the
antenna performance.

Richard Allen, W5SXD

Steve Diggs, W4EPI wrote:

> 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.
>
> Regards,
> Steve Diggs, W4EPI
> Area Coordinator - Atlanta, GA
>
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