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"Precision" Antenna Pointing (Was Ant. Comparisons)



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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