[amsat-bb] Re: Antenna setup

Robert Bruninga bruninga at usna.edu
Fri Mar 12 05:41:08 PST 2010

> About 15 degrees elevation is usually considered optimum.
> > ...and the booms angled 30 to 40 degrees... up

Yes, NEVER higher than 15 degrees.  You don't need the gain much
above the horizon where the satellite is up to 10 dB CLOSER.
You DO need the gain on the horizon where the satellite is 10 dB
further away.  See the plot on www.aprs.org/rotator1.html  Pay
particular attention to the scale drawing of a LEO orbit to the
ground station.

Tracking a LEO satellite is like sitting 100 yards from an
infinitely long east-west railroad track and using your beam to
communicate with a train.

1) You point your antenna almost due west (-15 degrees or so) so
that the infinite distance is still in the main beam PLUS all
the track almost up to where you are.  Then the approaching
train is ALWAYS in your main beam until it is 100 yards away
(where it is now 45 degrees from your location).  But then it is
so strong, you can hear it on a wet noodle.  

2) For those three seconds as it goes by, it is not in your main
beam, but it is so strong who cares.

3) Swing the beam  now due east (minus about 15 degres) and
again, for the entire rest of the pass, the train is in your
main beam.

If you put your beam in either case 30 or 40 degrees away from
the track, you would miss the MAJORITY of the time the train is
traveling, because the MAJORITY of the time, it is far far away
and not in your main beam.

We must dispell any literature that even hints that an angle
above 15 degrees is good for LEO satellites (of course there are
always exceptions)... For example, you live in a HOLE!  Then it
makes no sense to set your antenna at 15 degrees if that is
still pinting into surrounding dirt.  Common sense then applies.


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