[amsat-bb] Re: satellite average elevation
bruninga at usna.edu
Tue Apr 12 08:22:30 PDT 2011
> we used a horizontally polarized yagi fixed at 30
> degrees above the horizon. That worked very well..
Thanks for the confirmation. Yes, elevation rotation is simply not needed
at all for LEO spacecraft and modest beams. A mild, fixed tilt modest beam
is just perfect.
But, the "30 degree" angle myth is very pervasive throughout amsat, whereas,
the optimum angle is more like 15 degrees.
A 30 degree up-tilt gives up too much gain (-3 dB!) on the horizon where
signals are weakest and where satellites spend most of their time, and puts
the gain in an area of the sky where the satellite is already 6 dB stronger
and is rarely there (giving you max beam gain where you need it least).
If you look at the sketches on the web page, the optimum angle is more like
15 degrees up-tilt. It preserves max gain on the horizon within 1 dB (where
it is needed most) and focuses the breadth of its gain on the area of the
sky where the satellites spend something like 95% of their time. For the
missing 5%, the satellite is right on top of you and almost 10 dB stronger
without any beam at all. Oh, and the 15 degree up-tilt beam is also perfect
for Terrestrial operations as well.
See the sketch on: http://aprs.org/rotator1.html
In some future life, if we ever get back to HEO's and huge OSCAR arrays,
then elevation rotors have a place. These high-gain beams have such narrow
gain patterns, that higher precision tracking is a must. (Though it is
complete overkill for LEO's).
Using these OVERKILL arrays for LEO's adds significant complexity to LEO
operation requiring higher precision tracking, elevation rotors, better
timing, fresher element sets and automated operation.
Using a TV rotator and 15 degree fixed tilt beam is much more forgiving...
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