I'm kind of looking for an update from Bob, but can't find his email right now...
But the question is, in view of what appears to be some renewed interest in working the new cube sats, et al, is asking Bob to comment on his earlier thoughts on using antennae at fixed elevations. For me, I'm using my Elk on a Rat Shack rotor at a fixed el per Bob's recommendations. (I'm still struggling with PCSAT32...!!!%^&*!!) but, this antenna set up is very cost effective and seems to perform pretty well.
For example, Joel Black has asked for some advice in an earlier posting. My concern is that new operators or those returning run out and spend a bunch of $$$ on a new setup. No one knows how long the current crop will last or if a new crop is in the future, so probably some caution on the Visa is warranted.
Just asking (and especially Bob)
73, Ted K7TRK
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Bob Bruninga Sent: Tuesday, April 12, 2011 8:23 AM To: [email protected] Subject: [amsat-bb] Re: satellite average elevation
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...
Bob, Wb4APR
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