[amsat-bb] Re: satellite average elevation
gzook at yahoo.com
Tue Apr 12 09:40:12 PDT 2011
As I said, in the "goode olde dayes" we used 30 degree up tilt and it worked well for all passes including overhead passes. Lessening the up tilt may increase the gain for the lower angle passes but will also decrease the gain on the higher angle passes. So, it is a "trade off" no matter what you do!
Of course, back in the early 1970s we did not have computer simulations of antenna patterns, etc. readily available and experimentation showed that 30 degrees worked the best.
In terms of computers, if one had use of a computer what was generally done was to take the "keps" of a particular satellite (then primarily OSCAR VI and OSCAR VII) and make tables showing the azimuth and elevation for passes at various equator crossings giving a relative time for AOS and LOS. You just added this time to the actual "clock" time and adjusted your rotor direction to correspond with this.
During the early 1970s, members of the Richardson Wireless Klub (K5RWK - Richardson, Texas) had access to main frame computers at Collins Radio, Texas Instruments, and Electrospace that were used to compute the information. This information was compiled into a set of tables for OSCAR VI and OSCAR VII which was applicable to a radius of around 100 miles from the Dallas / Fort Worth area.
Using this rudimentary information I was able to acquire the 1st Satellite 1000 award issued in the 5th call area and the 3rd award issued anywhere. Also was able to acquire the achievement recognition number 32. Many of today's operators are not aware of the efforts that it took "back then" to work the satellites. Computer control of antennas, doppler, etc., make things much easier these days.
AMSAT 239/LM 463
--- On Tue, 4/12/11, Bob Bruninga <bruninga at usna.edu> wrote:
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).
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