[amsat-bb] satellite average elevation & new birds

[Ted]

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: amsat-bb-bounces at AMSAT.Org [mailto:amsat-bb-bounces at AMSAT.Org] On
Behalf Of Bob Bruninga
Sent: Tuesday, April 12, 2011 8:23 AM
To: amsat-bb at AMSAT.Org
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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