[amsat-bb] Re: satellite average elevation
bruninga at usna.edu
Tue Apr 12 12:49:28 PDT 2011
> Okay---but the 12-15 degree argument _assumes
> that the station has a view "to the horizon"
> that isn't tainted by trees, hills, and houses.
> In those circumstances, 30 deg might well be
> the better choice!... So, the 12-15 degree
> "optimum" assumes a clear view to the horizon...right??
Yes. Correct. But if one cannot see nor hear below 20 degrees, such a
station is missing out on almost 70% of all the times a LEO satellite is
above the horizon anyway. In that case, then there is little justification
for even having a beam, motors, tracking, and timing and a PC at all.
At 30 degrees and above, signals from LEO's are 5 dB or more stronger than
at the horizon, and a simple 1/4 wave whip over a ground plane (with a
pre-amp) will just about hear everything with no moving parts or tracking.
If you want even more gain, make the whip 3/4 wavelength long (still 19.5"
at UHF) and get nearly 7 dB antenna gain in a cone above 30 degrees. That
plus the 5 dB closeness gives you at least 10 dB gain over what a vertical
will hear of a satellite on the horizon.
But you are correct. If you really want to have a beam and you really want
to have motors and tracking, and PC's and updated elements, etc, then I DO
AGREE, tilting up to have the main lobe just over the tops of the visible
horizon is an improvement.
TO be clear. I am not arguing against a specific angle (say 30) just
because its 30, but I am arguing against how the choice of that angle is
presented. If it is presented in the absence of an appreciation of the
significant 4 to 1 difference in signal power over the angles from 30 down
to 0.. or does not reference the 1 to 4 times increase in VISIBILITY
DURATION over that same drop in angle, then I think it is worth pointing
I can see now that I should add a plot of visibility time versus angle as
well as the path-gain vs angle on the web page:
http://aprs.org/rotator1.html By the way, that is an old page, and you can
ignore the "how to build" a TV rotor controlled station, since no softare
currently drives it except mine (obsolete). But the information on the
geometery of LEO passes is what most satellite newbee's overlook.
On Tue, Apr 12, 2011 at 2:24 PM, Bob Bruninga <bruninga at usna.edu> wrote:
>> As I said, in the "goode olde dayes" we used
>> 30 degree up tilt and it worked well...
>> 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!
> Sorry to sound like I am quibbling... but that last sentence implies the
> idea of an equal "trade off". But the tradeoff is not equal at all and
> be missing the point here.
> A LEO satellite pass does not need gain at "higher angles" because the
> satellite is by definition 2 or 3 times closer to the ground station (+6
> +9dB stronger). But one does need the gain at lower angles where the
> satellite is much further away.
> An up-tilt of 30 degrees is throwing away excess gain where it is not
> (high angles) at the expense of low angles where every single dB -is-
> needed. So there is no real tradeoff... A lower angle (about 15 degrees)
> is more-or-less optimum for LEO's with fixed tilt and modest gain beams.
> To actually quantify the exact best angle (which will depend on the actual
> beam's own beamwidth), it is simply to up-tilt the antenna no more than
> angle at which the gain on the horizon LOSES say less than 1 dB. Note,
> is not half the published "antenna beamwidth" which is usually a "3 dB"
> beamwidth. It is much less than that, less than half the 1 dB beam width.
> You can measure this by setting the beam no higher than the upangle that
> loses less than 1 dB to a signal on the horizon....
> Something like that...
> Bob, WB4APR
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