[amsat-bb] Re: satellite average elevation
Mark L. Hammond
marklhammond at gmail.com
Tue Apr 12 11:39:55 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! I
know it would be where my array is at currently.
So, the 12-15 degree "optimum" assumes a clear view to the horizon...right??
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 may
> 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 to
> +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 needed
> (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 the
> angle at which the gain on the horizon LOSES say less than 1 dB. Note, this
> 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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Mark L. Hammond [N8MH]
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