[amsat-bb] Re: satellite average elevation
nss at mwt.net
Tue Apr 12 11:38:26 PDT 2011
Need to take into account also that the main lobe of a bean even flat on
the horizon the max center of the main lobe is still not also dead on
the horizon but elevated some due to ground reflections.
The Original Rolling Ball Clock
On 4/12/2011 1:24 PM, Bob Bruninga wrote:
> X-Antispam: NO; Spamcatcher 6.1.2. Score 1
>> 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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