[amsat-bb] Re: satellite average elevation

[Mark L. Hammond]

On Tue, Apr 12, 2011 at 3:49 PM, Bob Bruninga <bruninga at usna.edu> wrote:
>> 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.

On the contrary---all the more reason to get more gain on the uplink
and the downlink with directional arrays!  It helps with the leaves
and trees,and you can work through quite a bit of material--trust me!

Mark N8MH


>
> 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
> out.
>
> 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.
>
>
> Bob, WB4APR
>
>
> Mark N8MH
>
> 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]