[amsat-bb] Re: satellite average elevation
Edward R. Cole
kl7uw at acsalaska.net
Tue Apr 12 13:30:11 PDT 2011
Your discussion has prompted me to "throw up" a yagi (2m7) quickly in
anticipation of ARISSat-1. Viewing ISS from Alaska is much
simpler: Just point due south on the horizon as ISS rises no more
than 15 deg and usually half that angle. Living near 61 deg N
latitude makes the ISS 51 deg maximum sub-satellite longitude very
low in the southern sky and at maximum range.
I even have a preamp to use. Hoping to capture telemetry.
73, Ed - KL7uW
At 11:49 AM 4/12/2011, Bob Bruninga 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.
>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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73, Ed - KL7UW, WD2XSH/45
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
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