On Tue, Apr 12, 2011 at 3:49 PM, Bob Bruninga bruninga@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@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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