On Tue, Apr 12, 2011 at 3:49 PM, Bob Bruninga <[email protected]> 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 <[email protected]> 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 >> >> _______________________________________________ >> Sent via [email protected]. Opinions expressed are those of the author. >> Not an AMSAT-NA member? Join now to support the amateur satellite program! >> Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb >> > > > -- Mark L. Hammond [N8MH] _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
