Bob etal, 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 >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 <bruni...@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 > > > > _______________________________________________ > > Sent via AMSAT-BB@amsat.org. 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 > > > > > >_______________________________________________ >Sent via AMSAT-BB@amsat.org. 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 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 144-1.4kw, 432-100w, 1296-testing*, 3400-winter? DUBUS Magazine USA Rep dubus...@hotmail.com ====================================== _______________________________________________ Sent via AMSAT-BB@amsat.org. 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
