At 09:10 AM 11/19/2010, Robert Bruninga wrote: > > ... a typical 1/4 wave antenna... is super for > > terrestrial work, where we want to have as much > > power as possible going out to the horizon... > > but... from a station up 20 degrees or more, say, > > you'll find that you're working with much less ... > > And, say, 70 degrees... with an ideal 1/4 wave, > > you're putting out no power (and receiving none) > > (In reality, its not that bad, but its pretty darn bad.) > >I think the essence of what is being said is relatively correct >individually, but on closer inspection I think this is mixing >apples and oranges. What is said is true for *gain* omni's, but >not really true for the 1/4 wave vertical. In fact, the 1/4 >wave is about the best and simplest omni antenna for satellites. >Please see the detail explanation > >http://aprs.org/rotator1.html > >The argument being presented above *does* apply to a *gain* >verticla omni. Yes, that is NOT good for satellite work because >it does as stated, concentrates gain on the horizon and >drastically falls off at higher elevation. So that is why we >say "omnis" are not good for satellites. Because almost >everyone uses a *gain* omni. > >But the 1/4 ground plane antenna does not concentrate all of its >energy on the horizon and is why most people will not use it for >terrestrial work because too much of it goes out at higher >elevations. And even though it does drop off by more than 10 dB >at high angles above 60 degrees, one has to remember that the >satellite is 10 dB closer at that high angle! So it still works >great. AND the amount of time that a LEO satelite is above even >50 degrees is only 2% of all the access time. Nothing at all to >worry about. > >See the plot of gain on the above web page. It shows that a 1/4 >vertical has nearly constant gain for a satellite from about 10 >degrees up to over 70 degrees because of this range-gain. Of >course below 10 degrees the satellite is as much as 3 db further >away and hence weaker and most satellite link budgets were not >designed to operate with such 0 dB gain omnis AT the horizon. > >So, the 1/4 vertical is very hard to beat for a simple omni >satellite antenna. And by the same rationale, the terrestrial >gain omni is NOT. SO watch out for apples and oranges >comparisons... > >Bob, WB4APR
Just a note that I used a 19-inch mag-mount whip on a square sheet of steel sheetmetal on my roof to copy telemetry on AO-51 when it was first launched. This was UHF so the whip was working as a 3/4 wave vertical. The important part of that is that the 432-MHz preamp MUST be installed very near the antenna (I had about 10-foot of RG-58 as the standard cable for the magnetic base). If you are planning to use it for up and down link then you need a diplexer to separate the two frequencies and isolate the preamp from the 2m transmit signal. Since typically all you need is about 5w that is not difficult. 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 144-800*w, 432-100w, 1296-testing*, 3400-winter? DUBUS Magazine USA Rep dubus...@hotmail.com ====================================== *temp not in service _______________________________________________ Sent via amsat...@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
