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
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 [email protected] ====================================== *temp not in service