[amsat-bb] Re: Verticals on FM sats
Edward R. Cole
kl7uw at acsalaska.net
Fri Nov 19 11:28:09 PST 2010
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
>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
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 dubususa at hotmail.com
*temp not in service
More information about the AMSAT-BB