[amsat-bb] Re: Verticals on FM sats

[Edward R. Cole]

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
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