[amsat-bb] Re: Verticals on FM sats

[Robert Bruninga]

> ... 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


> What would be an ideal shape for our 'omnidirectional'
satellite
> antenna? Let's have a muffin instead of a bagel, lop its top
off and
> place that on the counter instead of the bagel. Now we have
increased
> power at the low elevations, but still some power at the
higher ones.
> Note, we don't want a situation where the power is all going
straight
> up, because satellites spend a good deal of time in the low
angles
> (depending on where you are). We just want to 'fill in' the
bits that
> we lose from our bagel pattern.
> 
> There are many designs that aim to provide something like this
muffin
> pattern. You can make your vertical longer than 5/8 wavelength
(the
> 19" at 440, e.g.); you can tilt the 1/4 wave vertical at
around 20
> deg. from perpendicular to its ground plane; you can use
lindenblad
> arrangements to circularize and redistribute. I have a 2m
qadrifilar
> helix antenna that does its job nicely.
> 
> One terribly important point on 70cm is that you have a low
noise
> preamp connected to the antenna on the mast, not in your
shack. I have
> a $5 70cm 1/4 wave groundplane that I made out of a female
> N-connector. I would much, much rather use it with my ARR
preamp than
> a 8 element yagi without the preamp. Way more fun. The reason
for this
> with omni antennas is that we are having to distribute all the
> 'receiving power' over all the elevation angles, as well as
the 360
> degrees of azimuth. In other words, your muffin has to be
smaller in
> diameter than your bagel half, because both have to have the
same
> total volume.
> 
> Finally, your question suggests that simpler antennas, like
1/4 wave
> groundplanes, are not necessarily easy to use on SSB/CW birds.
I find
> the contrary, especially on CW. FM satellites are easier
because they
> don't require as frequent tuning due to doppler shift, and
because
> many hams already have the equipment needed to operate them.
But they
> aren't 'easier' in the sense that their signals are easier to
hear
> with simple equipment. Heck, with any sort of antenna and an
low-noise
> preamp you'll hear the CW beacon of HO-68 from horizon to
horizon.
> 
> I hope you will forgive me if this reply was aimed at the
wrong level,
> and I wish you all the best in your satellite station
building.
> 
> 73, Bruce
> VE9QRP
> -- 
> http://ve9qrp.blogspot.com
> 
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