[amsat-bb] Re: Verticals on FM sats

Bruce Robertson ve9qrp at gmail.com
Fri Nov 19 11:57:07 PST 2010

Thank you, Bob, for clarifying this. The original letter made mention
of several types of antennas, including 1/4wave dualband and jpole,
both of which are, in your terms below, gain omnis at 440. (Jpole
being a 1/2wave antenna, e.g.)  I first wrote the letter with these
distinctions kept intact, but then, in an attempt to keep things
simple, used 1/4 throughout, which meant that the claims about 1/4
elevation patterns were not accurate. However they would be accurate
for the "1/4 wave vertical" that John originally mentioned, at least
on 70cm.

So for the purpose of practical advice, I hope we can agree that:
a) a *true* 1/4 wave gp vertical makes a fine and inexpensive vertical
omni for satellite work. But this means that the 70cm's vertical
element should be around 18cm long. If it's longer, you're likely
getting gain, and that ain't good in this circumstance.
b) using a gain omni such as a jpole (or, likely, a multiband vertical
on 70cm) will produce the effects described in my letter below
c) with such an vertical you will occasionally suffer drop-outs at
very high elevation
d) I could have been clearer :-)

73, Bruce

On Fri, Nov 19, 2010 at 2:10 PM, Robert Bruninga <bruninga at usna.edu> 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
>> 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
>> --
>> http://ve9qrp.blogspot.com
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