[amsat-bb] Re: Verticals on FM sats

Fri Nov 19 12:53:57 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. 

Actually, although a true 1/4 wave over an infinite ground plane
does have 3 dB gain over a dipole, very few people use an
infinite ground plane and so I consider the 1/4 "ground plane"
(with 1/4 radials) and 1/2 wave vertical dipoles and J-poles all
to be non gain omnis.  They basically have 0 dBd gain, and my
comments are pretty much still valid I think.  So I think a
J-pole also makes a pretty good omni antenna for satellites.
(Better than all other terrestrial "gain" antennnas.

When I refer to terrestrial omni "Gain" antennas, I am talking
about the 5/8ths wave (which has a complete null at only 15
degrees above the horizon) and other larger terrestrial gain
antennas up to 6 and 9 dBd gain.

Hope that clarifies it.
Bob, WB4APR

(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
> VE9QRP
> 
> 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
> >> VE9QRP
> >> --
> >> http://ve9qrp.blogspot.com
> >>
> >> _______________________________________________
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of
> >> the author.
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> >
> 
> 
> 
> -- 
> http://ve9qrp.blogspot.com
> 