[amsat-bb] Re: Verticals on FM sats
ve9qrp at gmail.com
Fri Nov 19 04:50:41 PST 2010
On Thu, Nov 18, 2010 at 9:28 PM, John Geiger <aa5jg at fidmail.com> wrote:
> I apologize if this topic has been covered recently, but I am asking again. I am in the middle of redoing antennas, and am looking to keep something up for the FM satellites. How well will a 1/4 wave dualband groundplane work on the satellites? I remember reading a review in QST in the past year or so about a dualband J pole,and the reviewer said he got into AO27 and AO51 just fine. What can I expect out of a 1/4 wave antenna?
> 73s John AA5JG
Hi, John --
If you picture the pattern of a typical 1/4 wave antenna, you will
recall that the it looks a bit like a half-bagel lying face down on
the kitchen counter. This is super for terrestrial work, where we want
to have as much power as possible going out to the horizon, because
the other, earth-bound stations are to be found at that low elevation.
But such an antenna improves its power on the horizon by 'stealing'
some from higher elevations, and once you think about transmitting to
or receiving from a station up 20 degrees or more, say, you'll find
that you're working with much less power. And once you're, say, 70
degrees from the counter-top, well, with an ideal 1/4 wave, you're
putting out no power (and receiving none) whatsoever. (In reality, its
not that bad, but its pretty darn bad.)
For this reason, a standard 1/4 wave (or J-pole) isn't ideal for
satellite work. It will certainly work, especially in the elevation
band where the path losses (which decrease with elevation) and losses
due to your antenna pattern produce a maximum signal. It's great fun
to listen for this effect, and a bit easier to observe with a CW
signal and s-meter. (You can treat a FM bird's signal as an FM signal
just by switching modes, of course.)
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
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.
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