Satellite Antennas
Satellite Antennas Part 1 by GM4IHJ
Looking at the possibilities for HF Satellite antennas which can be used
to listen to RS10 and transmit/receive to and from RS12 , soon convinces
the experimenter that there is no such thing as a perfect antenna. At
GM4IHJ the following have been comparison tested - Long Wire, Vertical
with counter poise, Sloped dipole , Horizontal dipole , Crossed dipole
and, 3 element Yagi beam. They all work - more or less, so it depends on
what the user can afford and fit.
The Long wire is cheap and easy to install but it is highly directional
and can miss a great deal unless you finish up as I did pointing it at the
pole from where I get most DX. The Vertical with or without a counterpoise
is much less effective, its high angle of take off is simply the opposite
of what you want for satellites. It works best when the sat is overhead ,
where you need least gain because the sat is very close to you. By
contrast it is least effective at low elevation when you want max gain to
catch the weak DX on your horizon. Many Amateur Radio handbooks flatter
the vertical by showing graphics of the ionosphere which are hopelessly
out of scale . One RSGB classic shows the ionosphere around 3000 kms high
instead of its true 300 km. This sort of nonesense makes any antenna look
good.
The dipoles are much better. A crossed dipole was perfect in 1976 for
Oscar 6, but a noisy micro computer came to live next door in 1985 , so
its lack of directivity became a problem rather than an asset. So since
then it has given way to a sloped dipole on the chimney as far away as
possible from the offending QRM. The sloped dipole being preferred to a
horizontal dipole because of its all round view.
But for the real DX on the horizon there is nothing to beat a 3 element
beam, if you can afford it and have somewhere nice and high to mount it .
Even without a rotator, fixed pointing north for most of my polar DX it
gets more signal and less QRM, covering the Arctic from about 330 to 030
azimuth, with if necessary the possibility to switch to the sloped dipole
for other bearings outside this arc. With the beams high gain you get
enough signal to follow your favourite satellite through the regular fades
and dips in signal strength due to Faraday rotation. Although Faraday at
10m is much faster than at 2m ( maybe 16 to 20 fades per orbit pass
horizon to horizon as opposed to 3 or 4 much longer more annoying fades on
2m ).
But having said all that if you can put your beam up on a tower and
control its rotation through 360 degrees you probably have the nearest to
a perfect answer you are likely to get - unless you are like IHJ who in
the excitement of chasing DX often forgets to move the beam. At which
point a switch over to the sloped dipole usually relocates the sat , and
also provides a handy feature when satellites appear from bearings which
are nowhere near the anticipated Great Circle path.
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