[amsat-bb] Moxon vs. Turnstile

Bob WB4SON at gmail.com
Mon Dec 11 16:25:16 UTC 2017


Probably best to start with the issue of antenna polarization.  Most
terrestrial antennas are either horizontal (HF) or vertical (VHF/UHF FM)
polarized.  Having a polarization match is critical, as a mismatch will
cause the signal to be about 30 dB weaker.  30 dB is more than enough to
take a signal that is "arm chair copy" to one that is below the noise
level.  Your proposed Moxon could be installed for Horizontal, or Vertical
polarization.

Now lets think about satellite use.  The satellite is orbiting above us,
and frequently has some sort of spin imparted on it.  That spin may cause
the antenna polarization to change over time.  Then as the satellite moves
across the sky there can be an apparent polarization change.  If you were
talking about the Space Station, then the signal might be bouncing off of
metal parts.  Very importantly for satellites, there can be a polarization
rotation or shift imparted by the ionosphere as well (at the frequencies we
use).  The bottom line is the polarization of a satellite is often
unpredictable from moment to moment.  This is why a more optimal antenna
will be one that has circular polarization.  It doesn't matter how that
antenna appears to rotate up in space, you still pick it up down at the
ground.  Deep fades are greatly reduced as a consequence.  Your turnstile
antenna will be circularly polarized.

Next issue is omnidirectional vs. directional.  The Moxon is a kind of beam
antenna, it is directional.  Most of the signals you hear and your RF
energy will be going in the direction it is pointed.  That's a good thing
as there is no reason to send your RF energy, or listen in directions that
you know you don't need to be.  But the drawback is that you need some way
to move the beam around (a rotor).  For satellite use it gets more
complicated, as not only do you need to know what compass direction to
point it toward (azimuth), you also need to know how far up in the sky to
point it (elevation).  Beams with fewer elements (2 or 3) can be used for
satellites by pointing them at about 15 degrees above your local horizon
(say you have a small hill or houses nearby that might cause your local
horizon to be about 15 degrees (not unusual), then set your beam to about
30 degrees elevation).  Having full azimuth and elevation control is ideal,
a requirement for beams with many elements, and can be real expensive for
the rotor and controller.  Your Moxon could be used with a fixed elevation
and use an azimuth rotor only.  Your turnstile antenna will be
omnidirectional -- no rotor required.

So now to what is "best".  Best is whatever works for you and meets your
budget criteria.  It might be a directional antenna on a fixed mount like a
camera tripod that is pointing at the location the satellite will be at 1/3
of the way through its orbit.  Then you could move the antenna one time to
the point it will be at about 2/3 the way through its orbit.  I do this
during Field Day -- it works every time -- but I'm generally only trying to
make one contact.

I've been using circular polarized omnidirectional antennas for 20 years.
No mechanical rotors and expensive controllers to worry about.  Very
simple, very reliable.  But you don't get something for nothing.  They
don't have "gain" and they gradually shift from circular polarization to
horizontal at the horizon.  In practical terms, if a LED satellite has a 12
minute orbital path, I will be able to work it with excellent results for
about 4 minutes (near the top of the orbit) and marginal results for maybe
another 2 minutes.  So I get about 1/2 the path.

If it were me, I'd go with the turnstile.

73, Bob, WB4SON


On Sun, Dec 10, 2017 at 9:37 PM, KD4ZGW <kd4zgw at gmail.com> wrote:

> What would be the better antenna: moxon or a turnstile?
>
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