[amsat-bb] Moxon vs. Turnstile

Rico van Genugten rico.van.genugten at gmail.com
Mon Dec 11 12:32:22 UTC 2017


It depends on the type of turnstile and your application. I'm assuming that
you want to use the antenna in a fixed position pointing upwards, e.g. not
on a rotor. In this case you want the gain in upward direction to be as
_low_ as possible, since this means that you will have more gain at lower
elevations, where you need it most.

A turnstile antenna consisting of crossed dipoles and crossed reflector
elements has maximum gain in upward direction, but the gain and impedance
depend on the distance between radiating and reflecting elements. At about
37 cm or 14,5 inch spacing the impedance is 50 ohm, but the gain is way too
high to use as a non-directional antenna. Increasing the distance means
decreasing the gain, but also increasing the impedance, so you need some
kind of match to get a good swr.

A turnstile antenna consisting of crossed dipoles over a ground screen
gives a better pattern than one that uses reflector elements, but can be
more impractical. The gound screen needs to be fairly large in terms of
wavelengths (about 2 minimum), but if you can mount the antenna on the
ground this might not pose a problem. At a dipole hight of about 70 cm
(27,5 inch) above the ground screen you can get an impedance of about 100
ohms per dipole and a good pattern (yes, I did quite some simulations in my
quest for the ultimate stationary antenna. :)), which with some trickery
can give you a good 50 ohm match when the dipoles are combined, see the
eggbeater design which also shows an impedance of 100 ohm per loop.

A moxon antenna has quite some gain in upward direction and therefore less
gain at lower elevations, therefore I think it is unsuitable for stationary
use unless you are only interested in passes above 50 degrees, which rarely
happen and if so, for only a very short time. It is more suitable to be
mounted at a fixed elevation as a small directional antenna if you have
azimuth control but no elevation control. I have recently built a crossed
moxon mounted at 25 degrees on a simple azimuth-only rotator. Tests
conducted from the yard are promising so far. I'm putting it on the roof
when the weather improves, I will send an update to this list when I have
some results.

Then there is the option of building/buying an eggbeater antenna, but in my
opinion it is bulky, difficult to build and does not offer many advantages
over a turnstile.

I've experimented a lot with stationary antennas, but didn't have a lot of
success so far. The following properties of most stationary antennas are
hard to overcome:
 - Most of the satellites currently active are linearly polarized and
unpredictable in orientation, so a ground-station antenna should be
circularly polarized or have changeable/switchable polarization. Most
stationary antennas are circularly polarized in upward direction, but
horizontally polarized at the horizon, resulting in deep signal fades when
the sat is at low elevations where the signal is lowest to begin with.
 - Intelligibility of a signal depends on the SNR or Signal to Noise Ratio.
Omnidirectional antennas have lower gain and therefore lower signal levels,
but due to the omnidirectionality also have a higher noise floor,
decreasing the SNR even more than gain figures alone would indicate.

That doesn't mean stationary antennas are not useful:
 - On linear sats you don't need the SNR required for FM-sats, so you might
have more luck with those.
 - VHF downlinks are much easier to hear than UHF ones (for this and the
above reason you can forget about SO-50)
 - As an uplink antenna they could be fine, usually hearing the sat is much
more difficult than getting in.
 - FO-29 has circularly polarized antennas and therefore does not give as
much fading with an antenna that is linearly polarized at low elevations
 - AO-85 and AO-91 have pretty strong downlinks, at higher elevations you
will hear them, at lower elevations you might too if you are lucky with
their orientation.
 - Use good quality coax and connectors, preferably the least possible.
Don't use RG-58 for anything over 1 meter (40 inch).

Phew, maybe that is a bit more information than you ask for, good luck
experimenting. :)

Rico van Genugten

On Mon, Dec 11, 2017 at 3:37 AM, KD4ZGW <kd4zgw at gmail.com> wrote:

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