What would be the better antenna: moxon or a turnstile?
Hi,
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. :)
Regards, Rico van Genugten PA3RVG
On Mon, Dec 11, 2017 at 3:37 AM, KD4ZGW kd4zgw@gmail.com wrote:
What would be the better antenna: moxon or a turnstile?
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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@gmail.com wrote:
What would be the better antenna: moxon or a turnstile?
Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
participants (3)
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Bob -
KD4ZGW -
Rico van Genugten