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 [email protected] wrote:
What would be the better antenna: moxon or a turnstile?
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