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EME/moonbounce polarity

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K3RW, Oct 28, 2015.

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  1. K3RW

    K3RW Ham Member QRZ Page

    I'm interested in EME/moonbounce, and I'm looking at using 2m SSB or CW for it. I don't really understand the issue of polarity though. As I understand, the polarity 'can change'. VE2ZAZ has a nifty presentation online.

    But I don't seem to get this idea of polarity. I see antennas that have polarity switch kits. M2 makes one. And M2 has circular, dual, and some other polarity (it doesn't say) that I'm guessing is horizontal, since it is a yagi.

    IF I get a circular polarized antenna, do I eliminate this 'not in the right' polarity issue? Such as, does the circular polarity essentially prevent the need to switch back and forth? I'm not sure if it is dual polarity if I have to 'know to switch it' and have to do it manually, or using a circular polarity one does both without the guesswork.
  2. KK4YWN

    KK4YWN Ham Member QRZ Page

    no circular doesn't resolve anything and it may cause more problems. you will have to manually switch. eme and weak-signal are not easy modes. you'll have to twiddle knobs and adjust things. you're shooting at a moving target with a triangular bullet while standing on a rolling ball. there is no "set it and forget it".

    this is worth studying
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Circular polarization has 2-different polarities: Right hand and left hand (clockwise and counter clockwise). In addition, once the signal is reflected from the moon's surface, the polarity is reversed! As such, having one antenna for transmitting, with one circular polarization, and another antenna for receiving, with the opposite circular polarization, is very common.

    Linear polarization, either horizontal or vertical, does not change polarization. However, "angle polarization", that is having the antenna somewhere between horizontal and vertical, usually at a 45-degree angle, does reverse polarity and, again, one would have to have 2-antennas with the angle being 90-degrees apart.

    Otherwise, when the polarization is opposite, in the "real world", there is usually around a 20 dB loss than if the polarizations are the same. In theory, the loss is infinite. However, practical applications usually show around 20 dB average with some losses being even more.

    Glen, K9STH
  4. KK4YWN

    KK4YWN Ham Member QRZ Page

    also helpfull

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    EME signals continuously change in polarization, so no given polarization is the right one other than by coincidence.

    This is one reason why most contacts take a while, waiting for peaks as polarity aligns in our favor. Nobody makes an EME contact in a few seconds.

    What most of us have done, and I was pretty active in EME in the late 70s through the late 80s, is use a whole lot of power, big and well aligned antennas, very good receivers, and clocks set to sequence transmissions to avoid 'doubling,' or transmitting while the other station is also transmitting.

    SSB EME is difficult; CW is a whole lot easier, and every EME contact I ever made was on CW. About 200 of them, over about half the globe.

    Now, JT weak signal digital modes are likely more popular but still involve time sequencing and having patience.
  6. K3RW

    K3RW Ham Member QRZ Page

    Ah--thanks everyone. This definitely makes it clearer. I'm a JT fan but can see how CW works better, given the lack of delays between tx/rx. I guess if you call CQ over and over on CW, at least you have a chance of them switching back and forth quick enough to know they aren't doing it right rather than waiting almost 2 min.

    Wish it didn't have that random chance element! Wish there was someone I knew in the Portland (OR) area that I could stand over their shoulder, but don't know anyone who does it.
  7. K6CLS

    K6CLS Ham Member QRZ Page

    no, it does not change continuously.

    Faraday rotation changes the polarization twice, on the way out and on the way back. Faraday rotation can change fast (50MHz) or slow (70cm), it can be stuck for hours and it can come and go. But that isn't continuous.

    The other change in polarization is from simple geometry. Draw it out: vertical from the left coast, reflect off the moon, comes in horizontal in EU.

    These days the hot setup is IQ+, a pair of SDRs running the same clock, one receiving from horizontal antennas, the other receiving from vertical antennas. Then with a little bit of math you can figure the actual phase angle of the received signal. that's worth 20dB, for just 3dB more (doubling) RX antennas. So that helps a lot with getting stuck by Faraday.

    Circular polarization does help, immensely, on 23cm and higher bands. But not because of Faraday. I am not sure why but the 10GHz folks are still on linear polarization for EME.

    The ARRL Handbook has a great chapter on EME.
  8. K3RW

    K3RW Ham Member QRZ Page

    Thanks for all that. Makes a lot more sense to me now. The geometry really helps that make sense.

    Sort of dumb question, but I wondered if a waveguide would help. Not sure what the polarity of that is (circular?). Probably because I"m not seeing that for 2m, tells me it is likely a bad idea.

    Looking at these M2 antennas, I see one that looks promising:

    This one looks equally good, budgetwise, but the gain isn't great.

    Really ignorant question, but I thought I read that you can run 2 feedpoints to a yagi and change its polarization. I could have misread that. If so, could I just have a switch between them to change the polarity, and not need one of these + or x shaped antennas?

    Might be cheaper to go to the moon and blast back to Earth with anything I want on a yagi!
    Last edited: Oct 29, 2015
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That one with a phasing harness system and relays, can do four different kinds of polarization (V, H, LHCP, RHCP) and is a winner. Waveguide for 2m doesn't make sense -- it would be very large, and fat coax has low enough loss on 2m.

    If the antenna needs to be pretty far from the station so coax loss would be higher than desired, a lot of guys "remote" the final amplifier and receiving preamp, placing them at the antenna (or right under the antenna), and just use regular coax and power from the shack out to the rest of the station which is in a weatherproof containment.

    CW contacts via e.m.e. are still time sequenced, at least at the start. If you work a BIG GUN under ideal conditions and signals are so strong they're literally "easy copy," some kill the timers and just operate like it was a terrestrial contact to completion. But usually they're still time sequenced, mostly to prevent doubling, which wastes a lot of time. Ditto for m.s. -- it becomes "PTT" when you catch a big burst, but otherwise both CQs and skeds are sequenced.
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    As mentioned the IQ+ equipment and software provide a direct indication of actual polarization strength and the ability to select the stronger mode. Amazing stuff.
    Last edited: Oct 29, 2015

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