EME Power Splitter Question(s)

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K5AKM, Jun 17, 2021.

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  1. K5AKM

    K5AKM Platinum Subscriber Platinum Subscriber QRZ Page

    Hello all,
    I am putting together (4) 20 elements M2 antennas with both horizontal and vertical elements for my EME station. The plan is to phase the vertical together and the horizontal together each going into separate preamps and into the shack into a dual antenna SDR for adaptive reception.
    So if I connect :
    4 antenna horiz. elements ===> 4 port power splitter 1 ---> coax relay 1 port C
    4 antenna vert. elements ===> 4 port power splitter 2 ===> coax relay 2 port C
    coax relay 1 port A ==> preamp ===> RX1
    coax relay 1 port B ===>2 port power splitter |
    |======> PA --- TX
    coax relay 2 port B ===>2 port power splitter |
    coax relay 2 port A ===> preamp ===> RX2
    So that means I need: (2) 4 port power splitters, (1) 2 port power splitter (2) preamps and (2) coax relays?

  2. WA7F

    WA7F Premium Subscriber QRZ Page

    I haven’t done this yet so I could be missing something but, I don’t understand why you will need the two port dividers. Here’s one way of wiring the system. If you Google KL7UW EME you should be able to find the full presentation.

    K0UO likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I’d be really tempted to use all four antennas at the same time, with one 4-port divider. If the coax is short or very low-loss, I’d skip the preamp (just another thing to fail when you need it most).

    Keeping the line short, like 50’ of LMR600, will keep the loss so low a preamp shouldn’t be needed; if the line has to be longer, 7/8” or 1-5/8” Heliax will keep the loss almost immeasurably low up to at least 100’ long.

    Since it’s great fun to hear your own lunar reflections (not necessary at all — just fun!) I’ve never run less than 1kW output power for EME on two meters. The power is pretty easy to achieve, and with all the $$ saved by skipping preamps and relays, the same budget would likely allow even a commercially built kW amplifier.

    The EME experience is different for each user, but to “demo” that it works to myself and visitors, nothing quite like sending a couple of Vs, switching back to receive, and hearing them come back with the delay.:)
    K5AKM likes this.
  4. WA7F

    WA7F Premium Subscriber QRZ Page

    WIK, the adaptive H + V receive is really quite impressive. At least that’s what l hear from the guys that are using it. Also, the ability to TX with H or V polarity has advantages.

    When my EME station comes together it will have at least H/V switching and ideally H + V adaptive receive.
    K5AKM likes this.
  5. K5AKM

    K5AKM Platinum Subscriber Platinum Subscriber QRZ Page

    If I am looking at this correctly (big if!) this appears the the TX portion is either Vertical or Horizontal. I thought by using a 22 port splitter that I would be able to pump a KW into both vert. and horiz...
  6. WB3BEL

    WB3BEL Ham Member QRZ Page

    I don't think you are looking at it correctly. If your Amplifier can only supply 1KW at the output, then with the two way divider
    (splitter) in the path, you would have a bit less than half of the power at each of the H and V antenna arrays. So maybe something like 480 - 490 Watts into the H array and similar into the V array. This is not really what you want. You want your full power into the correct array that matches the receiving conditions of the station you are trying to contact. Ditch the two way divider in the transmit path.

    Many people have been down this road before:

  7. WA7F

    WA7F Premium Subscriber QRZ Page

    I suppose you could do that but, I don’t think it would be effective as a single polarization. My understanding is that one or the other polarity works best at any given moment depending on the Faraday shift.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    But that constantly changes, as the reflected signal rotates in polarity. V+H full-time helps counter that effect.

    I wouldn’t want to be switching polarity via relays if I could avoid that; I can see all sorts of reasons to avoid it if possible.

    My old friend Wayne N6NB (ex-K6YNB) was an early-gen moonbouncer when it was 100% CW in the 1970s and used sixteen 3-L quads in a frame that aimed them all in the same direction but rotated on axis as well as for both elevation and azimuth, and actually had that all installed on a trailer with a crank-up tower pulled behind his camper. It could change polarity from H to angular to V by pushing a button (no relays) and worked really well. The setup was on the cover of QST and I’ve seen it up close (40+ years ago).

    It was a “mobile moon bounce system” he drove around to operate from places where nobody had an EME setup and as such was the first KL7 (Alaska) moonbounce activation ever, providing a new state to dozens of operators. He drove there from Los Angeles, just to activate Alaska!
    K5AKM likes this.
  9. WB3BEL

    WB3BEL Ham Member QRZ Page

    I think most of the people with polarization control on 144 MHz EME today are using relay based switching capability.
    You don't switch it hot in the middle of a transmission, you can easily switch it during your receive period and keep with the polarization that seems like the receiving station is hearing you. At least for a while until it shifts... It is a bit of a guesswork, but if you pay attention there are clues.

    You almost always want to be able to select H during periods when you have ground gain on your side unless you have a shot over salt water. If your local terrain is not conducive to ground gain, then do whatever works.

    Rotatable polarization is nice. You can see what polarization is best for RX and can do a calculation on the Moon Earth to DX geometry and make an educated guess for what to do for TX. But for the mechanical nature of this is a bit of a hazard for big antenna systems. On top of that the optimum polarization is continuously changing. It is easy to be wrong when there are lots of degrees of freedom in the control system.

    But putting TX power into both antenna arrays when you are not intentionally using CP, or unless you are doing it to create H or V from an X based system, is not generally recommended for 144 MHz. You are likely to be wasting power into the wrong polarization much of the time.
  10. NG1H

    NG1H XML Subscriber QRZ Page

    From what I remember talking with a friend who set up an eme station the thought was when transmitting:

    1) All one polarization: Max power is 100% and roughly 1/2 of the time you will be above 70% and 2/3 of the time above 50%.
    2) Cross polarization: Max power is 70%, min power is 50%.

    He chose to go with one polarization. Yes, 1/3 of the time the signal be weaker if not zero, but 2/3 of the time the signal will be as good or better. Another way to look at it is you have a 50% chance of a better signal versus a 33% chance of a weaker one.

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