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Thread: Placing a mono band beam above Tribander?

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  1. #1

    Default Placing a mono band beam above Tribander?

    Hi! I have a cushcraft A3S mounted on top of a 45' pump up mast. I have just put together a a 2 element Yagi for 12m and want to try and mount this above the tribander. Is there a minimum spacing I should try and aim for? What sort of detrimental effect can I expect on performance from each of these antennas, if any? Many thanks and Happy New Year! Cheers, Jess

  2. #2
    Join Date
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    I have 2 yagi's mounted above my 7 band trapped beam just above a 24' 6m then a 33 1/2' 2m these are all on a 15' heavy duty mast. You can figure the spaceing for yourself I have a little over 3' of mast in the tower. I do have a small amount of VSWR problem on 12m 2:1 after adding the new 6m yagi went from a 12' boom to a 24'.
    73 de Fred N0AZZ

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  3. #3
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    Quote Originally Posted by G8IYE View Post
    Hi! I have a cushcraft A3S mounted on top of a 45' pump up mast. I have just put together a a 2 element Yagi for 12m and want to try and mount this above the tribander. Is there a minimum spacing I should try and aim for? What sort of detrimental effect can I expect on performance from each of these antennas, if any? Many thanks and Happy New Year! Cheers, Jess
    This is entirely 100% a problem of mutual coupling.

    Since effects of mutual coupling vary with spacing and tuning of elements, as well as orientation, it would all depend on the antennas used and how fussy you are about the end effects.

    Since 12 meters is between 15 and ten meters, and since the one beam has 15 and 10 and the other 12, you could have a great deal of interaction even at a half wave of spacing. Of course, you could also have almost no harmful interaction with close spacing.

    So unless the eexact antennas were tested or accurately modeled, the best anyone can offer is really just a wild guess. :-)

    My wild guess is that you would want the antennas at least 1/4 wavelength apart. That would be about 3 meters of spacing. It might be more, it might be less, but if it were mine I would plan on at least 3 meters of spacing to not disturb patterns.

    You could also put the 12 meter antenna at right angles to the tribander, and probably use less spacing, if you ran into problems.

    73 Tom

  4. #4

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    Thank you both for your replies. I will go ahead and put it above the tribander and try it at right angles to it. I am assuming that if the setup was modelled using Eznec or similar I would get some better idea
    regarding loss or change in SWR from the mutual coupling? If that is the case, Would someone out there be willing to do that for me? Thanks in anticipation. Cheers Jess

  5. #5
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    It will always mess up the pattern more than the SWR.

    So if you see any SWR change, you can bet the pattern has some ill effect.

    Sorry but the modelling is too complex. The problem is in the unknown electrical details of the antennas. Just install it, and see if you even need to swing one of the antennas 90 degrees.

  6. #6
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    I have a 3 el Cushcraft 12/17 meter beam about 4 ft above my Mosley tri-band. SWR is high on 10M, must use tuner if I use 10M.
    And just Ok on 12, it’s a little higher than I would like. My plan is to get a Mosley 57 Pro and take down the other two.
    That should fix everything.

  7. #7
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    Aug 2001
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    College Park, MD
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    You can use G4HFQ's Polar Plot (http://www.g4hfq.co.uk/download.html#PolarPlot) with a sound card, your rig, and a nearby friend or small power signal source to measure before and after patterns on the tribander and the pattern of the 12m yagi.

  8. #8

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    I believe CC has a spec for their A3S and A3WS spacing so Id call and ask for suggestions.

    Carl

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