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SB-220 mismatch antenna line

Discussion in 'Amateur Radio Amplifiers' started by I0MOM, Apr 1, 2012.

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  1. I0MOM

    I0MOM Ham Member QRZ Page

    I changed antenna and I found that conneting directly to the transceiver SWR on all bands are almost null.
    When I connect in series to the transceiver my linear SB-220 SWR dramaticaly increase (eg. >3:1 on 10 meters).
    The linear is OFF so only the linear antenna relay in the "rest" position is involved.
    Any one has an idea of what could be the cause and (still better) the cure ?

    73 de I0MOM Marco
    in sunny Rome
  2. K1ZJH

    K1ZJH Ham Member QRZ Page

    Check the resistance between the antenna connectors. The only thing inline is the antenna relay, and
    it goes straight through when the amp is off. You might also want to check the grounds on the coax
    that is used between the SO-239 fittings and the relay. A bad coax ground could give you problems.

    Another possiblity is that you are changing the length of the antenna feedline, which will change the
    apparent SWR at the radio. If the antenna SWR isn't flat, the reflected impedance is affected by the
    coax length. Try a dummy load on the amp and see if the SWR is still high on ten meters.
  3. I0MOM

    I0MOM Ham Member QRZ Page

    Thanks for the suggestions
    I checked resistance between amp connectors : were 0 as expected
    Resistance between amp ground and imp. / out. connectors ground is 0 also.
    Added about 6 feets to the antenna feedline and without the amplifier the swr did not change and was very low, so I presume antenna SWR is flat.
    But strange anough : putting dummy load to the out of the amplifier SWR were 0 (I was expecting High SWR!) and changing the lenght of a coaxial cable put between antenna switch and output of the amplifier (from 6 feets to abt 2 feets) SWR returned low as if the SWR in the feedline were not flat!
    I' more confused than before ...
  4. K1ZJH

    K1ZJH Ham Member QRZ Page


    I suspect the antenna SWR is higher than you might suspect. If the SWR at the antenna is not 1:1, the coax length will affect the SWR
    reading at the transmitter. I think your research has proven this to be what is happening?

  5. I0MOM

    I0MOM Ham Member QRZ Page

    I think You are right Peter.
    I changed the coax leght adding a few feet and the SWR reading changed also without the amp.
    Thanks for You help,

    73 Marco I0MOM
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