Topband: Isolating a TX antenna

K9AY k9ay at
Thu Aug 13 07:40:30 PDT 2009


Interaction starts to be noticeable when the coupled tower or antenna is 
roughly 2/3 the size needed for resonance, and increases rapidly as the size 
approaches resonance. For example, I have a 72-foot tower 1/4-wave from my 
160M vertical -- not a problem with no antennas on top, but with a mast, 
tribander and 40M beam, it's darn near resonant and needs careful detuning.

As you have found, "floating" a 1/4-wave vertical or Inv-L is the best means 
of isolation. It is now half the length needed for resonance, so interaction 
will be small. Note that this may not be the case with Inv-Ls having extra 
length to simplify matching -- they will likely need further detuning.


> Greetings All:
> I just finished building a vertical RX array for 160 meters using
> K7TJR's Hi-Z pre-amps.  However, it looks like I am getting some
> re-radiation from my Inverted L.  The nearest vertical in the array is
> about 35 feet from the Inverted L.  Yes, I know that's quite close, but
> real estate dictates where these antennas go.  As my good friend, K5SCR
> says;  "you gotta dance with the one that brung you".
> I grounded the Inverted L at the base, and saw no reduction in noise.
> However, when I let the Inverted L float, that is open at the base, I
> see about a 2 S Unit drop in received noise in my FT-1000MP.  In a
> properly calibrated FT-1000MP, 2 S Units should represent 6 dB.  The
> question is:  How much noise should I expect, is 2 S Units reasonable,
> and what other methods should I consider to reduce, and hopefully
> eliminate this problem?  I'm ready for winter.
> Thanks & 73,
> Mike, W5UC

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