Topband: Isolating a TX antenna
k9ay at k9ay.com
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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