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[TowerTalk] Tower, 80m vertical and BC hash

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Subject: [TowerTalk] Tower, 80m vertical and BC hash
From: (Tom Rauch)
Date: Mon, 1 Oct 2001 20:26:48 -0400
> certain spots like 3.650 MHz I have S9+20db with 3 or 4 local stations
> mixed together (my rx is an ICOM 736). I found that if wound 60 ft or
> so of coax into a choke with a diameter of approx 3 ft and put it in
> the feedline near the base of the tower the noise is substantially
> reduced. The daytime noise drops to < S2 and the noise at 3.650 MHz 
> is reduced to < S4-5.
> Question:
> What is the most effective way to handle this problem?

Let's suppose nothing funny is going on with a poor ground 

It could be that the tower and feed system and receiver's BC band 
input impedance is "resonant" on the BC band while on 80 meters 
just by dumb luck. Adding the extra feedline could be changing the 
impedance at the rig, and changing the signal level.

Remember the receiver certainly overloads best with something 
other than 50 ohms of source resistance from the antenna, so it 
can like a really weird impedance.

I've had that happen, and measured several volts of BCB signal at 
the receiver input!

The way to sure that would be to detune the feedline for the BC 
band by shorting it. You could put a parallel tuned circuit for 80 
meters across the feedline, or a shorted 1/4 wl stub for 80 meters 
in parallel with the feedline.

Of course you may have an entirely different thing going on.

73, Tom W8JI 

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