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[TowerTalk] current imbalance on 450 ohm line

Subject: [TowerTalk] current imbalance on 450 ohm line
From: Jim Jarvis <>
Date: Fri, 16 Mar 2007 04:17:25 -0400
List-post: <>
I sent a direct response to Garry on this, but apparently
mangled the address, and it eventually bounced.  Let me try
again, this time taking into account the other comments:

Since the real complaint is the RF on the signal, my first thought
would be to fix the common mode/'ground' problem, and don't look at
the MFJ when operating on 17m. :)  But the meter may indicate another 
problem, AS WELL.

First, I would want a decent ground on the tuner chassis.  This 
should be less than 13' long, which is a quarter wave on 18.1.

Then, a ground strap between tuner and tx.  NOT just the coax.  
If that isn't the case, I would recommend the shortest possible 
equipotential (aka 'ground') strap between tuner and rig, and a 
serious common mode choke on the coax feedline between the two. 
(Radio Works has a good discussion of this, on their website) 
(If the tuner uses power for meter lamps...common mode choke 
on that power line would be a good idea, as well, to keep from
coupling RF into the power system or house wiring.)

The above should cure the RF on the signal complaint.  

That done, you need to return to the antenna/tuner system, and
try to understand what's happening.  The indicated imbalance may 
stem from a problem with the balun, or the environment around the antenna
and feeder, or the meter.  

If I've calculated correctly, the driving point impedance of
the 80m dipole on 17m should be about the same as a 17m dipole.
The legs are 5/4 lambda.  AND, the 108' of feedline is quite
close to 2 lambda, so the driving impedance is repeated at the
tuner input.  This SHOULDN'T be a problem for the onboard balun,
unless there's some godawful reactance involved. (I don't see that,
but I write this at 4am, and there's a lot I don't see.) 

If I've goofed in my calculation...or there's some unanticipated
variable in the system, adding or removing a quarter wave from the 
open wire line should change what the balun in the tuner sees...  
and .25 lambda is 13'.  

One assumes that the line isn't running alongside something which
is resonant on 18.1 MHz, which may be causing a genuine imbalance.
If that's the case, and you have a resonant rain gutter...I have
absolutely no idea how to change that, other than re-route the feeder.

Good luck!

Jim Jarvis, President
The Morse Group, LLC
We create high-performance organizations.
732 548 5573 office 


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