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Re: [TowerTalk] CM current and SWR meter changes

To: Tod Olson <>
Subject: Re: [TowerTalk] CM current and SWR meter changes
From: Steve Hunt <>
Date: Fri, 20 Apr 2012 18:37:41 +0100
List-post: <">>

Picture a dipole with no feedline; say the feedpoint impedance is 75 
Ohms (SWR=1.5:1).

Suppose we now connect a length of coax a multiple of a half-wave long, 
which is well earthed at the shack end; we will be connecting one side 
of the feedpoint to ground through a low impedance path and the 
resulting feedpoint impedance will be very different from 75 Ohms. Now 
change the coax length by a quarter-wavelength and the braid path is 
quite high impedance and the feedpoint impedance will be restored to 
close to 75 Ohms.

So even if the SWR meter is not affected directly by CM current flowing 
through it, it can still show a significant change in SWR reading when 
we alter the coax length simply because the feedpoint impedance has changed.

It helps to picture the coax braid as a third radiating antenna wire. We 
should expect that if we add a third dipole leg and start altering its 
length we would measure a changing SWR.

Steve G3TXQ

On 20/04/2012 18:14, Tod Olson wrote:
> Whoops.
> Net impedance at the 'feed point'S( ?
> Of the antenna, of the feed line at the station?
> I am not sure I understand the use of the term "net impedance' Steve.
> Mentally I am picturing two impedances the are combined to give a single
> impedance. The individual impedances are associated with the Common Mode
> [CM] path and the inside of the coax RF path.
> The inside the coax path is the one that the SWR measurement is taken on.
> The CM path is isolated from the SWR measurement path [if the SWR meter is
> a good one]. No matter what happens with the CM impedance, how does that
> change the 'independent' SWR impedance? Maybe they are not independent?
> Somehow I think that there are a lot of variables in play here and it gets
> challenging to figure out what is doing what and when it is doing it.
> Where are the guys who comfortably handle the equations? They should be
> jumping in here to set us straight.
> Tod

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