[TowerTalk] CM current and SWR meter changes

[Jim Lux]

On 4/20/12 7:46 AM, Steve Hunt wrote:
> Tod,
>
> Typical SWR meters are affected by any CM current that is present, and
> because the CM signal is a standing wave it can cause the meter to read
> differently for different cable lengths. In fact if someone ever reports
> to you that their SWR changed significantly when they changed their coax
> length, CM current is one of the likely "suspects".
>
> 73,
> Steve G3TXQ
>


What is the mechanism of the change here..

Say I have a box with a coaxial connector on each side.  there is no 
connection between the inside of the box and the outside of the box 
except "through" the wall.  Inside the box (i.e. inside the shield) I 
have a current and voltage probe, from which I can determine the 
direction and magnitude of power flow, reactive and active power flow, 
SWR, etc.  ).  The ends of the system are also boxes with the generator 
contained entirely within the shield, as is the load.  So there's no 
common mode path.

If I change the load or generator impedances, the I and V will will 
change, and my box can measure the SWR (or any other related parameter) 
jus tfine.


If I now put some current on the "system" that flows along the outside 
of the coax (i.e. a common mode current), that doesn't change the I & V 
I measure inside the box (because it's only measuring between the center 
conductor and the inside of the coax)



ANd if I change the load impedance so the power flow changes, it should 
be exactly the same as in the case with no common mode.


Now... the contention is that "real" devices don't do this (something 
I'll readily accept).  The question is "why".. it implies that the box 
is somehow also measuring the current on the outside of the shield.

I'm not sure, though, what kind of design defect would cause this.. 
Maybe something like leakage capacitance?

If we are looking at a balanced line configuration could easily see an 
imbalance of leakage C from internal components would result in a 
transfer of current from outside to inside.  But how would that happen 
in a coax type design..