[TowerTalk] Musing continued ----- Musing on G5RVs, baluns et al......

[Jim Brown]

On 2/3/2011 9:45 AM, Tod - MN wrote:

> If one reduces the coax common mode current using a common mode choke,
> should one expect that the 'common mode' component of the current on the
> open wire line above it going to the G5RV feedpoint is also reduced?

The best answer is "usually, but not always." Here's what I wrote 
yesterday about that on the RFI list with respect to sticking a choke on 
a conductor that might be receiving or re-radiating RF, or coupling to 
another antenna.  The original poster was saying that a current probe 
could be used to determine the best place to put a choke to do that. 
.Everything in my response applies equally to a transmission line.

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Not necessarily. Every wire (or conductor) is an antenna, whether we 
like it or not. A good ferrite choke acts like an open circuit, whereas 
a poor one (not enough turns) acts like an inductor,  and one with too 
many turns can look capacitive. A choke simply ADDS its impedance in 
series with the existing antenna. If it's an optimum choke (near its 
resonance and a high impedance), it acts like an insulator to stop 
current AT THAT POIINT, but if the wire is long enough, the choke might 
turn the remaining wire into a half wave and increase the current in 
that part of the wire. Ditto with the choke that is L or C -- it simply 
shifts the current maxima and minima along that particular antenna.

Bottom line -- measuring (or sensing) current in these conductors is 
certainly a worthwhile thing, but it's easy to mis-interpret the results 
if you don't view what you see in the light of the fundamental concepts 
of antennas.

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73, Jim K9YC