On Sun, 26 Jun 2005 13:37:46 +0000 (UTC), "Reg Edwards"
wrote:


Questions : How effective is it? To what extent can the percentage
unbalance current on the feedline be expected to be reduced? Has
anybody ever measured it before and after insertion?

Reg, I haven't measured it, however...

I am in the process of modeling a balun as follow up on my G5RV feed
systems analysis. The scenario is a G5RV at 10m centre height, a
vertical drop of open wire line (the so called "matching section") and
a balun at near ground height (with unbalanced shunt admittance
introduced by the balun to ground).

I have been jiggling the unbalanced shunt admittance introduced by the
balun and observing the current through that admittance wrt the
feedline currents, and the apparent distortion of the radiation
pattern. I could nearly as easily have been comparing the current in
one of the wires of the feedline wrt the other, it is just the format
of the currents report that makes the former easier. Another
interesting observation would be the changes to receive sensitivity to
vertically polarised waves (indicating possible feedline pickup).

The models indicate a few things so far:
- the effect of the balun is on the unbalance current distribution on
the whole feedline;
- current imbalance at the base of the "matching section" of better
than about -15dB caused little distortion of the pattern on 20m and
below;
- some configurations seem quite undesirable in terms of pattern
distortion even though balun current is quite low;
- raising the antenna / feedline / balun and lengthening the earth
connection from the balun shunt admittance changes the current
distribution.

This leads me to think that measurement of the imbalance current at a
single point might not give the "big picture". I found scenarios where
inserting an imperfect balun increased the current at a point (it
changed the magnitude of the standing wave and more importantly moved
the current minimum), so I could see someone with a handy-dandy
current probe measuring such a situation and insisting they have proof
the baluns don't "work".

Owen
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