Roy Lewallen wrote:
I'd also like to add that the induced current isn't unpredictable, as
you stated. It has to follow rules like all other physical phenomena,
so it's entirely predictable.

The key to reducing the unwanted feedline current is that knowledge that
"it has to follow rules". We can't change the rules, but we *can* change
the antenna/feedline configuration so that the rules will work in our
favor.

These are the well-known physical rules about the way that voltages and
currents distribute themselves on wires, for example:

* Antenna wires carry standing waves of voltage and current, and also so
do feedlines if they carry net (common-mode or surface) RF currents.

* A voltage maximum is located at a current minimum, and vice versa.

* Maxima and minima are a quarter-wavelength apart.

* Current goes to zero at the physical end of a wire; voltage goes to
maximum.

Whenever you change the antenna/feedline configuration, the rules will
then force the voltage and current to rearrange themselves into a new
configuration also. So how can we "play the rules" to reduce unwanted
feedline currents?

For the moment, let's talk about coax feedline and unwanted surface
currents, because it's easier to insert chokes and also easier to
understand the results.

The unwanted surface current on the feedline has two components:
* Conducted current, launched onto the feedline from the hard-wired
connection at the feedpoint
* Induced current, caused by the feedline being placed in an EM field.

Wherever you insert a feedline choke along the feedline (by winding the
coax into a coil, or using ferrite loading) then you are creating a high
impedance which *forces* a current minimum at that point. The rules will
then force the current distribution along the feedline to change, so
that it meet this new additional requirement.

A choke balun at the feedpoint is the single most effective change you
can make, because it almost completely prevents conducted currents from
being launched onto the feedline.

That leaves the induced currents to be dealt with. These are typically
less of a problem than the conducted current, but are very hard to kill.
It's rather like trying to squeeze the air out of a long inflated tube.
Squeeze in one place by forcing a current minimum, and a new current
maximum will pop up a quarter-wavelength away.

Your feedpoint choke has defined a current minimum at the top of the
feedline, but by squeezing there you have created an induced current
maximum a quarter-wavelength down the feedline. So place another choke
there too, and the rules will make it impossible for ay significant
currents to build up between those two chokes. By making the rules work
in your favor, you have effectively killed the feedline current between
the two chokes.

The current won't quite go to zero, and a current maximum will certainly
try to pop up somewhere else, if the rules allow... but it probably
won't be as large as before, and you can chase that one down too.

There are many other ways to look at the problem, but I've found this
viewpoint of "playing the rules" to be quite a helpful one. I hope it
chimes with some other people too.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek