[TowerTalk] W0IYH Feed line Choke Performance

[Jim Lux]

----- Original Message -----
From: "Chuck Counselman" <ccc at space.mit.edu>
To: <towertalk at contesting.com>
Sent: Wednesday, August 20, 2003 5:39 AM
Subject: Re: [TowerTalk] W0IYH Feed line Choke Performance


> At 9:55 PM -0700 8/19/03, Jim Smith wrote:
> >...if 500 ohms really is enough impedance to reduce the common mode
> >current to a negligible amount then none of this matters....
>
> It depends on the "impedance" (i.e., the ratio of the voltage to the
> current) of the common-mode wave at the point on the common-mode
> transmission line where you insert the choke.  (I put "impedance" in
> quotes 'cuz I'm glossing over the important facts that common-mode
> waves travel both ways on the line; the complex amplitude of the
> voltage at a point is the sum of the complex amplitudes of the
> voltages of the two waves; and the complex amplitude of the current
> at a point is the difference of the complex amplitudes of the
> currents of the two waves.)
>
<snip>
>
> 3. In general, the best places to insert chokes are where you know
> the common-mode current is near-maximum: wherever the transmission
> line connects to a much fatter conductor, such as your lightning
> "ground" or your rig.

Hmmm.. is a quarter wave actually how far apart you want them? I don't know
that putting two chokes at 1/4 wave apart guarantees that one is at a high
current place in the line (maybe they wind up nicely straddling the peak).
And how far is a quarter wave, since the propagation velocity of that
"transmission line" is going to vary somewhat, for the same reasons that the
Z does.   From an analytical standpoint this is an interesting (in a thesis
sort of way) optimization problem.Maybe someone will get ambitious and we'll
see a aper in IEEE Trans on Ant and Prop some day <grin>.

I suspect that empiricism will rule the day here.  One probably needs to
come up with a way to measure the common mode isolation, and then move the
chokes along the line until you find the best place.  For the "coil o' coax"
kind of choke, moving it is fairly easy: unwrap the tape, pull the slack,
and move somewhere else.  For the "beads on a string" choke, it's a bit
trickier, particularly if it's connectorized.

Let's consider why you're doing all this choking in the first place:
a) to reduce the perturbations from feedlines on the antenna pattern.  In
this case, breaking the line up into short isolated segments (like guy
wires) that are sufficiently "out of band" should work nicely.
b) to reduce imbalance.  Choking at the feed point works here, provided you
have already done a) so that you don't get common mode induced currents
coming in from the near field.


Jim, W6RMK