Hi,
I read in Walt Maxwell's (W2DU) book, "Reflections" about
his ferrite bead balun, a 1:1 current balun he claims will
provide absolutely equal currents to each side of a dipole,
or, I presume V-beam or rhombic, regardless of whether the
impedances are equal or not. Therefor (?) the legs do not
have to be exactly of equal length???
Maxwell asserts that Roy Lewallen, W7EL, has proven this
analytically. Lewallen has alleged to have shown that
"while choke baluns are current baluns, bifilar-wound 4:1
and trifilar-wound 1:1 baluns are voltage baluns......
"Lewallen has determined analytically that current baluns
force equal currents into opposite halves of a dipole,
independent of the impedance of either half. On the other
hand, voltage baluns provide only equal voltage to the
opposite halves, and thus do not provide equal currents
in each half if the impedances of the two halves are
not identical." "Reflections" Chap. 21, pg. 21-10.
Maxwell then goes on to describe an experiment proving
this using a 10" length or so of Teflon insulated
RG-303 threaded over with 50 ferrite beads, (0.197 in. ID
by 0.19 in. length); he shows that the voltage appearing
across two resistors of unequal R, connected to
ground at the balun output, have voltages appearing
across them that are directly proportional to the
values of the resistance of each individual R.
Thus, says Maxwell, we have exactly equal currents
flowing out from the terminals of the current balun
regardless of the resistances (impedances) of the
load seen at each balun output terminal.
As it is current that gives rise to radiation, and
the resulting "antenna field" pattern, it seems that
I do not have to be that careful to assure that I
have exactly equal lengths to the legs of my dipoles
or V-beam antenna legs. One V leg, say 560 feet long
on one side, and the other, say 575 feet long, would
still result in the same pattern as a perfectly balanced
leg length V? Not sure this is true; but it does seem
clear that each half could not radiate the same power,
as this goes as I-squared-R; thus even if the currents
in the two halves are equal, the R's would differ.
I am ordering all of my V-beam material from Press Jones,
The Wirenan. He also sells parts--ferrites, etc-- to make
up the W2DU baluns and others. Seems I am in for a time
of experimenting!
L.B. Cebik, W4RNL has sent me the following:
"Jim,
The copperweld wire should do the job. As far as matching goes, do not
use a balun if there is significant reactance on the line. Folks who do
that are missing a lot of what their systems can do because of losses in
the balun. Baluns like resistive loads only. Jerry Sevick stresses many
times to do all reactance canceling on the load side of the balun. With a
good ATU, a balun is unnecessary. Likewise, if you have a high reactance
on the line, the point at which you insert the ATU may show a low
impedance as much as it might show a high impedance. A 4:1 balun on a low
impedance is a still lower impedance and a worse match. Hence, parallel
feedline and an ATU is the system of choice. Yes, it is another
adjustment to make, but a wise one.
Once you have the system up and running in the way you want it to go, then
you can experiment with various baluns and compare signal strength with
the ATU system. Only when you can make a good comparison will you be able
to tell if a balun will do the job for you.
Given your terrain, there is no way to predict easily just what the
feedpoint impedance will be. An ATU let's you handle what you get. Once
you know what you have, then is the time to readjust for a "no tune"
system. My modeling figures were at best ball park for the ravine
situation you have.
-73-
LB, W4RNL"
I have one of Paul Shrader's XMatch Classic Low Band tuners. I want
to use it with the three V-beams going up. As it is single ended,
unbalanced output, I have got to get to a balanced feed system
somehow. The W2DU balun just outside the XMatch tuner seems the
way to go.
I know I already have advice from some very excellent mentor Elmers,
LB, Press and Paul, but nothing really quite adds up for my situation
just yet. Any other comments would sincerely be appreciated. I am
sure by the time I am finished with this project, I will have as
much money in it as I would were going out to buy a Force 12 Magnum
80/40 or Elite 20/40 beam, well, maybe not THAT much! I know
I am learning a lot, and I've only been licensed since 1950!
Probably should just build up a balanced Z-match tuner that I
read about in my 1956 ARRL Antenna Book(Green cover)! But,
today, from where would the parts come?
73, Jim, KH7M
On the Garden Island of Kauai
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