Stuart Rohre wrote:
> W3DCG and group,
> If you have a "Windom" type antenna, then there is no "balun" in the world
> that will balance what is inherently an unbalanced antenna (doublet) set up.
Not impossible, but very difficult.
What is required is a current balun with very high
common mode impedance, able to withstand very high
common mode voltage, which can be many times
the antenna feeding voltage.
I did this on a very asymmetric antenna (half wave wire for 7 MHz,
fed at 2.5 m from one end). It took me almost a month
to come up with a suitable balun using 4 of FT240-61 cores.
Commond mode voltage was above 3000 V with ~600 V at feedpoint
at 1500 W drive (compare this to <140 V c.m. voltage
in the case of symmetric resonant 50 Ohm dipole).
Wouldn't do that again :-)
> Consider that one leg is shorter than the other leg of the antenna. That
> means each side has a differing impedance to the feedline. There are
> unequal currents in the antenna legs.
With a suitable current balun (see above), currents will be equal,
there is nothing in principle forcing currents to differ,
apart from a non-perfect balun.
> A balun is a balanced to unbalanced converter. It MUST have an equal sided
> antenna to work with on the balanced side, and then it allows use of
> unbalanced line like coax on the other side.
That appplies to voltage baluns only.
A current balun is just a choke, it requires nothing to work.
Whether it will work well enough, depends on circumstances,
as explained above.
> What you are seeking is an LC network that will force equal currents into
> both sides of the Windom. No such animal has been common in the ham
> literature
A current balun (choke balun) is precisely the animal needed.
> What might work better, is just a cable choke type solution
> to keep the unbalance current off the outside of a coax feeder.
That's it, and if there's no unbalance current,
both legs will have the same current.
> Either beads or coiled coax can choke RF.
Yes, but... one must look at the common mode impedance
of the balun, and the commoin mode voltage presented to
the balun by the antenna. Asymmetric antennas present
some real challenges in this respect :-)
> In the case of the Windom, you inherently will have some residual unbalance
> in the doublet, and thus you have an un-un situation, unbalanced to
> unbalanced. Typically, this is NOT what the built in 4:1 balun of a
> transmatch is designed to handle.
Absolutely so. Built-in baluns can be safely used only
with non-reactive symmetric loads, which is rarely found
in practice, expect by design.
> Remember, the true Windom was a single wire feed antenna. Any balancing of
> the antenna currents involved accepting radiation off the single wire
> vertical feeder. So called windoms fed with coax or parallel lines are a
> different animal, where usually you do not want radiation off the line.
Quite right. A genuine single-wire-fed Windom has no problem
with common mode currents. In fact, it is designed to use them
to advantage, environment permitting.
> As a practical matter, studies by L. B. Cebik, W4RNL, have shown only 10 per
> cent imbalance for the parallel line currents in the balanced feeder of an
> end fed Zepp doublet.
I wouldn't rely on that information.
There are limits on what can be safely deduced from simulations,
and that's stretching well above the limits.
Best 73,
Sinisa YT1NT, VA3TTN
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