Owen Duffy wrote:
John Smith I wrote in news:esg31b$n0a$1
@nnrp.linuxfan.it:

John Smith I wrote:
Practical example of winding a guanella type balun ... (good detail in
picture depicting green/white wires)

http://www.n0ss.net/qrp_4-1_guanella-type_balun.pdf

Notwithstanding all the focus on current baluns, a current balun does not
(by itself) prevent common mode current on an OCF Dipole feedline. The
feedline has assymetric mutual coupling to the dipole and can not be
expected in any configuration of significant length to be balanced wrt the
dipole over a wide frequency range.

Owen

Wide freq range?

Well, 40-50uh should provide 500 ohms impedance to common mode currents
on the outside of the braid of 50 ohm coax at ~2mhz, this is about the
minimum usable (in my humble opinion), although I have seen designs only
providing a 5:1 "common mode impedance to coax impedance", at lowest
freq, in common use (and especially when spanning wide bandwidth. And,
all figures used here are approx. and taken from my guess/memory/past
experiments--although I am sure they are in the ballpark)

At ~10 meters, the same inductance (same current balun) will present an
impedance of 7,000+ ohms to common mode currents (impedance increasing
with freq though the HF bands)

It is true, coupling to the coax, past the current balun can occur (and
probably most noticeable at low freqs/high power.) I seen a post by
Richard addressing that very problem, I believe, and he is using ferrite
beads along a section of coax leading away from the antenna, and
apparently in addition to some other current balun at the antenna. I
have seen others using an additional 1:1 current balun 1/4 wave away
from the antenna on the coax, or near their rig.

However, you will recognize that "magical" point where you realize
minimum/reduced/tolerable common mode currents as the coax can be moved
about without wild swings in SWR performance and you have a good match
over the freqs/bands which the antenna is designed for (given that you
don't have one end of the antenna close to a large metal building or
some other structure/object providing an object to couple to and set up
a LARGE imbalance.)

Now, the above is only VERY general. As even the
bifilar/trifilar/quadfiler windings and their spacings are of importance
to a properly designed and constructed balun and will affect its'
performance.

I make only VERY GENERAL statements in the construction of these baluns
and I don't wish to accept ANY responsibility in the results of someone
using them.

There are plentiful designs, many-many tried and true, available on the
net and I leave finding those as an exercise to anyone wishing to use them.

There is also abundant formulas/equations and examples given, on the
net, on how to design your own (material to use for rod/toroid, wire,
spacing, etc.)

And someday we will talk about the "rest of the story", those guys who
can hook a coathanger to their rig with a wet thread and realize good
characteristics!!! (well, almost grin)

Personally, I use the current balun whether it appears I need it or not,
I find the "insertion loss" tolerable. But, I agree, in a perfect world
there would be a better solution ...

JS
--
http://assemblywizard.tekcities.com