So, Tom, how would you comment upon a transistorised rig
of 50 ohms impedance unbalanced output connected to an
unbalanced pi configuration antenna coupling unit
(variable C1 between input and ground, variable series L
in the hot lead, and variable C2 between output and
ground) followed by a balun consisting of about 20 or so
turns of coax around a large diameter coil former and the
output of the balun connected to a 450 ohm window line
running to an inverted vee antenna slightly longer than
that required to be resonant at 80 metres and used on 160
metres and all the HF bands?
Kevin,
We are a bit off topic for this reflector and I am severely
time limited but I'll try to answer.
Regardless of which side of the unbalanced network the balun
is on, you will need the same common mode impedance in the
balun. It is ONLY the common mode that heats the core. The
differential mode only places an electric field between the
conductors, the magnetic flux all cancels fully for
differential modes, so the core is totally unaffected by
normal "push pull" transmission line currents. It is the
impedance from end-to-end of the winding that matters in a
true transmission line balun.
That impedance will have to be high enough to have minimal
current flow, otherwise the output terminals will never be
able to "float' to the proper voltages with respect to earth
at the tuner. The required impedance has nothing at all to
do with the output impedance of the rig. The impedance and
length of the transmission line to the antenna and the
antenna certainly affects it, but the transmission line
impedance does NOT determine the balun impedance you need in
almost any case.
For example if you have a dipole 1/4 wave above earth and
bring a transmission line directly down to earth where one
side is grounded, you would have virtually no unbalance at
all in antenna or feedline currents. This would be true if
the feedline was coax or open wire line. The common mode
impedance of the line itself would act like a nearly perfect
balun.
If you added a balun at the earth, you could easily make the
system worse of it was fed with coax! The better the balun
at earth with coax, the worse the feedline radiation would
be in this case. This is why a balun has to be at the
transition point between balanced and unbalanced parts of
the system.
You could, with the "wrong' length of feedline in
combination with a certain antenna, need many thousands of
ohms of common mode impedance in the balun. You will never
get that with an air core balun on any more than one band,
and that would be the band where the balun is self-resonant.
Air core baluns are fine when dealing with low controlled
impedances like the feedpoint of a dipole, but they aren't
very good in multiband systems with uncontrolled impedances.
You would probably need a low loss tangent or "high Q" core
(at the operating frequency range) with very high impedance
for high power. That might be 20 turns on a 3 inch high
stack of 61 material (Q=350 at 1 MHz). For really high
power, higher frequencies, or higher impedances you might
want 67 materials, with a Q of 400 at 2 MHz. 65 and 43
materials are generally adequate at lower power and can use
a smaller core size.
I used a binocular stack several inches tall of 68 material
(Q=350 at 7 MHz) with just a few turns (maybe 5 or 10, I
can't recall) of HV insulated twisted pair for high power SW
BC baluns.
The only place I ever use air cores are on my yagis or
dipoles. Monoband applications with controlled modest
impedances.
One way to solve all this without a special balun design is
a true balanced tuner, but that is expensive. The other
thing is most people don't know and don't care if the system
radiates from the feeder or not. If nothing smokes or arcs
and people answer them...they are happy.
Sorry I'm out of time for now, but I hope this helps get
some experimenting with the right cores. You absolutely do
NOT want 73, 65, 33, or 43 cores at high power or with high
voltage from the feeder to earth, and while an air core
might not heat it likely won't be doing much balancing
either unless in a 50 ohm system right at the balanced to
unbalanced transition.
73 Tom
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