[Elecraft] balanced tuner

[Tom W8JI]

> 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