To choke balun Gurus.

There is, in fact, a transformer in a choke balun which you have ALL
forgotten about, or never knew it exists.

It has nothing to do with 1-to-1 or any other turns ratio.

It is a transmission line transformer comprised of the short length of
line (coax or just a pair of wires) which passes through the balun. It
is the length of line which is wound on the balun to form the choke.
(All lengths of line behave as transformers.)

This line has an impedance Zo. If it is coax line then its impedance
is usually 50 ohms. If it is a pair of wires laid alongside each
other then its impedance depends on wire diameter, wire spacing and
permittivity of the insulation. If it is similar to twin speaker cable
then it has an impedance of about 130 ohms. The presence of ferrite
has hardly any effect.

When the balun is located between the main transmission line and the
tuner then on the antenna side of it there can be seen, say, 300, 450
or 600 ohms.

This is transformed to another impedance on the tuner side of the
balun.

The transformation depends on line Zo and its length in wavelengths
which depends on frequency. On the 160m band very little happens. The
frequency is too low and the line is too short. It is only a few turns
of wire around the ferrite core.

On the 10 meter band, at the higher frequencies, the winding length on
the balun can approach 1/4-wavelength and the 450 ohm main-line
impedance can be transformed to something considerably different.

But the tuner couldn't care what it is. Its purpose is to transform
whatever it is presented with to 50 ohms. The presence of the balun
merely causes a change in tuner L and C settings. Insofar as the tuner
and operator are concerned the transformer in the balun does not
exist.

When designing a choke balun the number of turns on the choke and the
length of wire involved are best restricted to something less than
1/4-wavelength at the highest frequency of use. 1/8-wavelength is
suitable.

Restricting turns has the effect of reducing choke inductance and the
choking effect at the lowest frequency of use.

But a 2-inch diameter ferrite ring with a modest permeability of 200
or more, wound with 18 awg stranded twin speaker cable, will allow
operation from 1.8 to 30 MHz.

With a balun constructed of a number of ferrite slugs over a coaxial
cable there is no transformer action at any frequency because the
short length of 50-ohm 'transforming' line is terminated at both ends
with a 50-ohm line.
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Reg, G4FGQ.