Roy and Walter,

CHOKE BALUNS.

Keep it simple. Why make a song and dance about it?

The pair of wires wound on a choke balun behave as an inductive choke
for longitudinal currents and as a transmission line for balanced
currents.

The two circuits, line and choke, behave independently of each other.

The actual current on a wire, if anyone should ever wish to know, is
just the arithmetical sum of the other two.

All authors omit to say that the the pair of transmission line wires
need not be coaxial. They can be a balanced twin line such as a length
of stranded flexible speaker cable. Or just a pair of insulated wires
laid alongside each other. Or twisted.

Not only is the twin-line mechanically more convenient to construct,
it has electrical advantages. Its Zo is intermediate between the high
and low terminating impedances. Typically Zo is about 120 -130 ohms.
The geometric mean of 50 and 300 ohms is 122 ohms. Not that this
matters very much because the device is never terminated in its
nominal impedances.

Twin line also has a higher velocity factor than 50-ohm coax.

The length of line wound on the choke behaves as an impedance
transformer. So it seriously, but not harmfully, affects the impedance
presented to the tuner. The tuner LC settings may be beneficially
affected. Otherwise the transformation ratio, which heavily depends on
frequency, is not of consquence.

Preferably, the length of twin-line wound on the choke should be less
than 1/8th of a wavelength long at its own velocity at the highest
operating frequency.

It is not too difficult to construct a choke balun which has
sufficient inductive choking reactance at 1.8 MHz without the
transmission line exceeding 1/8 wavelengths at 30 MHz.
----
Reg, G4FGQ