An unbalanced-to-balanced tuner is an over-complicated and
unwieldy-to-use collection of components. It is usually operateable
over only one or two adjacent bands. The only nice thing which can be
said about it is that it looks good when outside of its box.

A choke balun can be inserted between a high-Zo balanced transmission
line and the tuner to allow an ordinary, simple, unbalanced tuner to
be used.

In the range 1.8 - 30 MHz, the balun consists of a relatively few
turns of twin, flexible speaker cable wound on a ferrite ring about 2
inches in diameter. Or a pair of 18-gauge plastic-insulated wires can
be wound alongside each other around the core. This is mechanically
more convenient, more robust and less lossy than using coaxial cable
with its very small-diameter inner conductor.

Furthermore, the twin-line balun, with its higher Zo impedance of
roughly 120 ohms, has electrical advantages over the usually
recommended 50-ohm coax. 120 ohms is intermediate in value between
450 and 600 ohms of the transmission line and the 50 ohms impedance to
which it is eventually to be transformed by the tuner. This presents
to the tuner a more manageable range of impedances it has to handle.

The line wound on the balun has to withstand the same high voltages
which arise on the main transmission line with VSWR.

It is not generally appreciated that a choke balun behaves as an
impedance transformer. Insofar as the tuner and transmitter are
concerned it is a short length of 120-ohm line in tandem with the main
balanced line which has a different impedance. The transformation
ratio depends only on the length of line on the balun and on the
operating frequency. It operates on the input impedance of the main
line which can vary with frequency over a very wide range in a random
manner. Therefore there is no fixed impedance transformation ratio
between it is used. It can be considered indeterminate.

In general the transformation ratio increases linearly with frequency.
To keep things under control during design the length of line on the
balun should not exceed about 1/10th or 1/8th of a wavelength at the
highest frequency of use. The propagation velocity on the balun line
is about the same as when it is drawn out straight. At 30 MHz this
limits line length to about 1 metre or 3 feet or preferably less.

So for the balun to be effective as a choke at 1.8 MHz the
permeability of the ferrite material must be large enough to produce
sufficient inductance with the number of turns on the core permitted
by the restriction on line length.

Here is another advantage of twin-line over coax because the
propagation velocity on coax is much lower than twin line which
further reduces available line length and also the possible number of
turns.

Even at 30 MHz, loss in the balun is very small because of the short
length of line and wire thickness. A small line loss in the core
material does occur because of flux which leaks outside the immediate
vicinity of the line into the ferrite core.

Most heating loss in the core is due to longitudinal current in the
choke at the lowest operating frequency when, insofar as longitudinal
current is concerned, the pair of wires behave as a single
inductance-producing wire. But the power available from longitudinal
currents is already lost from the antenna anyway. To mimise
longitudinal current make sure there is sufficient inductance - which
is the sole purpose.

For the design and peformance of a choke balun download program
BALCHOKE from website below.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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