The performance of a coaxial choke inserted at the antenna end of the
feedline cannot be properly studied without knowledge of the
feedline's length in wavelengths and how the feedline is terminated at
the transmitter end. No two systems are the same.

Choke performance depends on the condition of the system before the
choke is inserted. The choking effect depends on the impedance seen
by the choke looking into the outer conductor of the coaxial feedline.

The input impedance of the coaxial outer conductor depends on Zo, 400
to 550 ohms, of the single-wire line, its length in wavelengths, and
on the very uncertain resistance of the ground connection at the
transmitter end.

If the ground connection at the transmitter end is very low and the
line is an odd number of 1/4-wavelengths there will be a very high
impedance at the antenna end and longitudinal current will negligible.
Insertion of the choke will have no effect.

If the line is a whole number of 1/2-wavelengths, with a low
resistance ground the input impedance of the coaxial outer conductor
will also be very low and longitudinal current in the line will be at
a maximum. Insertion of the choke will be very effective.

With the usual random line length and on different bands, choke
performance will always be completely random and unpredictable. Which
explains the different opinions and arguments about whether a choke is
necessary or not.

Program SELFRES3 models choke behaviour.
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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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