cliff wright wrote in news:47a1325a$1
@clear.net.nz:

....
I came across a Z match circuit on optushome.com.au by VK3YSF which has
a 25 pF variable capacitor from one side of the open wire feeder to
ground. However the article gives no further information about this
component except to call it a "Balance" capacitor.

It is no surprise that the Z match is susceptible to a difference in the
magnitude of the voltage on each balanced port wire as described by
Lloyd, VK5BR.

A possible explanation of a certain construction of the Z match circuit
is that the earthy n turns of the main inductor and the output link form
a two wire transmission line.

Looking at Lloyd's article at
http://users.tpg.com.au/ldbutler/SingleCoilVK5BRCct.htm , the two wire
transmission line is 4*pi*62mm in length which represents about 30°
electrical length at 10m. Such a phase delay suggests that lumped
component analysis (eg as an ideal transformer) will probably not
adequately capture the distributed RLGC nature of the two wires.

Analysis as a transmission line offers an explanation of why the currents
flowing in both ends of the 'link winding' or transmission line conductor
into the centre tapped load are not equal. To minimise the high end loss
of balance, the transmission line section needs to be shorter, and an odd
mode Zo closer to the half load Zo (but not necessarily equal to it). TL
analysis also suggests a more symmetric network (with two cascaded TL
sections and the earth connection between the two) that will have more
equal voltage magnitued and phase changes at both ends of the TL.

The balance capacitor introduces another current path which will change
the voltage across the upper half of the load, a crude form of
compensation to try to extend performance.

Owen