Cecil Moore wrote:

Furthermore, the choke does NOT do what the old-wives say it does, ie., stop
radiation from the line and prevent noise pick-up. It might even make
matters worse. The choke merely shifts the volts and amps standing waves to
other places along the line.

I use a choke to reduce common-mode problems in the shack and it does
that apparently by causing reflection of common-mode waves back toward
the antenna which, as you say, wouldn't decrease feedline radiation
between the choke and the antenna.

It isn't really helpful to think of it as "reflecting common-mode waves
back". It is simply Ohm's law: a high series impedance reduces the
amount of current that will flow.

Forcing a current minimum at one location on the feedline (usually right
at the end of the coax) will change the current and voltage distribution
not only on the feedline, but also on the antenna proper. The whole
antenna-feedline system readjusts itself to take account of the fact
that the choke impedance is there. This readjustment will also change
the feedpoint impedance, so it's actually true that a balun will change
the SWR of the antenna - it has become a different system that no longer
involves the feedline.

Reg is right to say that the choke shifts the common-mode voltage and
current standing waves to other places along the feedline. Depending on
the location of the 'victim' equipment (TV etc) relative to those
standing waves, the change can sometimes make RFI problems worse. Even
so, a choke balun right at the end of the coax is almost always the
right place to start. If it doesn't help, it may *still* make sense to
leave it there, and try a second choke somewhere else as well.

A clip-on RF current probe can work wonders in showing you what really
is happening.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek