"Richard Fry" wrote in message
...
There is a general belief that a balun should be used where coax connects
to the feedpoint of a 1/2-wave dipole (for example), to avoid RF current
on the outer surface of the coax outer conductor -- which can affect
antenna input match, radiation pattern etc.

But does even a perfect balun fully remove these effects? The outside of
the outer conductor of the coax feedline still will be coupled into the
received and/or radiated fields, even when there is no current flowing
into the coax outer conductor via a metallic connection directly with the
dipole itself.

As an illustration of this, consider the effect of a 1/2-wave dipole
suspended near, and parallel to another 1/2-wave dipole. Only one dipole
is
driven. For simplicity of concept, let's say the active RF device (either
a tx or rx) is a physically small unit built in to the center insulator of
the
driven dipole, e.g., no feedline. Standard equations, and NEC-2 analysis
show that considerable interaction exists between the two dipoles. The
input match of the driven dipole changes, and the radiation pattern of the
simple dipole is strongly affected. Yet the only coupling between these
two dipoles is by radiation.

So how important is the balun in the total RF system?

RF

Richard

I'd sure like to see radiation pattern from testing or modeling you do on
this "dipole pattern". I have been wondering about how the feed line can
be disassociated from effecting the dipole's radiation pattern.
I finally decided to make my feed for a 137MHz dipole by using two 50 ohm
coax cables to form a 100 ohm balanced line. That alowed me to locate the
balun at a place where its effectiveness is not important.

Jerry