On Sun, 07 Oct 2007 15:08:45 -0000, TF3KX wrote:

I am working on a topband vertical, approx. 12m/36ft tall, with four
top hat wires sloping downwards, approx. 6m/18ft long each. Just
about midway up this vertical I am contemplating mounting a dipole for
the higher frequency bands, and running a coax from this dipole down
along the vertical.

Hi Kristinn,

This may be the time to rethink both antennas. Hoist the dipole to
the TOP of your vertical radiator and use IT for the top hat (after
all, being lower will present proximity problems of the current top
hat with the dipole). This, of course, will complicate things (like
how to connect the dipole for dipole operation, and connect it for top
hat operation).

Now, I am worried about the interaction between the vertical antenna
and this dipole coax. If I feed the vertical through an inductor at
the base I presume there will be fairly high voltages on the entire
vertical (I may run power up to a KW) and this may couple RF into the
dipole coax, which in turn will give me RF where it ends, in the
shack.

The solution (again, redesigning things) is to convert the vertical
from base feed to gamma feed. This allows BOTH the vertical radiator
and the dipole coax to be grounded at the bottom and to share the same
current and voltage levels (and will solve half the top hat problem if
you put the dipole at the top).

When both have the same potentials, there is no coupling between them
- no high voltage problem except at the dipole feed point.

The solution here is to make the dipole one piece of wire connected at
the middle to the top of the radiator (the top hat of the vertical),
and to feed it with a small resonant loop. This will provide the
voltage standoff to allow the coax to float near the vertical during
its operation.

This is complex description in a short space, so I hope you can follow
the outline of it and see the whole.

73's
Richard Clark, KB7QHC