Topband: High voltage and coupling from a vertical to adjacent coax

Kristinn Andersen kristinn1 at
Sun Oct 7 12:58:21 EDT 2007

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.

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

So, I would like your comments on:
a)  Am I correct that there will be high RF voltages along the entire
vertical, all the way from the "top end" of the base inductor?
b)  What effects will these voltages have on the coax that runs along
the vertical?  High degree of RF coupling into the coax?
c)  Would any kind of choke or filtering on the coax, on its way from
the vertical into the shack, be efficient in removing this common-mode
d)  What if I move the inductor up the vertical, above the dipole?  In
that case all the high voltage will be above the dipole and instead I
would have low voltage / high current below that inductor.  Less
coupling there?
e)  If this is going to be messy, one solution might be simply
disconnecting the coax at the base of the vertical, so that the section
going up to the dipole, and the dipole itself, are "floating" when I am
using the vertical, i.e. their RF potential is not tied to the shack or the
RF ground....

Any comments would be appreciated.

73 - Kristinn, TF3KX

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