yes to all your suspected problems. high voltages may arc over the
insulation of the coax, there will be very high induced currents on the
shield and the dipole, and in general it will be a mess. the worst part is
one you seem to have missed, with the coax running along side the vertical
it will be essentially like running a ground wire very close to the vertical
and will change it's impedance and likely cause all sorts of grief with it.


"TF3KX" wrote in message
ups.com...
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
shack.

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
RF?
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?

Any comments would be appreciated.

73 - Kristinn, TF3KX