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On Jun 24, 7:18 am, Nick wrote:
Hi All,

I keep reading on various websites and in various documents that HF
verticals should have a "line isolator" (a choke/current balun)
located near the antenna's feedpoint to prevent coax feedline re-
radiation, disruption of the antenna's radiation pattern, sub-optimal
vertical performance, and RFI in the shack. My question is this: just
how important is a choke balun when using a ground-mounted HF (for
40M) quarter-wave vertical antenna with 32 quarter-wave buried
radials? And if it is important, why wouldn't the choke be placed at
the shack's input, so that the coax's outer shield could act as
another radial? (I had always assumed that a vertical didn't need a
balun at all, and that only dipoles used them. Am I wrong about
this?).

Thanks!

Nick

I think line isolator is slightly different from balun. Here line
isolator would be necessary for lightening protection etc and hence
better kept at antenna base. Balun is for converting a balanced
structure like dipole antenna to unbalanced structure like coax. Since
monopole antenna is unbalanced structure it does not need balun at its
input.

A "line isolator" (a choke/current balun) will provide no lightening
protection.

What is being talked about is a choke to prevent currents from flowing on
the outer of the coax.
It is not a balun in the purest sense, but it does stop unbalanced currents
from flowing on the coax.

The problem with an HF vertical is that the ground plane will never be
anywhere near prefect, so it will
not be possible to feed with coax without some current flowing on the outer.

I guess the only problem with putting the choke at the antenna base will be
pickup onto the outer as is
crosses the groundpalne. so I can see the idea of putting the choke at the
edge of the GP might be better.
I guess it either suck it and see, or try to simulate it with NEC.

73
Jeff