On Mon, May 1, 2017 at 1:08 PM, Jim Brown <jim@audiosystemsgroup.com> wrote:
> One common interaction is between a resonant antenna and a nearby antenna on
> the second harmonic of the first. For example, 40M interferes with the 20M
> pattern.
>
> There's an easy fix for this -- the 40M antenna should be shorted at the
> feedpoint on 20M.
Well, just how "easy" interference can be mitigatged is certainly up
for discussion. When working against a hex beam, there are as many
potential interference modes, as there are bands that the hex beam
covers. That's five to analyze in the ones I've seen. I personally
don't know what they are selling now. Six meters on those those these
days?.
The simplest situation that there are only one or two bands affected
assumes that there is an effective balun, and that the 40 m feedline
shield cannot be separately induced by the hexbeam. I've seen many 40m
dipoles fed just with coax, and the shield connected directly to one
side of the dipole. That makes interference modes interesting.
A 40 m dipole is normally close to resonant (3 1/2 waves) on 15 m and
is often used that way. To defeat interaction on that band requires
that dipole halves be disconnected, from each other and from coax
shield and center conductor at the dipole feed to defeat it, a task
not assisted by a proper balun since some degree of the unwanted
coupling will be differential and passed by the balun.
This causes the no-interference task to include switching fixes in and
out on the 40 dipole depending on the hexbeam's band in use at the
time.
Assuming that all the bands that the hexbeam covers are desirable to
the operator, the easy, simple, solution is separation.
73, Guy K2AV
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