Topband: antenna interaction

[Rudy Severns]

The discussion on the interaction between 160 m verticals and other antennas
and structures is very appropriate.  I learned that lesson again just
recently.

Last summer I erected a two element vertical Yagi from a 150' wooden pole.
It seemed to work great and had a good F/B. When I was done I decided I
needed an 80 m dipole also.  Because I have other poles that high 300' and
400' from the array I simply hung the dipole between two of the poles as far
away as possible from the 160 m array.  As a check I modeled the 80 m dipole
in conjunction with the 160 m array.  The 80 dipole, with its feedline
hanging straight down, formed a top loaded vertical on 160 m which, with
small alterations, could be either a director or reflector.  Great! But the
problem was that a bit of wind was quite sufficient to change the tuning
from one to the other.  Beaming East and inserting a reflector in that
direction killed the gain and/or loused up the F/B.  Totally unacceptable!
For daily operation I carefully detuned the 80 m dipole plus feedline so
that it did not interact very much but during 160 m contests I have to
completely lower it to be sure.

In addition I have a telescoping 90' tower, with a large array on top, about
500' away from the 160 m array. The structure is close to quarterwave
resonance on 160.  At a certain height near maximum extension, it will
strongly interact with the 160 m array.  I have to crank it down to detune
it.

Any structure, close to resonance at 160 m, within several wavelengths, can
distort the pattern.  If you have gone to the trouble to erected a
directional array, that can be very discouraging.  Even if you have only a
simple vertical, you don't want any notches in the pattern, which will
surely be in your most desired direction, as required by Murphy. The answer
of course is to carefully detune any other structures nearby where possible
as well as spacing them as far away as you can.

73, Rudy N6LF