[TowerTalk] Guy Wire Affect On Beams

[n4kg@juno.com]

Interesting observations Ken.

Clearly, the directivity of a Yagi minimizes coupling to
conductors above and below the antenna in the far
field, but I still have concerns about conductors in 
the near field at spacings of less than 0.25 WL.

For continuous wires, I wonder if the guy wire length
may come into play, i.e., could there be frequencies
where no interaction is observed and other frequencies
where there is pronounced coupling?  This could be
a major concern for multi-band antennas.

With insulated guys, care must also be taken.  I recall
a friend commenting that his tribander worked very well
on 10 and 20 meters but seemed less effective on 15M.

His guys were broken up with insulators, but the first
insulator was placed 10 ft from the tower, several feet
below the antenna.  Consider the case where the boom
is parallel to one of the guys.  The other two guys are
offset 30 degrees from the driven element and provide
a continuous conductor 10 ft to each side plus 1 ft
through the tower.  A 21 ft conductor looks like a 
DIRECTOR at 21 MHz.  In his case it was spaced
approximately 0.15 WL BELOW the Driven Element
which I expect coupled tightly to the DE being closer
to the DE than the director on the Yagi.  Intuitively,
I would expect considerable energy was being directed
straight DOWN from the antenna on 15M.

Bottom Line:  I would expect it is still good practice to
use non-conductive guys or very short (<3/8 WL at 
the highest frequency)  insulated guys in the near 
field of the antenna.  As you stated, lower guys are
well isolated from TOP mounted antennas.

When antennas are side mounted on towers, I expect
that ALL guys need to be non-conductive or electrically
short in the near field of the lower antennas.  Note that 
very often, upper guys come quite close to the elements 
of lower side mounted antennas.

de  Tom  N4KG




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