Thanks to everyone who responded .The posts by K7TJR and W8JI were most
helpful in understanding what is going on .I should add that the phasing
box in the shack is continuously variable, (NCC-1) so changes in the
phasing caused by changes in the antenna surroundings came be compensated
. It works quite well but I think there maybe some benefit to change the
elements to top loaded style.
A system like the NCC-1 will indeed compensate any changes. Fixed phase and
level systems will have a problem. For people with fixed phased arrays using
passive elements (that cannot compensated for drift), this is an important
effect.
When I looked at this stuff years ago when I was planning the passive
elements for my directional arrays, element stability was a major concern.
The problem you observed has little to do with the wood's resistivity. You
already know it isn't a conductivity issue, since you insulated the wood
from the element and still have a problem.
The problem has much more to do with the wood's dielectric constant
properties. Anything other than thick Styrofoam (or something mostly air)
will tend to concentrate the electric field in the area of the dielectric.
Wet wood and other high dielectric constant insulators make the issue worse.
This means any changes in that area, either the hand-effect you observed or
moisture changes in the wood, will cause the system to drift.
A similar problem occurs at the ends of hat wires. Unless the insulator is
thin, long, and sheds water, the element will detune in rain. The hat cools
the base area down, and creates a new issue up at the hat wire open ends.
It's just mechanically much easier to handle there.
This is unimportant to you with the NCC-1, but can cause issues with
resonant fixed-tuned systems.
73 Tom
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Topband Reflector
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