Topband: 160 receive antenna puzzle update

[Tom W8JI]

> 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