Topband: A 160 Meter antenne puzzle

[Tom W8JI]

 I am only moving my finger a fraction of an inch and my hand not at all 
when this happens .The feed lines are disconnected when this is going on 
.There is no dc continuity between the antenna and any mounting hardware or 
th>>>>

You have the very high impedance part of the antenna near the post. This is 
part of the reason I always use large hats, which self-resonate the antenna 
near 3.6 MHz.  With no hat and a 30 ft vertical above the loading, the 
antenna has an almost triangular current distribution, so voltage gets 
pretty high. You didn't say (or I missed) the length of the post, but 
impedance problems like that are also aggravated by post height. If the post 
was 8 feet above ground, that would be a substantial fraction of element 
length.

Not using a hat on a short vertical is just generally a bad idea. Not using 
a hat makes the antenna behave more and more like a mobile whip as you go 
down in frequency. It's a good design esthetically, but a bad design 
electrically.

Insulate the element like you have 50 kV at every point (even though you 
don't) and it will start to stabilize with moisture and other stray effects. 
This includes insulation of the entrance to the box containing the loading 
coil. As long as the coil is inside a dry large metal box and stable 
mechanically (low temperature coefficient), it won't detune with weather.

Things are a whole lot better and easier electrically with the element self 
resonant at 80 meters or lower by virtue of element length and a hat.


> e post .I disconnected the base loading and put  the analyzer on it and it 
> is resonant at about 8 mhz and touching the post causes no change with 
> this setup . There are eight 30 foot radials laying on the ground and a 2 
> inch wide copper strap about 3 foot long  in the hole that the 4x4 is in .
> What is going on here and should I make an effort to limit the coupling 
> between the 4x4 and the antenna?
>

On 8 MHz the base impedance is low. That makes the impedance low, and 
altering the electric field near the low impedance point has minimal effect. 
Almost any insulation or dielectric is good enough. On 16 MHz, you could be 
back to the same problem you have on 160, because base impedance would be 
high again (assuming a thin element).

73 Tom