N4KG comments inserted below.
On Mon, 30 Jul 2001 "WA9ALS - John" <wa9als@starband.net> writes:
>
> I'm interested in this too, so will jump in now. My main question
> now is:
>
> I have a C31XR at 72 ft - The elements are -not- grounded. Thus I
> assume much less (or none?) capacitance hat effect.
There will be LESS capacitance than if the elements were
grounded. There is still the loading effect of the BOOM
which I assume is electrically connected to the tower.
And don't forget the capacitive coupling between the tower
and the COAX which is connected to the driven elements.
Tom N4KG
I therefore assume I
> can still work out a shunt feed for -some- frequency(s), but they will
likely
> be higher freq than if I had grounded elements and therefore more
> capacitance at the top?
TRUE
>
> What do you think would be reasonable to shoot for with a 70 ft
> tower with the ungrounded elements of the C31? 40M? Maybe 80M?
> Doubt 160M??
Your best solution for 40M is a rotary dipole or 2L40.
You could also short the center of the end elements
to the boom and match the boom as a 40M end loaded
dipole. I have posted my method on this several times.
It's a KILLER. N4KG
You could make separate shunt feeds for both 80 and 160.
An antenna does NOT need to be resonant to radiate.
I have matched a 50 ft tower topped by a TH3 on both
160 and 80M.
de Tom N4KG
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