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[TowerTalk] finding resonant frequency of a shunt-fed tower for 160

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Subject: [TowerTalk] finding resonant frequency of a shunt-fed tower for 160
From: (
Date: Mon, 30 Jul 2001 12:44:42 -0600
N4KG comments inserted below.

On Mon, 30 Jul 2001 "WA9ALS - John" <> 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
> be higher freq than if I had grounded elements and therefore more 
> capacitance at the top?

> 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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