Topband: Tophat? Does it have to be at the top?

[Milt Jensen]

My experience with using towers with large directional antennas as the "top
hat" follows the line of thinking that has been expressed in this thread
thus far.

For example, at W7MCO ( http://www.eaars.com/~n7gp/ for photos) the C4XL on
top of 80' of Rohn 25 above an insulated section at 40' AGL is a bit short,
resonating about 1900 kHz.

It takes 20' of 1 1/4" conduit stinger above that to bring the resonance
down to the 1850 kHz range.

My original design and installation of this antenna system was 90' of tower
over the insulated base at 30' AGL.  The resonant frequency was a bit below
1700 kHz.  By moving the insulated section up 10', thereby shortening the
tower above to 80', raised the resonant frequency to the 1900 kHz.

So, 10' of change in tower length below the C4XL made a +- 200 kHz change in
the resonant frequency whereas twice that length of stinger above the C4XL
is needed to fine tune the whole array down 50 kHz..

Good day to all de Milt, N5IA

----------------------------------------------------------
> G'day,
>
> If the "tophat" effectively isolates the section of tower or what have you
> above it, then those folks matching towers with stacked beams are surely
> loosing quite a bit of effective structure.
>
> Regards,
>
> Mike VP8NO
>
>
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