I always thought the 468/f was for dipoles supported at the ends due to
"end effect" Using 492/f gets you closer, but disregards the effect of the
diameter of the tubing. Adjusting for that probably puts you spot on what
you measured.
Chuck W5PR
On Mon, Oct 25, 2021 at 11:07 AM <john@kk9a.com> wrote:
> Did you model this? I believe the 468/Mhz formula only works on small wire
> dipoles. Telrex used through the boom element uninsulated elements so how
> did you feed this? Measuring the SWR near the ground is pointless.
> Perpendicular to the boom would mean that the elements are in line with the
> KT34, I am not sure how they affect each other being that close.
>
> John KK9A
>
> w5wz wrote:
>
> I have built a full-size 30m rotatable dipole using parts from an old
> Telrex yagi. Taper from 1-1/4" OD to 5/8" OD. The length of each side
> is 23 ft 1 inch.
>
> Mounted on tilt over tower, perpendicular to the boom and 4 feet above a
> KT34XA. With the tower tilted over and dipole about 8 feet high, SWR at
> 10.125 was 1.16 with >1.5 across the band.
>
> Stood the tower back to vertical with the dipole about 30 feet above the
> ground. Across the band, SWR >3.5, and the new SWR minimum is now at
> 10.906 Mhz.
>
> Same 70 ft length of feedline used for all tests at all heights.
>
> What's causing this big change? Is the KT34XA creating a "too close to
> the ground" effect?
>
> Mathematically, this indicates I'm roughly 19 inches short on each side.
>
> As built:
>
> 468/10.125=46.22 ft total length
>
> 46.22/2=23.11 ft per side
>
> At SWR minimum:
>
> 468/10.906=42.91
>
> 42.91/2=21.45
>
> 23.11-21.45=1.66 feet difference
>
> --Scott W5WZ
>
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