[TowerTalk] Full-size 30m rotatable dipole has big swing in low SWR point

[W0MU Mike Fatchett]

Did you model this with software and input the taper lengths etc?  The 
Antenna will certainly look much longer close to the ground.

Using the free space calculation of 492/Freq us about 24.2 ft per side.



On 10/25/2021 8:03 AM, w5wz at w5wz.com wrote:
> Stepping back from the problem to see the entire forest, not the 
> single tree.......
>
> 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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