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

[w5wz at w5wz.com]

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