Must be some strange interaction, especially that SWR is high.
JK Antennas recommend testing the antenna prior to installation a couple of
feet off the ground, stating that resonances should be lower than at height
but SWR not too bad. My JK Mid Tri 40 at 5 feet had a resonance on 40m
around 6.9, that changed to 7.03 at 100 ft. Your change from 10.1 to 10.9
was proportionally much bigger.
I had a problem with the resonance of one 80m wire vertical for 4 sq. It
turned out to be an interaction with coax to a dipole between a tree and
the tower.
Ignacy NO9E
On Mon, Oct 25, 2021 at 12:08 PM <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
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|