[TowerTalk] SWR jumps on a 6m beam

[Rick Kiessig]

I have an UltraBeam UB-50 (SteppIR-like dynamic elements covering 6 to 40)
on my tower. When set for 6m, the antenna analyzer shows an SWR of less than
2:1 across the band. I set my TX for 50W output, and my Expert 2K-FA
auto-tunes fine, reporting a tuned SWR of around 1.05:1.

 

When I TX with less than about 250W output, the reported SWR stays in the
original range, maybe bouncing up to 1.5:1 at times. However, if I increase
power to 500W or so, the amp complains about high SWR and kicks out of
operate mode. This happens reliably, every time.

 

Everything works fine on all other bands with a full 1000W. It's only 6m
that's causing me grief.

 

I'm also using the 4-element add-on kit for 6m on the UB-50, and I've
adjusted the now-unused dynamic reflector to a non-resonant length of 1.5m.

 

The feedline is about 50m of LMR-600, with a segment of BuryFlex running up
the tower. I have a Power Master meter at the output of the amp, with a
section of LMR-400 running from the amp to the meter that passes through a
common mode choke (three loops through eight #31 toroids). I'm using a Hofi
lightning protector tied to a ground rod at the shack entrance. The coax
shield is also tied to ground at the base of the tower.

 

Any suggestions on what might be causing the problem or how to chase it down
would be appreciated. Could it be RFI getting into the amp? Maybe I need a
larger choke (or a second one)?

 

73, Rick ZL2HAM