[TowerTalk] HF2-V Doorknob Capacitor Failures

[Tom Rauch]

> I have a Butternut HF2-V 80/40m vertical that has served me well for over
> 10 years.  The only problem I have experienced is the periodic failure of
> the 200 pF, 10 kV, door knob capacitor on the 80m coil assembly.  I have
> had to replace this cap about every 3 years or so.  The failure mode is
> usually rather spectacular, occurring with a loud bang and pieces of the
> capacitor ending up several feet from the antenna. It appears that maybe
> moisture is getting into the capacitor causing a flash over which ends up
> destroying the cap.

DON'T use RTV on the cap. Don't seal the cap!

We have a similar problem in high power amplifiers operated in 
humid environments, and the capacitors are not in the rain!!! I can't 
imagine having one exposed to weather!

What I would do is make a sealed "rain hood" or inverted cup that 
hangs over the capacitor with clearance on all sides.

There are dozens of ways to do this, but you have to pick a 
material that will withstand UV exposure. I'd suggest using a thin 
sheet of UV resistant plastic clamped to the antenna tubing, 
formed as an upside-down cone-shaped hood over the capacitor 
and coil.

If you try to seal the capacitor, you are asking for many different 
problems!


   


73, Tom W8JI
w8ji@contesting.com

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