[TowerTalk] Balun for Force 12 6 Element 20 Meter

[jimlux]

On 7/21/20 11:23 AM, dj7ww at t-online.de wrote:
> Before WX0B switched to use feed through insulators I replaced two of his
> baluns on Optibeams at a friend.
> Baluns were made from RG393 wound through ferrite toroids.
> Around the stainless steel screws the boxes were burnt out, I guess that was
> caused by current, not by voltage and went into a loose connection and
> thermal runaway.

Did the nut on the screw come loose or similar? I can see a temperature 
cycling slowly loosening it, and then once it starts to get loose, the 
temperature cycling could get a lot worse.

The "classic" way to avoid this would be to make sure there's enough 
torque that there's always sufficient preload. I'm not sure what that 
would be. For 1/4-20, it would be pretty high compared to #6 or #10 
machine screws. I'm thinking single digit ft-lbs, which means with a 6" 
long wrench, you're pushing pretty hard on it.