[Elecraft] KPA500 RFI problem

[Jim Brown]

On 1/9/2013 10:59 AM, David Gilbert wrote:
> More importantly, a balun with a ruined core is going to look bad for SWR

Please delete the word "balun" from this discussion, and use the words 
"unknown black box" instead.  That's because the word "balun" is used to 
describe at least a half dozen circuit components, most of which are 
very different from each other.

A so-called "current balun" is really a common mode choke formed by 
either placing a lot of ferrite beads on coax, or by winding multiple 
turns of coax around/through a ferrite core. The most common failure 
mode of such a choke is overheating of the coax, causing it to deform, 
and in the extreme, to short (between center and shield).

A common mode choke fails when it's choking impedance is too low to 
limit common mode current to a value that does not limit dissipation to 
a safe value.  Excessive common mode current is caused by high common 
mode voltage, which results from antennas that are unbalanced 
(especially "Windom" style antennas, and, of course, by running higher 
power.

Many years ago, before I had learned about what was inside an unknown 
black box called a balun at the feedpoint of a dipole, I fried one by 
tying both sides of the feedline together to feed it as a top-loaded 
long wire, and running 100W for a weekend 160M contest.

73, Jim K9YC