[TowerTalk] Balun Overheating?

[Jim Lux]

On 4/20/14, 6:34 PM, Tom Hellem wrote:
> Fellow TTers:
>
> I built a balun for my triband yagi consisting of 50 type 73 ferrite beads
> slipped over a piece of RG 303
> coax, enclosed inside of a piece of 1-1/2" pvc. Had one of the wires break
> inside and I surmised it must have
> been a result of wind-caused vibration, etc. So after I repaired the broken
> connection, I squirted some of that
> canned foam insulation (the kind that is sold for foaming around windows &
> doors, etc.)into the pvc enclosure,
>   encasing the entire piece of coax & ferrite beads in the stuff thereby
> preventing any vibration , etc in the future.
> Upon further thought I am wondering now did I invite the balun to overheat,
> especially while running high power,
> as any heat generated has no way to escape.
> Does anyone have an opinion one way or 'tother?
>
>


It kind of depends on what the choking impedance is.

It's all about I^2*R, after all

  if the impedance is very low (e.g. not a very good choke/balun), then 
there's not much dissipation.  Lots of RF current flows, but most of it 
just goes right on through and isn't absorbed.  That is, the R is small.

Likewise if the choking impedance  is suitably high, ( a good 
choke/balun), not much current flows, so the absorbed/dissipated power 
is small.

The worst is when the impedance is just wrong.. low enough to let 
current through, high enough to dissipate power.