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.
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