[TowerTalk] Balun Overheating?

[Roger (K8RI) on TT]

On 4/20/2014 10:26 PM, Jim Lux wrote:
> 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

Just stick 'em in place with a dab of epoxy every few inches.
It will prevent them from moving around, but let them breath.  The stuff 
for plastic will keep the wires in place and set up in just a few 
minutes. You might have to work a bit fast with the 5 minute stuff. If 
so, they make a 15 minute version.  As they "Prevent" current flow" I'd 
not expect a lot of heat.

73

Roger (K8RI)


>
>  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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