[TowerTalk] Balun Overheating?

[Roger (K8RI) on TT]

On 4/21/2014 1:16 AM, Jim Brown wrote:
> On 4/20/2014 9:24 PM, Roger (K8RI) on TT wrote:
>> As they "Prevent" current flow" I'd not expect a lot of heat. 
>
> Not quite.  First, let's call this critter by it's real name, which is 
> Common Mode Choke. Dan and Jim Lux got it right -- Effective common 
> mode chokes are resistive at their operating frequencies, and heat is 
> I squared R, where R is the resistive component of the choking impedance.

Sorry.  I was thinking more of what I do. I either super glue, or epoxy 
the cores into a single, rigid stack, but I also wind the turns of Davis 
BuryFlex around the core to get close to 5000 ohms resistance. With the 
small, Teflon coax,(forget the nomenclature) I can enclose them in about 
a 3" tube open on both ends, with no heating problems so far.  A 75M, 
sloping, fan dipole takes two to keep RF out of the shack.

73

Roger (K8RI)

>
> If you go back and study W2DU's original paper describing the "W2DU 
> Balun" you will see that he  built and described (at least) three 
> versions -- one with 50 beads, others with 100 and 200 beads 
> respectively. The #73 beads resonate at HF (that is, they are 
> resistive), and the choking Z for the entire choke is the Z of one 
> bead multiplied by the number of beads. One of the older Power Point 
> presentations on my website shows this in detail.
>
> When guys started building W2DU Baluns, they wanted to make money, so 
> they built Walt's smallest design with only 50 beads. That choke does 
> not provide much noise suppression, and it allows four times more 
> current than the choke with 200 beads, so the choke dissipates 4X the 
> power of the choke with 200 beads. If you're running 100W, you may get 
> away with that, but running power will likely fry something.
>
> Bottom line -- chokes fry because their choking Z is too low for the 
> voltage across them. That voltage is proportional to the square root 
> of power, increases significantly if the antenna is badly unbalanced 
> (like Windoms), and also if the feedline is a resonant length.
>
> Power handling is also reduced by surrounding the choke with an 
> enclosure that inhibits air flow.
>
> 73, Jim K9YC
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