[TowerTalk] Balun Overheating?

[Jim Brown]

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.

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