On 4/20/14, 10:16 PM, 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.
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.
Just to elaborate on the math..
50 beads is 1/4 R of 200 beads..
So I is 4 times greater, for the same voltage.
I^2*R is then 16* (1/4) .. the total dissipation is 4x greater.
I think the other factor in overheating is that W2DU had the beads
hanging (literally) in the wind. A commercial product is often something
that looks neater: inside a nice tube, or covered in shrink tubing, etc.
but which also has poorer thermal properties.
The real sticky issue when trying to calculate stuff is "what is the
voltage". And that's something that is hard to determine in a simple
way. One can do a numerical model (using NEC, for instance), but in
practice it's highly installation dependent.
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