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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