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Re: [TowerTalk] Balun Overheating?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Balun Overheating?
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sun, 20 Apr 2014 22:16:25 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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
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