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Re: [TowerTalk] W2DU Balun Bead Source

To: "AD5VJ Bob" <>,
Subject: Re: [TowerTalk] W2DU Balun Bead Source
From: "Dan Zimmerman N3OX" <>
Date: Wed, 10 Jan 2007 02:20:48 -0500
List-post: <>
> I jerked my hand back rather quickly, however, because I had in no time at 
> all burnt a bluster > on my thumb and forefinger because
> the ferrite was so hot.

This is a problem with ferrite baluns if the choking impedance is too LOW.

Maybe you had amps of RF current flowing on that coax, and you put a
one ohm impedance in series with it.  It's not enough to cut down the
common mode current significantly,but it is enough to dissipate some

It's definately a problem with *beads* because they're really
physically small and they don't take long to heat up.   If you've got
only a moderate choke impedance but a high RF voltage driving those
common mode currents down the line, you better have a massive core to
take the dissipation.

If you have a very large impedance choke, no current flows, and you
don't dissipate any power.  If you have zero impedance choke, no
voltage drop develops, and you don't dissipate any power.

That said, I have good luck with bead baluns at the 100W level.  I
only use them because I got a freebie box of several hundred of the
size of #43 material core that fits over RG-58 coax, so I use them a

Anyway, I've heard advice against bead baluns for high power because
they are too easy to destroy with heat ( you crack a core because it
gets hot, its impedance disappears, more current flows through the
string, another couple go, more current flows... )  Work great for my
100W HF antennas and 50W VHF antennas though.


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