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Re: [TowerTalk] Bead Balun Choke Question

To: "Arthur Moe" <>,"tower" <>
Subject: Re: [TowerTalk] Bead Balun Choke Question
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Wed, 20 Oct 2004 17:34:05 -0400
List-post: <>
> What determines the power handling of ability of a Bead
Choke Balun.  I am talking
> about the beads NOT THE COAX. Is it the length (number of
beads) or the diameter??

It is the length, thickness, and material of the bead in
conjunction with the frequency and load characteristics that
determine the power handling.

> Also, What is the right mix for HF???

Depends on the application. For low power and low common
mode voltage stress 73 or 77 beads are OK. A foot long
string of 73 material will normally handle a KW output into
very controlled loads like a dipole with the feedline
dropping straight away.

As the voltage across the beads increases you have to change
to a really long string of lower permeability beads or use
another method entirely.

Strings of beads are really not that good unless you have a
very controlled situation or run low power. Again it all
comes back to how much common mode current flows through the
bead string, and that varies wildly with load impedance.

One VERY common misconception is placing a string of beads
on the input of a floating network tuner magically relaxes
bead requirements. It does NOT. The beads might as well be
on the output if the network floats, because the common mode
voltage across the beads (and current through them) will be
essentially the same. It doesn't help balance, it doesn't
help heating.

73 Tom


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