Balun Burnups

[W8JITom at aol.com]

In a message dated 96-09-25 22:32:31 EDT, you write:

>W7LR did some test of current baluns and overheated them as well.  Does
>anybody know what the loss mechanism is?  Eddy currents or somesuch?  I
>kinda thought, from reading Maxwell's book, that the ferrite beads
>increased the reactive impedance looking down the outside conductor of
>the coax and that this increased impedance reduced the current flow.  If
>there is heating (ala KT34XA traps :)) its gotta come from somewhere.  
>
>73, Fred, KE7X
>
>
The loss mechanisim is this. The bead looks like a resistance in parallel (or
series) with a reactance. It's the thing we call Q. Hi initial permeability
materials have low Q at higher frequencies. So high permeability beads look
more and more like a resistor than an inductor as frequency is raised, and
can even look capacitive if the frequency is high enough! It does relate to
eddy currents INSIDE the bead. Heating is a fuction of Q and the voltage
across each bead (current through the bead). You can translate this to
"common mode volts per bead" at a given frequency.

The beads are so small, you don't need to lose much power to overheat them.
Fractions of a dB will make them very hot at 1500 watt power levels. Even
though they are getting hot, it doesn't mean they are eating up all your
power. But it does mean they can't be used.

Papers by W7EL and AI1H explain the application better. But for high power,
you'll be better off with an air core choke baluns.

73 Tom

>From aa4lr at radio.org (Bill Coleman)  Thu Sep 26 15:01:04 1996
From: aa4lr at radio.org (Bill Coleman) (Bill Coleman)
Date: Thu, 26 Sep 96 10:01:04 -0400
Subject: Power and offspring
Message-ID: <960826095824.JAA15974 at gate.iterated.com>

>From:        Larry Tyree, tree at lady.axian.com
>
>  And when I was at W7NI, I didn't inhale.
>
>Tree N?TR

Sheesh! Not only can the big guns go 48 hours with no sleep, but they 
don't have to breath either! 

Bill Coleman, AA4LR           Mail: aa4lr at radio.org
Quote: "Not in a thousand years will man ever fly!"
            -- Wilbur Wright, 1901