On 9/28/98 9:00 AM, w8ji.tom at w8ji.tom@MCIONE.com wrote:
>I NEVER use 73 material beads with higher power, or with high common mode
>voltages across the balun in the system. Despite what you read, it is the
>wrong material for your application
>
>The problem is loss tangent. 73 material has a Q of one at 2 MHz. The Q
>gets even lower as frequency is increased. that means each bead looks like
>a little resistor at 7 MHz. If the voltage is high, you get heating of the
>beads. As the material in the beads heats, the curie temperature of the
>material is reached. When the curie temperature is reached, the impedance
>of that area of ferrite drops like a rock increasing current through the
>other beads and causing more heating.
>
>An air wound choke would work much better, or a very long string of 43
>material beads. The air wound choke is the least expensive method, with
>multi-turn cores as an alternative. I don't like the "string of beads"
>concept because the impedance adds in linear proportion to the overall
>length of the string. If you use an air wound balun with good form factor,
>or a large 43 core with multiple turns, impedance goes up by the square of
>the turns.
I know I've suggested this before, but why not use a mix of bead
materials? Have the beads closest to the feedpoint be 43 material, and
the ones away from the feedpoint be 73 material.
This has the advantage of requiring fewer overall beads. (Which is why
you'd use 73 material in the first place) But still has the power
handling of 43 material.
If you had an infinite variety of bead materials, seems the optimal way
to approach the bead balun would be to start with a low-permability
material at the feedpoint and increase the permability as you move away
from the feed. Done right, this would evenly distribute the loss (and
thus heating) across all the beads. None of the beads would get "hotter"
than the rest, and fewer beads would be needed overall.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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