Hi Pete,
> Yesterday, I was inspecting the choke after bringing the antenna down for
> modification. To my surprise, there was a hole through the tape wrap at
> about the middle of its length. I surmised that it might have been
damaged
> by the birds that perch on the antenna in great numbers, but when I
removed
> the wrapping I'm not sure. First, at the point where the tape was
damaged,
> 2-3 beads are missing, the teflon jacket is open, and the braid is
severely
> disrupted. The cable retains continuity and is not shorted. Looking
very
> closely, the teflon appears to have been melted, and there is some
evidence
> of burning inside the jacket (though this may be discoloration from
> oxidation of the exposed braid).
>
> Even more mysterious, about 10 of the beads adjacent to the opening, on
the
> antenna side of it, have been shattered into 4-6 pieces each, even though
> the tape outer wrapping is intact. There is no evidence of heating other
> than the teflon right at "ground zero."
>
> Anyone out there experienced a similar failure, or understand what's
going
> on here?
Yes. Been there done that many many times over the years.
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.
73 material string-of-bead choke baluns are great in low power low common
mode impedance systems, but not with medium or high power. Get them out of
the high power feedline, save them for Beverage transformers.
73 Tom
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