----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <topband@contesting.com>
Sent: Wednesday, August 31, 2011 10:34 AM
Subject: Re: Topband: Fabricated common mode chokes - Sourcing
> On 8/30/2011 10:30 AM, ZR wrote:
>> Did you open up a Cushcraft balun to discover that method? I believe some
>> other companies do similar and Teflon wire is used so it doesnt char as
>> does
>> THNN.
>
> "These methods" are well documented in the literature, especially Jerry
> Sevick's work (W2FMI). In one of his later books, he discussed some
> chokes wound with THHN, noting that their Zo was on the order of 100
> ohms.
With Sevick you had to wait an edition or two until he made corrections to
some of his "assumptions". There was also a series in CQ that were often
questionable. There would often be long discussions on the air with folks
taking him to task.
What I have NOT found in the literature are discussions of the
> significance of choking impedance, nor of measurements of choking
> impedance. I may have been the first to do that for common mode chokes
> outside of the military.
Far from it, you havent been looking hard enough. As far back as the 80's it
was published in ham journals, particularly the NCJ and some individual club
contest journals as the subject was important to contesters. When I was
selling large 43 mix beads in 85 I was sent a breakdown of impedance by
band. I later did some work on my own of the FT-240 43 and 72/77 cores. At
some point in the mid-late 80's it was a presentation at Dayton by one of
the top contesters with a strong RF engineering backround; I dont remember
who for certain so wont do guesswork but several are on this reflector.
A few years ago, after I published my work,
> someone sent me a military research document that did so in the 70s (and
> reached the same conclusions that I did).
I had documentation from Mitre published around 81 when I worked at Wang
Labs RF Group and earlier at Sanders Associates (late 70's) who was involved
in the Tempest program which include EMI/RFI interference from a wide range
of equipment.Lots of ferrites were involved and Fair-Rite was just a small
player then.
> As to charring -- that's an issue if the device in question is seeing a
> lot of dissipation. Properly designed and applied, chokes should NOT
> see a lot of dissipation unless they are being used in series/parallel
> combinations to do impedance transformation.
A strict sleeve balun with no impedance transformation can get very hot when
the antenna VSWR increases. Sevick didnt understand the real world when he
wrote his assumptions on power handling capabilities based strictly on 50
Ohm lab results. Unfortunately many ham antennas are not 50+J0 over the full
band.
For this reason Teflon wire or coax is often used when the capability of
many readily available amps is involved.
Carl
KM1H
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