On Thu, 17 Jun 2004 20:38:38 -0400, Tom Rauch wrote:
>As for beads, people just throw a few beads at the problem and assume they
>are making some profound change in the system. Actually most beads, like
>most cables, have little effect on the system. You might remove medium
>amounts of RFI, but won't likely make any large canges.
My 6m SSB radio was getting into the landline in my wife's office. A 1" long
Fair Rite #43 clamp-on took it right out.
You might be surprised to learn that multiple turns around large ferrite
cores yield choke impedances of more than 1K ohms over a couple of octaves.
For the HF bands, Fair Rite #31 and #43 are the weapons of choice. Yes, a
single turn is ineffective at HF, but the interwinding capacitance of
multiple turns around a toroid or other cylindrical core will pull the
effective frequency down into the 7-25 MHz range! And a dozen turns around a
#78 2.4" toroid hits well over 1K in the AM broadcast band!
The principle is well documented in the Fair Rite catalog, available for
download as a pdf on their website. See the applications note in the back of
the catalog on the use of ferrites for suppression. In my hard copy of the
catalog (14th edition) there's a graph showing the impedance of multiwinding
ferrite chokes on page 181. Three turns around a 28mm OD, 14mm ID #43 core
yields Z>1K from 7 MHz to 70 MHz!
I'm currently in the midst of documenting the behavior of a variety of the
larger Fair Rite products with an eye toward suppression at HF. My data is
quite consistent with that Fair Rite graph.
Jim Brown K9YC
http://audiosystemsgroup.com
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