On 8/30/2011 2:06 AM, DJ7WW wrote:
> My chokes are wound through 4.5" toroids #61 mix.
#61 is a TERRIBLE material for a common mode choke, because it is a
relatively high Q, low loss material on the HF bands. It is designed
for use as a TRANSFORMER or inductor at HF, NOT as a suppression
material. A common mode choke must be very LOW Q (high loss) to
function effectively. It is the RESISTANCE of the choke in the region of
its resonance that makes it effective, NOT its inductance. Chokes wound
on #61 have very narrow resonance (typically only a single ham band) and
are VERY difficult to measure, both because their impedance at resonance
is so high (typically 10K or more) and because the stray capacitance
associated with that resonance is only 1-2 pF.
#61 IS an effective suppression material in the low UHF spectrum
(400-900 MHz).
These principles have been well known by engineers who work with ferrite
materials since the 1940s, and references to them can be found in
applications notes from major manufacturers of ferrites from Europe and
North America. The classic texts on ferrites were written between 1950
and 1975 by a Dutchman, E. C. Snelling, who worked for one of those
companies.
See my RFI tutorial for a much more detailed discussion of these
principles.
#43 material, made by Fair-Rite, and similar materials made by several
other companies, is effective for multi-turn chokes down to about 4
MHz. #31 material, made only by Fair-Rite, is a new material developed
around 2000 that is effective for multi-turn chokes down to the AM
broadcast band. Both of these materials are good for suppression up to
about 200 MHz. These are very low Q materials, typically about 0.4 in
the HF spectrum.
73, Jim Brown K9YC
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