making a common mode filter
 

Ed G wrote:

I am having a heck of a time locating a source of good information on
how to make an effective common mode filter using the snap-on type ferrite
beads such as available at RAdio Shack. Planned usage is for RG-8 coax
feeding an SGC tuner, with primary operation on 75 & 40 Meters, but
possibly up through 10M. Any help?


Ed K7AAT

http://www.yccc.org/Articles/W1HIS/C...S2006Apr06.pdf makes
reference to the RS snap-on ferrites being mix 43 (page 25):

"One 'turn' on (i.e., one pass through) a Fair-Rite mix-43, splitbead,
'Cable Snap-itT ' about 1.25 inch long by about 3/4 inch square, able to fit
around a cable of diameter up to 1/4 inch. Radio Shack sells these, and many
are found at hamfests.
Freq.(MHz) / Z(ohms)
3.5 / 40
14 / 115
28 / 171
There is no angle data here; but note that the variation of Z as a function
of f here is similar to that seen above for the string of mix-43 toroids."

Using that info, you'd stack ferrites in order to achieve the impedance
you're seeking. At 80m, if you want 400 ohms, you'd use 10 of them.
However, I believe it'd be cheaper to stack ordinary ferrite toroids such as
Amidon FT82-43 (u=850, AL = 557mH/1000T, 0.516" ID). With these, 400 ohms
at 3.5 MHz = 18.2 uH. At 557mH/1000T and one turn per core, you'd use 5.68
cores (round up to 6 cores). Amidon sells 'em for $1.20 ea

Bryan WA7PRC