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.audiosystemsgroup.com/RFI-Ham.pdf will answer a LOT of these
sorts of questions. It has a cookbook for building chokes, etc.

The challenge is that the Radio Shack ferrites might be a mix more
intended for suppression of VHF spurious signals than HF. I believe
that the Radio Shack ones are either #43 or #61. Most of the snap
together ones are #43 (optimized for 30 to 400 MHz, i.e. to reduce RFI
for FM broadcast and TVI with rabbit ears).

For myself, I use 5 2.4" toroids of #31 material (Fair Rite
#2631803802), and run a half dozen turns of the coax through it. It has
about 30 dB of attenuation from 2-30 MHz. These run about $5 each in
moderate (10-30 pcs) quantity, so the choke is around $25. Get a bunch
of people together in a group buy so you can get 1000 of them, and they
get down to the $2.50 each range.

http://www.fair-rite.com/