Common mode current (CMC) is current that flows along the outer shield (braid) of coaxial cable used as a feedline for driving certain types of antennas. It is undesirable because it can introduce EMI inside the home environment affecting not only your transmitter/transceiver but other nearby electronics as well.
By increasing the impedance along the outer shield, the current is effectively "choked", and the problem is reduced or eliminated. This can be accomplished by placing ferrite beads around the outside of the coax or wrapping the coax itself around a toroid to increase the inductive reactance. Note that the inner conductor and inner shield current paths of the coax are not affected.
I decided to go the toroid route for my project. I used a 4' length of RG58 and wrapped it around an FT240-43 toroid twelve times. This particular toroid is a type commonly used for the HF amateur bands (3-30MHz).
Next, I measured the attenuation by connecting it to my vector network analyzer. From 3 to 20MHz the attenuation was at least 30dB (ie a 1/1000th reduction). The minimum attenuation occurred at 30MHz but was still greater than 25dB. Objective achieved.
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| Attenuation achieved with RG58 coax w/ toroid (12 turns) |
As a baseline for comparison I then took a short piece of the same type of coax and measured its attenuation. The maximum value was under 7dB. Obviously, quite useless as a choke.
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| Attenuation achieved with RG58 coax only (no toroid) |
Finally, the completed choke was installed inside a water resistant case and inserted in-line with my feedline.
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| Completed CMC choke in water resistant case |
