RFI-DIGITAL – for Digital Audio/PSK31/FT8, etc. These RFI kits are specifically selected to cure most household computer device RFI problems. It contains Snap On ferrite split beads selected to eliminate RFI from 1 to 300 MHz. The beads are easy to use, don’t require modification of the protected equipment and work in almost all cases. What causes RFI? Strong signals from nearby amateur radio, CB, AM/FM broadcast stations and other “transmitters” (such as microwave ovens, cell phones, desktop, laptop, and tablet computers, wireless phones, baby monitors, fish tank heaters/pumps, etc) that are picked up by long wires (“antennas”) running through the house including electric power wiring, telephone cables. alarm system wires, speaker wires, whose signal may be coupled into the computer connected to the digital interface with your radio causing intermittent operation. How to cure RFI – RFI can be stopped by suppressing the signal from the radio “transmitter” or suppressing the signal on the “antenna” feeding the affected device. What is included in the kit? The RFI kit is meant to be used with a DIGITAL interface between your computer and the radio. These digital interfaces consist of sound cards, or Signalink type devices using USB or serial cables interfaced to the computer and a RADIO cable to the radio or driven device. Installation The 3/8” snap on is used on the USB/Serial cable between the interface and the computer. Use 1 snap on at the device end of the cable and another at the computer end of the cable and wrap 2 or more turns thru the center of the core and snap shut. Two 3/8" snap-ons included. Similarly, use the ½” diameter snap on beads for the RADIO cable between the device and the radio. Two 1/2" snap-ons included. 
Computer Radio Frequency Interference Kits
For RFI common mode suppression use, mix 31 is effective from 1-300 Mhz, mix 43 works from 25-300 Mhz, Mix 61 is for 200-2000 Mhz, and mix 77 favors .1-50 MHz. These frequencies are those of the interfering signal to be eliminated, not the operating frequencies of the equipment to be protected. See Mix Selection for other applications.
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Bead dimensions are shown in the picture below and in the table as A, B, C
Ferrite Bead Dimensions
Application Note: Use impedances for each bead at your desired frequency to select number of beads needed for desired Z. For example if you have a 50 ohm RG-8 coax cable and want a choking impedance of 500 ohms at 10 Mhz, you can check the FB102 column (since FB102 will pass RG-8 through its center ID) at 10Mhz and find that Mix 31 has a Z of 108 ohms/bead and Mix 43 has a Z of 91 ohms/bead. For 500 ohms we would need 5 mix 31 beads or 6 mix 43 beads to have greater than 500 ohms. Mix 31 has better response below 10 Mhz but above 10 Mhz mix 31 and mix 43 are very close. If you need more Z, just use more beads in series on the cable or use more windings thru a larger ID bead (e.g. USE FB102-31 for 4 turns of RG-8X for 16X increase in Z per bead – see pictures below for examples). Use Mix 61 for VHF and above for RFI/EMI suppression and below 30 Mhz for multi-ratio impedance transformers (baluns/ununs). See Mix Selection for other applications.
For extra large cables use the FB400-31 which has an inside diameter of 3 inches (76mm).
Frequency Range Comparison Chart
(FB56-xx size)
Multi-turn Common Mode Choking Impedance Increase Example (FB56-43)
Ferrite Bead relative size comparison
Left to right sizes: FB56 (ID = .25″), FB102 (ID=.505″), FB103 (ID=.77″)
SLIP ON Sizes for Heliax and Coax Cable
Cascaded Common Mode Chokes/Line Isolators/1:1 Baluns
 RG58 4 turns on 2 FB102-31 for 2000 ohms choking impedance |
 RG58 12 turns on FT240-43 for 4000-6000 ohms choking impedance |
Handy Wire Size reference for ferrite bead sizes needed
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Connector already on cable? See Snap On Ferrite Beads for convenient installation on cables with connector already installed.