Computer Radio Frequency Interference Kits
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.
With the RFI-DIGITAL kit, you run the wires entering the device through ferrite cores right next to the interface. Ferrites do not affect the signals going through the wires but they resist the passage of RF keeping the RF out of the appliance and stopping the RFI.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.
Use two or more turns thru the snap on ferrite and use one at the device and another on the end of the cable near the radio. If your interface device uses a DC supply you can wrap the DC supply cable in the ½” ferrite with the radio cable. Use at least 2 turns for best results.A completed installation will look like the picture below (remember to add similar ferrites at the radio end of the cable).
Often it is difficult to slip beads on a cable because of a plug or connector. Split beads solve this problem. They come in two halves that fit over the cable. A plastic snap cover holds the two halves together and holds the assembly on the cable.
For RFI use, mix 31 is effective from 1-300 Mhz, mix 43 works from 20-250 Mhz, Mix 61 is for 200-2000 Mhz, and mix 77 favors .1-20 MHz. These frequencies are those of the interfering signal to be eliminated, not the operating frequencies of the equipment to be protected.
Not sure of what sizes you need or solving a RFI problem with devices using multiple size cables? Try the Ferrite Snap On Combo Pack which has sizes from 1/4″ to 3/4″ with either Mix 31 (1-300 MHz) or Mix 61 (200-2000 MHz) or Mix 75 (150 KHz – 10 MHz).
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Bead dimensions are shown in the picture below and in the table as A, B, C
Ferrite Split Bead Dimensions
Application Notes
Application Note for using these Snap On split beads: How Many Chokes do I need?
You can very effectively increase the choking impedance of smaller cable by using a large hole ferrite bead and wrapping the cable through several times. For particularly difficult common mode current/RFI problems you may need a choking impedance as high as 5-10K ohms. Using multiple beads and multiple turns through each bead will give higher impedances up to a point. At higher frequencies, when the number of turns approaches an electrical wavelength, the choke becomes resonant due to stray capacitance and the inductance of the cable and limits the high end impedance values to less than the above table points.
Some example of ways to wrap cable are shown below.
 FSB-1/4 – 1 turn |
 FSB-1/2 1 turn |
 FSB-1/2 Open Shell |
 FSB-1/2 2 turns |
 FSB-1/2 3 turns |
 FSB-1/2 4 turns |
—–//—–
More examples of split bead use
Ethernet Cable on FSB-1/4
Ethernet Cable on FSB-1/2 – 3 turns for 9X Impedance good filter for 15-30 mhz between 2K-3K ohms choking Z
Three Effective Radio Interference Filters
 FSB-1/4 or FSB-1/2 4 turns for 14-50 Mhz 2K-3K Z |
 FSB-1/2-5 turns for 10-30 Mhz 2K-4.5K choking Z |
 FT240-43 – 14 turns for 3-14 Mhz or use two FT240-43 for 1.8-10 Mhz |
These snap-on chokes are useful for reducing RFI in public address system amplifiers, speaker wires, mic cables, mixer panels, coax cable, telephone wires, computer sound card speakers, dsl/cable connections, remote control heads for ham transceivers (Especially FT-100, FT900, IC-706, IC-7000), AC power cables, DC power cables, home theater cables, video cables, HI Fi cables, guitar amps, linear amp cables, etc.
They are also helpful in suppressing common mode noise/RFI in radio and TV receivers, audio and RF amplifiers. They can also help suppress noise generated by treadmills, exercise equipment, fish tank heaters, oil heater pumps, fluorescent lights, motors, cable modems, DSL modems, computers, and many other electrical devices. Common mode current reduction in computer cables often helps speed up computers.