EMI? Use The Proper Ferrites!

Transmitters and computers often don’t get along.  Cables going to computers, routers, ethernet switches, USB hubs (and anything else with wires) can pick up RF and disrupt operations.  This RF can be coming from your near-by antenna, from leaky coax, or from the transmitter itself.  Usually when this happens the easiest fix is to add ferrite chokes to the cables.  Sometimes it takes several chokes on a cable.  You will end up experimenting.

The ferrites you will find on eBay (etc.) are probably designed for VHF signal suppression. They will work after a fashion on the HF bands, but you will have better results if you use ferrites made from a material optimized for HF frequencies. Cores using “31” material from the “Fair-Rite” company are excellent performers. These come in all shapes and sizes, but the clamp-on core shown below has proven to be quite useful.



Fair-Rite Products Corp
Part Number: 0431164181
Lower & Broadband Frequencies 1-300 MHz (31 material)

You can buy these from Mouser:

Here are some guidelines for ferrite core use:

  • Put the core as close as possible to the equipment.
  • If possible take two or three turns through the core. Up to a point, the suppression effect is proportional to turns² (turns-squared).
  • If possible, run power and ground together through the core.
  • Before buttoning things back up, secure the heavy core to protect the wires and connectors.  This advice is definitely appropriate for boat radio installations, but perhaps not necessary when in a stable environment.