Q Colin, VE1CSM, asks, “I’ve built a computer interface for use with my Kenwood transceiver. The interface works, but it seems to be generating interference. Can you help?
A Computer interfaces and their connecting cables are all potential sources of interference, both in terms of RF radiated from the cable and in terms of RF feedback into the devices the cables are connected to.
The fix for both involves two parts: proper shielding and common mode noise suppression via the use of ferrites. If you haven’t put the interface into a metal box, that’s the first thing you want to do. Second is that you would want to use a cable where all the signal-carrying conductors are enclosed in a foil (preferably) or high-density braid shield. A stranded braid wound around the outside of the other wires is not sufficient.
Even with all that, you may still have problems. The next step is the judicious application of Type 43 ferrite cores. To suppress common-mode signals (where the interfering signal is being picked up by the whole cable), you would take a suitable-size ferrite and wrap a number of turns of the cable around it. Donutshaped (toroid) cores are best in that they couple the energy back into the core, whereas rods tend to “leak” a little at the ends. As far as the number of turns goes, more is generally better, but there is a limit. If you wrap three turns around the ferrite and it helps, that’s great, but if it doesn’t, it usually means you need more turns. However, if you get a dozen turns around the ferrite and it doesn’t help, chances are two dozen turns won’t be any better.
From QST June 2000
