I have a good friend who has a J430 aircraft. From day one it was plagued with radio ignition noise. We tried all the usual fixes with little or no results. I did some experiments at home and the source of the problem became apparent. The problem is the negative side of the electrical system, that's why anything done to the positive side of the electrical system yielded no results. Essentially what happens is, the ignition units and the firewall act like transmitters and a semi tuned aerial system. So all Jab aircraft have a built in transmitter and aerial in the negative earth system. This happens because there is a thin sheet metal firewall attached to a non metal fuselage. In a metal airplane the charges leak into the rest of the airplane. In a non metal airplane, the charges radiate off the sharp edges of the firewall. Any radio worth it's salt could hear it.
There are some individual differences because of holes cut through the firewall for cables and controls. The particular size of the firewall tunes it to the ignition boxes.
Other non metal aircraft with jab engines may experience the same radio noise.
Fixing the problem, turned out to be fairly easy. The firewall was taken out of the earth system (electrically) and becomes electrically inert. All braided cables and controls have to be insulated off the firewall not just the battery cables. The end point comes, when a multimeter across the engine and firewall shows an open circuit. This modification dubbed the "Rochedale modification" was carried out on the J430. No more radio noise.!
All the ferrite clamps were thrown out.