The RF or EMF if you will, is generated from several sources in the early Fords. Ignition systems are generally the largest contributor but the generator can also be a heavy source depending on how hard its working at any given time. Resistor spark plugs & suppression type high tension leads are used for the same purpose on modern vehicles. A capacitor can be installed in the generator circuit to suppress RF from that system. This stuff was done to get rid of the noise on the car radios back in the day too except they used resistor caps on the spark plug high tension leads.