Others are wound so that the current flows in opposite directions. The magnetic field created by one winding is multiplied with that created by the other, resulting in a greater net magnetic field. Some bifilars have adjacent coils in which the convolutions are arranged so that the potential difference is magnified (i.e., the current flows in same parallel direction). For three strands, the term trifilar coil is used. Wire can be purchased in bifilar form, usually as different colored enamelled wire bonded together. It is commonly used to denote special types of winding wire for transformers. In engineering, the word 'bifilar' describes wire which is made of two filaments or strands. First, will decreasing the size of the enameled wire significantly affect the feed line impedance? In the guide the 12 gauge enameled wire is about 50 ohms.A bifilar coil is an electromagnetic coil that contains two closely spaced, parallel windings. The other question is, if all things remain equal (toroid mix 31, number of bifilar turns, wire type, etc) and I simply stack a second toroid core and wind my choke on 2 cores instead of one, will this increase the choking impedance significantly without changing anything else? If I wind the same choke with 14 gauge wire will it remain close to 50 ohms? First, will decreasing the size of the enameled wire significantly affect the feed line impedance? In the guide the 12 gauge enameled wire is about 50 ohms. So winding and testing isn't an option for me. I do not have sophisticated test equipment (it would be far, far cheaper to buy a commercially made choke). What I want to do is make a common mode choke with these materials that has enough impedance to avoid overheating with legal limit power (4-5K ohms), has close to 50 ohm transmission line impedance, and keeps this high choking impedance on 80, 40, 20 meters primarily. I have on hand 3 of the FT240-31 ferrite toroids, some 14 gauge enameled wire (not 12) and a suitable enclosure. Looking at the charts in the guide, it appears that the enameled wire has a lower overall common mode impedance than the THHN. The guide mentions that closely wound enameled 12 gauge wire has an impedance closer to 50 ohms. However, with the choke inline with my antenna (indoor mounted screwdriver) my radio's internal tuner cannot match the antenna. The choke reduces the noise floor as measured on my radio's meter from S9 to S6, so it is definitely having a beneficial effect. I get 100 ohms on my antenner analyzer when connected to a 50 ohm dummy load through this choke. I built one of these chokes and it seems to follow the data in the guide. The guide mentions that theoretical impedance should be around 100 ohms. In this guide, he lists a formula for winding a choke on a 2.4' mix 31 toroid core using 14 gauge THHN house wire, 14 bifilar turns. I have been reading up on a few different websites about common mode chokes.

Bifilar wire