Common-mode choke
| Common-mode choke | |
|---|---|
| Also called | Choke balun, 1:1 current balun, line isolator |
| What it does | Blocks RF current on the outside of a coax shield, or on any cable, while passing the signal inside |
| Common types | Coax wound on a ferrite toroid; ferrite beads or clamp-on cores; coiled coax |
| Ferrite commonly used | Mix 31 for HF, especially the lower bands; mix 43 for upper HF and VHF |
| In use | |
| Main location | At the antenna feed point |
| Other locations | Where the coax enters the building; on power, USB and audio cables in the shack |
| Cures | RF in the shack, hot microphones, receive noise, unstable SWR |
A common-mode choke stops radio-frequency current from flowing on the outside of a coax shield, or along any other cable where it doesn't belong. It is usually nothing more than several turns of coax wound through a ferrite ring, or a coil made from the coax itself, yet it cures a long list of station problems, from a microphone that tingles on transmit to a computer that locks up whenever the radio keys. A 1:1 current balun is the same device under another name.
Common-mode current
Coax normally carries signal current along two paths, the center conductor and the inside surface of the shield, with the two currents equal and opposite. Their fields cancel outside the cable, so it neither radiates nor picks up signals. This is the differential mode, and it's how coax is meant to work.
Because RF current flows only on the surfaces of conductors, the outside of the shield is effectively a third conductor, and current can flow on it independently. That is common-mode current. It gets there in several ways: coax connected directly to a balanced antenna such as a dipole; an antenna that isn't symmetrical; a feed line that runs alongside the antenna instead of leaving it at a right angle; or an end-fed or vertical antenna without enough counterpoise or radials, which then uses the coax in their place.
Why it matters
With common-mode current, the feed line becomes part of the antenna. The usual symptoms are:
- RF in the shack. The radio case, microphone or key is "hot," giving small RF burns to lips or fingertips, or transmit audio that other stations report as distorted or buzzing.
- Misbehaving electronics. Computers, USB audio interfaces for FT8 and household devices lock up or act strangely on transmit.
- Noise on receive. The coax runs through the house and picks up noise from chargers, televisions and power supplies, raising the noise floor.
- A distorted pattern, with power radiated, or simply lost, by the feed line instead of the antenna.
- A changing SWR that moves when the coax is touched, rerouted or changed in length.
Types of choke
Ferrite chokes are the most effective type and work over the widest frequency range. Several turns of coax are wound through a large ferrite toroid, or a long string of ferrite beads is slipped over the cable. Ferrite is made in different mixes, formulations whose properties vary with frequency. Mix 31 is widely recommended for HF chokes, particularly on the lower bands, and mix 43 is a common choice for the upper HF bands and VHF. At the frequencies where it works, a good ferrite choke behaves mainly as a resistance, absorbing common-mode current as a little heat rather than resonating, which is what makes it broadband. Widely followed guidance for transmitting chokes aims for a common-mode impedance in the thousands of ohms on the bands in use. Less still helps, but may not cure a stubborn problem.
Clamp-on cores are split ferrite cylinders that snap over a cable without unplugging it. A single core on a straight cable adds only a little impedance at HF, so they work much better with the cable looped through two or three times, or with several cores in a row. They are ideal for the cables in the shack: power leads, USB and audio cables, and microphone cords.
Coiled-coax chokes, sometimes called "ugly baluns," are a handful of turns of the feed line wound into a coil and taped together. They cost nothing and help, but only over a limited range. A coil of coax is an inductor whose own capacitance makes it resonate, so a coil that works well on one or two bands may do little on others. A neat single-layer coil works better than a bundled scramble. At VHF, where little inductance is needed, a few tight turns of coax at the feed point of a J-pole or ground plane make a reasonable choke.
Where to put them
The first choke belongs at the antenna's feed point, where common-mode current starts. A second choke where the coax enters the building, or near the radio, deals with current picked up along the way and keeps noise collected by the outside of the shield from reaching the receiver. On an end-fed half-wave antenna, the choke usually goes a short distance down the coax from the transformer, so that the stretch of shield before it serves as the counterpoise.
Chokes are not a substitute for proper grounding, and grounding is not a substitute for chokes: a ground wire from a second-floor shack is usually too long to do much at RF.