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RF Chokes - 1:1 Current Baluns for HF | Greyline

Feedline System · Common-Mode Control · HF 160–10 m

RF Chokes for HF Verticals: Where They Go, How They Work, What to Buy

An RF choke, also called a 1:1 current balun or common-mode choke, stops RF from flowing on the outside of your coax shield. That stray current carries noise into your receiver and puts your own signal into the shack, the TV and the computer. It goes at the feedpoint with a shack tuner, or on the radio side of a remote tuner at the antenna. A second, at the radio end, is optional and lowers noise further.

Where the Choke Goes

It goes at the feedpoint with a shack tuner, or on the radio side of a remote tuner at the antenna. Its job is to keep RF off the coax running back to the shack.

• Tuner in the shack: at the feedpoint, where the coax connects at the base of the pole.

• Remote tuner at the antenna: on the radio side of the tuner, between the tuner and the coax. Same job.

A second choke is optional, at the radio end of the coax, to lower noise the coax picks up on its way back to the shack. That's it.

Beads or Wound? The Two Kinds of Choke

Beads on the coax. Ferrite strung along a straight run of coax: the bead choke Walter Maxwell, W2DU, published in QST in 1983. Slim and light, which is why tower stations use beads at a feedpoint a hundred feet up. Each bead adds a little, so it takes many beads to choke well. The five snap-on Mix-31 cores in every Greyline feedline kit are this kind, and they are a good start.

Coax wound through the core. A few turns of coax through one or more ferrite rings, the approach Joe Reisert, W1JR, made popular and Jim Brown, K9YC, turned into a cookbook. Turns multiply: choking rises roughly with the square of the number of turns (ARRL Antenna Book), so a wound choke gets far more from the ferrite. It is bigger, and on a Greyline that doesn't matter, because the coax connects at the base of the pole, within reach.

What to Look for in a Wound Choke

• Coax, not two-wire line. Wire turns that aren't evenly spaced add capacitance that works against the choke. Teflon coax such as RG400 takes the most heat.

• Ferrite chosen for series resistance, the part of a choke that absorbs the unwanted current. K9YC's guide sets a target of about 5,000 ohms; #31 is the HF baseline, and the best builders go further.

• More cores for more power, with air around them in the enclosure so heat can leave.

• Tested, by the builder into a 50-ohm load, and by you: no change in SWR after a long transmission.

How You'll Know It's Working

Your signal stops showing up on the TV, the computer and the speakers. The noise floor on receive comes down. And the SWR holds steady through a long FT8 or CW session, because a choke that isn't running hot isn't changing.

Our Recommendation

The Artisan Hand-Made Choke, 160 Through 10 Meters

Custom-made by hand, one at a time: RG400 Teflon coax wound through ferrite chosen for series resistance, SO-239 on both ends in a waterproof box, each one bench-tested before it ships. It's large, it's beautifully made, and it works. We've operated a big multi-tower contest station that runs them, and the bands were quiet.

On our network analyzer, an all-band unit measured 38.6 dB of common-mode rejection at 1 MHz, 27.8 dB at 10 MHz and 12.9 dB at 28 MHz: it cuts the unwanted current about 85 times just below 160 meters, about 25 times on 30 meters, and about 4 times on 10 meters.

500 W, Two Cores · $119 →High Power 1.5 kW, Three Cores · $159 →

500 W is the one most stations want. High Power is for legal-limit amplifiers. Hand-made means patience: each one is built by hand, and at times there will be a backlog. Your order holds your place in line, and every choke ships within our three-week window.

Hand-made · WA1BXY

Questions Owners Ask

Does a Greyline need a choke?

Yes, and every Greyline ships with one: five snap-on Mix-31 ferrite cores that go on the coax. For an amplifier, a long coax run or a noisy neighborhood, a wound choke is the step up.

I have a remote tuner at the base of the pole. Where does the choke go?

On the radio side of the tuner, between the tuner and the coax. It does the same job: keeping RF off the coax running back to the shack.

Do I need two chokes?

One, at the feedpoint or on the radio side of a remote tuner, does the main job. A second at the radio end is optional and lowers the noise the coax picks up on its way back to the shack.

Beads or a wound choke?

Beads are slim and light, which matters on a tall tower. A wound choke gets far more choking from the ferrite because the turns multiply, and on a Greyline its size doesn't matter, since the coax connects at the base of the pole.

Can I run full power on FT8 through a choke?

Rate the choke above your power, not at it: the 500 W Artisan for stations up to 500 watts, the High Power for more. Hours at full legal limit and 100% duty is the hardest job any enclosed choke can do; tell us your station in the order notes and the build is matched to it.

What does "dB of common-mode rejection" mean?

How much the choke reduces the current on the outside of the coax. Every 20 dB is ten times less current: 40 dB is a hundred times less, 12 dB about four times less.

Sources: Walter Maxwell, W2DU, QST, 1983, and Reflections; Jim Brown, K9YC, A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing (k9yc.com); the ARRL Antenna Book.

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