Balun and Common Mode Choke Selector

A balun and a choke are the two least glamorous components in an antenna system and between them they cure most of the mysterious problems in amateur radio: the SWR that moves when you touch the coax, the tingle from the microphone, the computer that crashes on transmit, and the receive noise that appears only when the antenna is connected. All of those are the same fault, and it has one fix.

The problem a choke solves

Coaxial cable carries current on three surfaces, not two. There is current on the centre conductor, an equal and opposite current on the inside of the shield, and a third current that can flow on the outside of the shield. That third one is common-mode current, and it is not part of the transmission line at all.

Feed a symmetrical dipole with coax and you have connected a balanced load to an unbalanced line. The two dipole legs draw equal currents from the two coax conductors, but the outside of the shield is a third conductor sitting right there, and some of the return current takes it. The consequences follow immediately:

• The feedline radiates. It is now part of the antenna, with a pattern nobody designed, running vertically down the side of your house.
• The pattern is distorted. The nulls fill in, the gain moves, and the antenna does not behave the way the modelling predicted.
• RF appears in the shack. Burns from the microphone, tingles from the radio case, and equipment misbehaving on transmit.
• Measurements become meaningless. The SWR changes when you touch the coax, move it, or coil it, because you are altering the antenna.
• Receive noise rises. A radiating feedline is also a receiving feedline, running past every noise source in the house.

A common-mode choke inserts a high impedance in the path of that third current while doing nothing at all to the differential current inside the cable. The transmission line keeps working normally and the unwanted current stops.

Turn counts on a type 31 core

The mechanism behind those numbers: a wound choke has both inductance and self-capacitance, and it self-resonates where they balance. Choking impedance peaks at that resonance, so more turns moves the peak lower in frequency. Twelve turns puts the peak in the middle of the HF range, which is why it is the default recommendation.

A turn is one complete pass through the core. Wind them side by side rather than overlapping, leave a small gap in the winding rather than closing the circle completely, and secure the ends so the cable cannot work loose in wind. An FT-240-31 ferrite toroid core$11.67 costs about twelve dollars and this is the single best value component in an antenna system.

Balun ratios and what each one is for

Impedance transforms as the square of the turns ratio, which is why the numbers jump the way they do. A 7 to 1 turns ratio produces a 49 to 1 impedance ratio, and 50 ohms times 49 is 2450 ohms, which is where a typical end fed half wave sits. A 2 to 1 turns ratio gives 4 to 1 impedance, and 50 times 4 is 200 ohms, which is a folded dipole.

The important thing to understand about the transforming ratios is that they do not make an antenna resonant. A 9:1 unun on a random wire brings a wildly varying impedance into a range where a tuner can finish the job; it does not replace the tuner and it does not make the wire a good antenna. The end fed 49:1 case is different only because the wire genuinely is resonant and the transformer is matching a known impedance.

Voltage balun or current balun

A voltage balun forces equal and opposite voltages at its two output terminals. If the load is perfectly symmetrical that produces equal and opposite currents and everything works. Real antennas over real ground, with one leg nearer a house and the other over a garden, are not symmetrical.

A current balun forces equal and opposite currents regardless of what the load impedance does. That is what actually suppresses feedline radiation, because feedline radiation is caused by unequal currents. For amateur wire antennas the current balun is the better choice in nearly every case, and it is what a coax-wound choke inherently is.

The practical consequence: a length of coax wound on a ferrite core at the feedpoint is a 1:1 current balun. You do not need to buy anything else for a dipole, a Yagi driven element or a vertical. Commercial versions such as the Radiowavz B11A 1:1 air core balun$52.94 use an air core, which cannot saturate and therefore survives legal-limit power, and the Balun Designs 1171 1:1 current balunnot on amazon is the type 31 ferrite equivalent at 5 kilowatts. A Fumei 1:4 HF balun, 200 W$28.99 covers the 4:1 case for a folded dipole or a doublet.

Where to put it

• At the feedpoint, always. This is where the imbalance originates and where the choke does the most good. On a dipole it is at the centre insulator; on a vertical it is where the coax meets the radiator and the radial bond.
• A quarter wavelength down the coax, for an end fed half wave. The current maximum on the shield sits roughly there, and a second choke at that point is considerably more effective than a second one at the transformer. The end fed calculator gives the distance for your band.
• At the station entry point, as insurance. A choke where the feedline enters the building keeps anything that survived the first two from reaching the shack. This is also where the bonded entry panel and the arrester go.
• On the DC and control cables, if RF persists. A few turns of the power lead through a ferrite core at the radio catches the path people forget.

A commercial in-line choke such as a Palomar Engineers feedline common-mode chokenot on amazon does the mid-feedline job in a sealed package if winding a toroid outdoors is unappealing. Full field on baluns and common mode chokes compared.

Power rating, saturation and heat

A ferrite core has a power limit, and exceeding it does something specific rather than simply failing: the core saturates, its impedance collapses, the choke stops working, and the energy that was being reflected is now being absorbed as heat. A saturated core gets hot enough to crack, and it does so while the choke is silently no longer doing its job.

The rule of thumb worth carrying: if a choke or balun is warm after a normal transmission, it is undersized. Warm means it is dissipating power that should have gone to the antenna, and hot means it is about to stop working entirely.

Diagnosing whether you actually have a problem

Four tests, none of which needs equipment you do not already own.

1. Touch test. With the radio at low power into the antenna, watch the SWR while touching the coax outer at various points. A reading that moves is common-mode current, full stop.
2. Coil test. Loop a few turns of the feedline loosely and see whether the SWR changes. It should not.
3. Route test. Move the feedline away from the wall by a foot. If the match shifts, the feedline is radiating.
4. Sweep test. A NanoVNA H4 vector network analyzer$89.90 sweep with ripples superimposed on an otherwise sensible curve is showing feedline resonances, which only happen when the shield is carrying current.

If any of those is positive, add a choke at the feedpoint before doing anything else to the antenna, because every measurement you take until then is measuring the wrong thing. The full diagnostic sequence and the cures for RF that persists after choking are on common mode current and RF in the shack.