How a Bad Choke Can Turn a Great Antenna into a Terrible One

You can build the most carefully tuned, beautifully matched antenna in the world — and still make it perform worse than a clothesline if you choke it wrong at QRO levels.

The problem isn’t the antenna itself. It’s the return currents sneaking back on the outside of your coax braid — what most call “common-mode currents,” but are really stray return currents that escape when the choke isn’t doing its job. These currents re-radiate, change your radiation pattern, increase noise pickup, and can even make the feedline part of the antenna. In other words: your coax starts transmitting too.

When “a little ferrite” isn’t enough

Many hams throw one FT-240-43 or FT-240-31 core on the feedline and call it a day. At 100 W, that might work — but at the legal limit (1.5 kW), it’s a recipe for RFI, distortion, and burned ferrite. A single core typically offers only 1–3 kΩ of choking impedance across HF. That’s not enough to stop the coax from becoming part of the radiating structure.

At high power, even modest stray return currents can heat a small choke dramatically. Once a ferrite core reaches about 100 °C, its permeability (μ′) starts to collapse. Push past that, and the damage is permanent — the choke’s impedance drops, losses rise, and what was meant to suppress current becomes a warm, lossy radiator.

What happens when the choke fails?

• Pattern distortion: your carefully modelled lobes shift or disappear as the coax radiates.
• RFI outbreaks: feedline radiation couples into shack wiring, microphones, and USB cables.
• Ferrite fatigue: repeated heating/cooling cycles change µ′ and µ″ permanently, killing suppression performance.
• Impedance drift: a once-quiet antenna becomes noisy, asymmetric, or hard to tune.

At that point, it’s not the antenna’s fault — it’s the choke’s.

The physics behind the failure

On transmit, the antenna drives two current paths: the differential feed current inside the coax, and the unwanted stray return current flowing on the braid’s exterior. The choke’s job is to block that outside path by adding a high common-mode impedance Z_CM — ideally tens of kΩ. At RF.Guru, we recommend ≥ 10 kΩ for 160–80 m and ≥ 6–8 kΩ for the higher HF bands. That’s why our QRO chokes use multiple stacked FT-240 cores — not to look impressive, but to keep core temperature and stray currents under control.

The deceptive “cool coax” myth

Even if your coax stays cool, the choke itself may be frying. The outer braid current creates magnetic flux in the ferrite — and if Z_CM is too low, that flux turns into heat inside the core, not outside on the cable. It’s the invisible failure: no smoke, but the next time you transmit, your pattern has changed and your RFI problem has grown.

Fixing it the right way

• Use more ferrite volume: stack two or more FT-240 cores; impedances add, temperatures fall.
• Split by band: #31 for 160–40 m, #43 for 30–10 m.
• Target 6–10 kΩ: below that, you’re only half-choking the current.
• Measure it: a clamp-on RF ammeter tells the truth — < 50 mA is excellent, > 300 mA is trouble.

Takeaway

A good antenna deserves a good choke. At QRO levels, undersized or mistuned chokes don’t just “waste a few watts” — they wreck your radiation pattern, raise your noise floor, and cook your ferrite.

So the next time you’re tempted to save ferrite, remember: in QRO, a bad choke makes a good antenna bad.

Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.

Questions or experiences to share? Contact RF.Guru.