RFI troubleshooting guide
Radio frequency interference in your station. Identify and fix common sources of RFI, feedline radiation, and RF in the shack.
Identify and fix radio frequency interference in your station.
Symptoms
- Computer reboots or freezes when you transmit
- Audio hum or buzz on transmitted signal
- Mouse cursor jumps during transmit
- Hot microphone chassis
- Neighbor's TV or internet is disrupted
- SWR changes when you move the coax
- Touchscreen behaves erratically during transmit
If you have any of these symptoms, you have RF getting where it should not be. The cause is almost always feedline radiation — common-mode current flowing on the outside of the coax shield.
The cause: common-mode current
In a properly functioning antenna system, RF current flows inside the coax: on the center conductor and the inside surface of the shield. In an improperly functioning system, current also flows on the OUTSIDE of the shield. This outside current radiates from the feedline, which means your coax is part of the antenna. The RF follows the coax back into the shack and gets into everything.
The fix: choke the common mode
A choke balun (current balun) at the antenna feedpoint blocks common-mode current from flowing on the coax shield. This is the single most effective RFI fix available.
Option 1: Ferrite choke
Wind 8-12 turns of your coax through a ferrite toroid (FT-240-31 or FT-240-43) at the antenna feedpoint. Cost: $10-15 for the toroid. Effectiveness: excellent on HF.
Option 2: Snap-on ferrites
Clamp snap-on ferrite chokes (Fair-Rite #31 material) on the coax near the radio. Less effective than a wound choke but easier to install. Use 6-10 snap-ons for meaningful suppression. Cost: $3-5 each.
Option 3: Commercial choke balun
Buy a commercial 1:1 choke balun (DX Engineering, Balun Designs, Palomar Engineers) and install it at the antenna feedpoint. Cost: $40-80. Effectiveness: excellent.
Troubleshooting sequence
- Add a choke balun at the antenna feedpoint. This fixes 80% of RFI problems. If you do nothing else, do this.
- Add snap-on ferrites where coax enters the shack. Belt-and-suspenders approach.
- Check your antenna SWR. High SWR means more current on the feedline. An antenna tuner hides the SWR from the radio but does NOT reduce feedline radiation.
- Add ferrites to affected equipment cables. USB cables, speaker wires, monitor cables, ethernet cables — anything that enters or leaves the shack can carry RF.
- Check your station ground. A proper RF ground (not electrical safety ground) provides a low-impedance path for stray RF. A ground rod with a short, wide strap to the station.
The tuner misconception
An antenna tuner does NOT fix feedline radiation. The tuner makes the radio see 50 ohms, but the coax between the tuner and the antenna still has the mismatch and the common-mode current. A tuner masks the problem from the radio while the feedline continues to radiate. Fix the feedline radiation with a choke, then use the tuner if needed for the remaining mismatch.