Common-Mode Noise on 2m: How to Detect and Eliminate It
Many operators on the 2-meter band experience unexplained noise when switching from a handheld to a base station antenna. The most likely culprit? Common-mode (CM) pickup on the coax feedline. This article explains how to detect CM pickup and how to mitigate it effectively.
How to Check for Common-Mode Pickup
Use a Portable Radio as a Reference
Tune your handheld transceiver with its original rubber duck antenna to the same frequency where you experience noise on your base antenna. Walk around your station location and listen. If the handheld is quiet, but your base station receiver with a 5/8 lambda vertical picks up noise, that’s a strong indicator of CM noise.
Use the Handheld with an External Antenna
Connect your handheld to the base antenna via coax. If the noise reappears, it’s likely coming in via the coaxial cable's shield acting as an antenna.
Dummy Load Test
Connect a dummy load at the radio end of the coax. If the noise remains, it is likely entering via the shield. If it disappears, your antenna is likely picking up the noise differentially.
Touch and Move the Coax
If touching or moving the coax changes the noise pattern or level, it confirms the coax is acting as a pickup source.
Understanding the Cause
Common-mode currents are caused by unbalanced sources of RF noise, such as solar inverters, LED lighting, or switch-mode power supplies in the near field. These induce RF onto metallic conductors like coaxial cable shields. On 2 meters, where wavelengths are short (about 2 meters), even short coaxial runs can act as efficient CM antennas if not choked.
Mitigation Techniques
Always Use a Double 1:1 Current Choke (Line Isolators)
Use two ferrite chokes made from Type 43 or 31 material. We have a wideband choke or a 2m/70cm specific one for this purpose. Install the first choke at around 0.05 lambda (10–15 cm) from the antenna feedpoint. Place the second choke close to the radio.
This double choke strategy ensures CM suppression both at the antenna and along the feedline.
Keep Coax Away from Noise Sources
Route coaxial cable away from power cables, solar inverters, routers, wall warts, and LED drivers. Keep at least 30 cm separation where possible.
Use Bonding and Shielding
Bond your equipment grounds together with flat copper braid. Avoid ground loops. In extreme cases, ferrite sleeves on equipment power cables can help prevent RF ingress.
Test With a Different Coax Run
Temporarily substitute the coax with a shorter or better-shielded cable (e.g., LMR400) and check if the noise floor drops. Sometimes the coax type or installation method is part of the problem.
Conclusion
A handheld radio is naturally immune to common-mode pickup due to its lack of extended ground or feedline. A base station with an external 2m antenna is not. By understanding where noise enters and applying double current chokes and smart coax routing, you can bring your 2m receive noise floor back to where it belongs.
Quiet signals are good signals. Use your ears, test methodically, and choke that noise before it chokes your QSO.
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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.