Where Does the Noise Come From?
Understanding Near-Field, Far-Field, and Common-Mode QRM in Amateur Radio
When you're chasing a weak DX contact or trying to enjoy a quiet noise floor, there's nothing more frustrating than persistent QRM. But not all QRM is created equal—and more importantly, not all of it comes through the air.
Many hams are surprised to learn that a lot of the QRM you hear isn’t actually radiated, but picked up directly through near-field coupling, conducted paths, or common-mode currents on your coax. Let’s break that down.
What’s the Near Field?
The near field is the region close to your antenna, typically significant up to about λ/2 for practical diagnosis, though technically the reactive near field extends to λ/2π and the transition zone may reach up to 2λ. In this zone, your antenna behaves like a sensor of electric and magnetic fields—not just radiated EM waves. This means it can pick up interference from nearby devices, even if they aren’t radiating efficiently.
If you’re hearing broadband noise or hash on 17 m (λ ≈ 17.6 m), the likely source is within ~8 to 9 meters of your antenna. That includes your house, your neighbor’s garage, or even your own coax cable.
Common near-field offenders:
- LED lights and dimmers
- Switching power supplies (wall warts, chargers)
- Ethernet cables and network equipment
- Solar charge controllers
- VDSL, PLC, and inverters
The Far Field: True RF Interference
The far field begins around λ and beyond, where electric and magnetic fields become orthogonal, propagate together, and follow the inverse-square law. This is the region of classical radio propagation.
Far-field QRM might come from:
- Broadcast harmonics
- Nearby industrial or amateur transmitters
- Over-the-air interference from distant sources
- Faulty insulators or arcing on power distribution systems
If another ham 1–2 km away reports the same noise, it’s likely far-field radiated interference.
The Missing Piece: Common-Mode Pickup
Even if the noise source isn’t strongly radiating, your own coaxial cable can become an antenna and bring the noise into your shack.
This is called common-mode pickup. It occurs when the outside of the coax shield carries RF currents because of poor decoupling, poor grounding, or imbalanced antennas. The coax unintentionally becomes part of the antenna system.
This happens when:
- Coax is not properly choked or decoupled
- Feedline is part of a non-symmetric or unbalanced antenna
- The environment near the feedline contains strong near-field sources
The common-mode path is most sensitive to noise sources within λ/2 of the coax run.
So if you have 20 meters of coax entering your house and you're operating on 40 m (λ ≈ 20 m), then your entire coax run is within the coupling zone for that band. Nearby devices such as routers, switching supplies, or power cables can inject noise directly onto the coax.
This is often worse on:
- 160 m: Long coax runs with no choking act as noise antennas
- 80–40 m: Very sensitive to wall wiring, routers, and network gear
- 20–10 m: Noise sources must be physically closer (2–10 m) but still couple effectively
- VHF/UHF: Less susceptible to ambient E/H fields, but still vulnerable to conducted noise and local hash if coax runs near digital equipment
How To Tell: Near Field, Far Field, or Common Mode?
| Symptom | Likely Cause | Tip |
|---|---|---|
| Noise changes when rotating loop or beam | Far-field | Rotatable RX antennas help null far-field sources |
| Noise drops when moving coax, not antenna | Common-mode | Add or improve chokes on feedline |
| Noise appears only on specific bands | Near-field or common-mode | λ/2 defines coupling radius—source proximity matters |
| Noise persists even on a dummy load | Conducted or internal | Receiver or power system likely at fault |
Fixing Common-Mode Pickup
To reduce common-mode noise:
- Use a 1:1 current balun (choke) at the antenna feedpoint
- Add a second choke at the shack entrance (for added suppression)
- Add a third one close to your transceiver or pa.
- Use high-permeability ferrite (type 31 or 43) with multiple turns of coax through a core
- Keep coax clear of walls, power wiring, Ethernet cables
- Shorten coax runs where possible and avoid running coax near noisy gear
Remember: the coax shield is not a perfect barrier. Without effective choking, it can act as an unintentional receive element.
Real-World Band Examples
| Band | Key Issues | Common-Mode Tip |
|---|---|---|
| 160 m | λ/2 = 80 m; noise pickup from power lines and shack wiring is dominant | Always choke at feedpoint and where coax enters building |
| 80–40 m | Very susceptible to wall-born noise, SMPS, and network gear | Ferrite choke required near shack; avoid wall-adjacent coax |
| 20–10 m | Local electronics within 2–10 m dominate | Diagnose room-by-room with portable RX antenna |
| 2 m–70 cm | Mostly conducted noise via DC power lines or cables | Keep shack power clean and isolate gear with ferrites |
Final Thought
Ask yourself: How long is λ? That determines how close a device must be to couple noise into your antenna or coax.
Noise isn't just "on the air"—it's often on your cable or in the room next door.
Diagnose smart, choke often, and think in wavelengths.
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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.