Where Does the Noise Come From?
Why All QRM Is Not the Same
When you’re chasing a weak DX contact or trying to enjoy a quiet noise floor, persistent QRM/RFI can be maddening. The key point is that not all “noise” arrives as a clean, radiated radio wave through the air.
A large part of what we hear couples into our system locally (near-field), rides in on wiring (conducted), or arrives as common-mode current on the outside of the coax.
In practice, three paths dominate:
- Near-field coupling into the antenna or feedline
- Far-field (true radiated interference)
- Common-mode currents on the outside of the coax
Wavelength Sets the Scale
A quick wavelength reminder helps frame everything that follows:
λ (meters) ≈ 300 / f (MHz)
This relationship defines what “nearby” really means. On the low bands, nearby can mean your entire house. On the high bands, it may mean only the same room.
Near-Field Coupling
The near field is the region close to an antenna or close to a noise source where the electric (E) and magnetic (H) fields are not yet locked together like a plane wave. In this zone, antennas behave much more like E-field or H-field sensors.
There is no single sharp boundary where the near field suddenly ends. The transition depends on wavelength and on the physical size of the antenna or noise source. For practical RFI hunting:
- The closer a device is (within a fraction of a wavelength), the easier it couples.
- Coupling falls rapidly with distance, but never drops to zero.
Example: On 17 m (≈ 18 MHz), the wavelength is about 16.7 m. Anything within roughly 5–10 m of the antenna or feedline can couple strongly. Broadband hash on this band usually points to sources inside your home or the nearest neighbor’s building.
Common near-field offenders include LED lamps and dimmers, switching power supplies, Ethernet cables and switches, PoE injectors, solar inverters, charge controllers, PLC adapters, and smart meters.
Far-Field Interference
In the far field (also called the radiation zone), the E and H fields have settled into a stable propagating wave. The radiation pattern no longer changes shape with distance; only signal strength decreases.
Far-field QRM may originate from nearby or distant transmitters, industrial equipment radiating via long cables, or infrastructure faults such as arcing power lines.
Sanity check: If other stations in different locations — and especially different directions — hear the same noise at the same time, the source is likely radiated or tied to shared infrastructure rather than something local to your station.
Common-Mode Pickup on the Coax
This is the part many amateurs underestimate.
Coax is intended to carry RF in differential mode: current flows on the center conductor and returns on the inside of the shield. But if the antenna system is imbalanced, RF current can flow on the outside of the shield. That current is common-mode current.
When common-mode current exists:
- The feedline becomes part of the antenna system.
- The coax can both receive and radiate noise.
- Any noise the feedline passes near can be delivered straight to the receiver.
This commonly occurs when antennas are unbalanced, effective 1:1 current chokes are missing, feedlines run close to house wiring, or grounding is relied upon instead of proper common-mode isolation.
Why Band Matters
A feedline does not need to be exactly resonant to cause problems. Being a significant fraction of a wavelength is often enough.
Example: On 40 m (≈ 7 MHz), half a wavelength is about 20–21 m. A coax run of similar length without a proper choke can be an efficient common-mode pickup path on that band.
Recognizing the Dominant Path
- Noise changes with antenna direction — often far-field, but loops can also null nearby sources.
- Noise drops when adding a choke or rerouting coax — strong indication of common-mode pickup.
- Noise appears only on certain bands — near-field coupling plus harmonics or resonant wiring lengths.
- Noise remains with antenna disconnected and input terminated — conducted or internal station noise.
Fixing Common-Mode Problems
- Add a 1:1 current choke at the antenna feedpoint.
- Add another choke where the coax enters the building.
- Add a third choke near the radio if USB/Ethernet density is high.
- Route coax away from wiring, power strips, network cables, and LED drivers.
- If rerouting is impossible, treat the noise source with ferrites as well.
Ferrite guidance: Mix 31 is an excellent general-purpose HF suppression mix, especially on the low bands. Mix 43 is often more effective higher in HF and into VHF. Use enough turns and enough cores to achieve meaningful common-mode impedance.
Band-by-Band Perspective
| Band | Center freq | λ (m) | 0.5λ (m) | What this implies |
|---|---|---|---|---|
| 160 m | ~1.83 MHz | ~164 | ~82 | House wiring and feedlines dominate noise pickup. |
| 80 m | ~3.6 MHz | ~83 | ~41 | SMPS and neighborhood sources are common. |
| 40 m | ~7.1 MHz | ~42 | ~21 | Feedline length and routing become critical. |
| 20 m | ~14.1 MHz | ~21 | ~10.5 | Local electronics still couple easily. |
| 15 m | ~21.2 MHz | ~14 | ~7 | Very local sources dominate. |
| 10 m | ~28.4 MHz | ~10.6 | ~5.3 | Room-scale devices and harmonics matter most. |
Final Thought
Always ask yourself: “How big is a wavelength on this band?” That sets the scale of what “nearby” really means. Noise is often not “on the air” at all — it’s on your feedline, in your wiring, or in the room next door.
Diagnose smart, choke generously, and think in wavelengths.
Mini-FAQ
- Is all QRM radiated? — No. Much interference enters via near-field coupling or common-mode currents.
- Why do chokes help so much? — They prevent the feedline from acting as a noise antenna.
- Why is noise band-dependent? — Because wavelength defines coupling scale.
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