Where Does the Noise Come From?
Where Does the Noise Come From?
The waterfall can show a problem without showing its origin. Separate the source, coupling path and receiver response before deciding whether to move a cable, filter a supply, add a choke or investigate the receiver itself.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
Noise does not arrive by one road. It can be a real field collected by the antenna, common-mode current brought along the feed line, a conducted signal on station wiring, or a product generated inside an overloaded receiver. Those cases can look similar on a spectrum display and need different remedies.
This is why my first question is not “Which ferrite mix should I buy?” It is: what changed, where does the unwanted energy enter, and can I make the observation repeat?
The Receiver Shows a Result, Not a Cause
A raised trace, hash pattern, comb or impulsive line is the output of the complete receive chain. Antenna pattern and polarisation, feed-line loss, preamplifier, filters, attenuator, mixer, automatic gain control, detector, resolution bandwidth and display averaging all influence what appears.
| Observed category | Possible mechanism | What would distinguish it |
|---|---|---|
| External field at the antenna | Atmospheric, cosmic, broadcast, industrial or local man-made radiation | Changes with antenna direction, position or polarisation under repeatable receiver settings |
| Common-mode ingress | Current on the feed-line exterior, mast, control lead, USB cable or station wiring couples into the receive path | Multi-position current measurements and controlled routing/choke changes alter the result |
| Conducted station noise | A supply or digital device carries noise through power, data, shield or bonding conductors | The signature follows a connection and survives or disappears in a restored-baseline disconnect test |
| Receiver-generated response | Blocking, gain compression, reciprocal mixing, intermodulation or aliasing creates or lifts apparent noise | Input attenuation, preselection or gain changes produce a response inconsistent with a linear external signal |
| Internal receiver noise | Thermal noise and noise added by the receive chain set the sensitivity floor | A correctly terminated input and calibrated bandwidth/gain measurement establish the baseline |
More than one category can exist at once. A nearby switcher may conduct noise onto a USB cable, drive common-mode current on the coax through the computer and then overload a wideband receiver. Removing one path may reveal another rather than “failing.”
Internal Noise and External Noise Need Different Tests
External radio noise is power delivered by the antenna system from the surrounding electromagnetic environment. Internal receiver noise is added by losses and active devices after the antenna reference plane. Comparing the antenna input with a suitable shielded termination can help separate them, but the termination changes source impedance and removes every external signal, so it is a diagnostic boundary—not an antenna-performance comparison.
Noise power depends on measurement bandwidth. A wider receiver or analyser bandwidth collects more noise power, and detector/averaging choices change the displayed statistic. Record frequency, resolution or receive bandwidth, detector, averaging, preamplifier, attenuation, AGC state and reference level before calling a trace “six decibels noisier.”
ITU-R P.372 separates atmospheric, galactic and man-made radio-noise contributions and treats them statistically by frequency, time and environment. Its curves are planning references, not a prediction for one garden or one hour. ITU-R SM.2093 likewise specifies receiver, antenna, measurement-condition and processing records for reproducible indoor radio-environment measurements.
Receiver Overload Can Masquerade as Noise
A strong signal does not need to sit inside the displayed passband to cause trouble. It can compress an amplifier or mixer, raise the apparent floor through reciprocal mixing, or combine with other signals to create intermodulation products. An SDR can also show aliases or digital artefacts when its analogue front end or converter is overdriven.
Use a known input attenuator or suitable preselector as a controlled test. In a linear chain, wanted signals and external noise should change predictably with inserted loss after display/gain effects are accounted for. If a noise pedestal or spur collapses disproportionately, moves, or disappears while nearby signals behave differently, overload or a generated product becomes a serious suspect. Repeat with the starting state restored; one screenshot is not proof.
Differential and Common Mode Are Not Interchangeable
Inside coax, the intended differential transmission-line current flows on the centre conductor and the facing inner surface of the shield. Current on the outside of the shield belongs to another circuit. Its return can include the antenna, mast, station equipment, protective conductor, other cables and distributed capacitance to the surroundings.
On a two-conductor cable, differential noise exists between the conductors. Common-mode noise drives both conductors relative to a third return structure. Asymmetry, connector geometry, interrupted reference planes and parasitic capacitance can convert energy between modes.
A conventional SWR or differential-port measurement does not measure exterior coax current. A clamp current probe or suitable current transformer, used at several positions and within its calibrated range, answers that question more directly.
Conducted and Radiated Are Parts of One Route
“Conducted” and “radiated” describe portions of a coupling path, not permanent labels for a device. Noise may leave a converter through a power lead, travel as common-mode current and then radiate from a longer cable. The receiver’s feed line can collect that field and conduct it to the input. Follow the complete route.
Near-field electric and magnetic coupling depends on source type, source dimensions, loop area, orientation and distance. Wavelength provides scale, but it does not create a fixed “everything within five metres is local noise” boundary. A small H-field loop can null or peak on a nearby source without proving how the station antenna receives it.
Direction finding is also evidence, not a verdict. Reflections, house wiring, utility conductors and common-mode pickup can make a local source appear to arrive from another direction. Corroborate it with position, current and on/off tests.
A Reproducible Noise Hunt
Record frequency, mode, bandwidth, gain, attenuation, AGC, antenna, time and a waterfall or level trace before touching the installation.
Compare antenna and terminated-input states, test overload, map exterior current, and localise E-field or H-field coupling with the right probe.
Make one safe change, capture the result, restore the baseline, then apply the change again. Causation must survive A/B/A.
Establish the Baseline
Choose a representative frequency and preserve all receiver settings. Record wanted-signal level and noise in the same bandwidth and detector state where possible. If the goal is reception, the useful outcome is SNR or intelligibility—not the lowest S-meter value with the wanted signal also attenuated.
Check the Receiver and Antenna Port
Apply known attenuation and preselection to look for overload. Then compare the antenna with a suitable shielded termination at the receiver input. If the signal remains with the input correctly terminated, investigate receiver, power, control, computer and enclosure coupling before blaming the antenna.
Remove Local Sources Safely
Run the receiver from an appropriate battery where practical and switch household circuits only through normal controls and safe electrical practice. Log the exact device state; standby supplies may remain active on a powered circuit. Never open mains equipment, improvise live measurements or disconnect protective earth to improve a noise trace.
Trace Cables and Current
Map common-mode current at several points on the feed line and connected cables. Reroute or disconnect only permitted low-voltage leads one at a time, then restore them. A change after moving coax can mean altered common-mode pickup, altered antenna current distribution, altered coupling to the source—or a combination.
Use Probes According to the Field
An H-field loop helps localise current loops and magnetic coupling. An E-field probe responds strongly near high-impedance, high-dv/dt nodes. Neither is a calibrated compliance antenna unless the complete probe, receiver, correction and geometry are characterised. Probe loading and cable pickup can otherwise create false conclusions.
Verify the Fix, Not Just the Change
Repeat the baseline measurement across the affected frequencies, device operating modes and station configurations. Confirm wanted-signal SNR, receiver linearity and equipment function. Keep raw traces, settings, locations, current readings, photos and uncertainty notes so the result can be reproduced later.
Choose the Remedy After the Path Is Known
- Differential conducted path: correct source layout or use a suitable differential filter designed for the port, impedance, current, voltage and safety class.
- Common-mode cable path: reduce mode conversion, improve the intended return geometry or add a characterised common-mode impedance at the measured current boundary.
- Enclosure or aperture path: address seam, penetration, connector and shield continuity as one enclosure system.
- Receiver overload: use appropriate preselection, attenuation, gain distribution or a more linear front end while preserving the required sensitivity.
- External radiated source: work on source suppression, separation, antenna directivity/polarisation or the applicable interference-resolution process.
Ferrite mix, number of turns and placement follow the measured mode and frequency-dependent impedance. More cores at a guessed location are not automatically better. A choke can also change the antenna system if the feed-line exterior was part of its RF return, so remeasure impedance, exterior current and SNR after installation.
The decision rule: call the emitter identified only when its on/off or state change is repeatable, the entry path is supported by current/field/connection evidence, receiver overload has been excluded, and the final remedy improves wanted-signal SNR under restored conditions.
Primary Measurement and Engineering References
- ITU-R P.372-17, Radio noise: in-force atmospheric, galactic and man-made radio-noise terminology and statistical planning data.
- ITU-R SM.2093-0, Methods for measurements of indoor radio environment: reproducible receiver, antenna, measurement-condition and processing practice.
- ITU-R SM.1753-2, Methods for measurements of radio noise: measurement types, site/frequency selection, analyser settings, uncertainty and result reporting.
- Keysight, Fundamentals of RF and Microwave Noise Figure Measurements: receiver-added noise, repeatable measurement and SNR degradation.
- Texas Instruments, Fundamentals of EMI: source–path–victim analysis, differential/common mode and conducted/radiated paths.
- Texas Instruments, AN-643 EMI/RFI Board Design: coupling mechanisms, multiple paths and differential/common-mode definitions.
Mini-FAQ
- Is every raised waterfall trace external radio noise? No. It may be an external field, conducted or common-mode ingress, receiver-generated intermodulation or overload, an alias, or internal receiver noise.
- Why must bandwidth and receiver settings be recorded? Noise power and displayed level depend on bandwidth, detector, averaging, gain, attenuation, AGC and calibration. A level without those settings cannot be compared reliably.
- How can I test for receiver overload? Insert known attenuation or suitable preselection and observe wanted signals, noise and spurs with controlled gain/display settings. A disproportionate collapse suggests a nonlinear receiver response.
- Does a choke-induced noise reduction prove common mode? It is evidence, not proof by itself. Confirm exterior-current change, restore the original state, repeat, and check that antenna impedance and wanted-signal SNR were not changed adversely.
- Can wavelength alone prove a nearby origin? It supplies scale, but source size, field type, wiring, reflections and coupling geometry matter. No fixed fraction of a wavelength identifies the source.
- What makes a noise diagnosis convincing? Stable receiver settings, an A/B/A source-state result, a supported entry path, an overload check, matching current or field evidence and improved wanted-signal SNR after the remedy.