"Radio interference" covers a lot of ground, but almost every case is settled by two questions. Is the interference broadband noise leaking from equipment, or a narrowband signal from a transmitter? And is it coming from inside your own site or from outside it? Answer those two and the fix usually picks itself.
The first question: broadband or narrowband?
On a spectrum analyser the two look completely different, and that difference points straight at the cause.
Broadband interference shows up as a raised noise floor smeared across a wide span. It's almost always incidental — a by-product of electrical equipment doing its job. Anywhere power is switched hard and fast is a candidate: variable-frequency drives, switch-mode power supplies, LED drivers, brushed motors, welders and plasma cutters. Nothing is trying to transmit; it's leakage. The fix is about containing the noise — filtering, shielding, bonding and earthing — or putting distance between source and victim.
Narrowband interference shows up as a discrete spike at a specific frequency. That usually means an intentional transmitter, a harmonic of one, or an intermodulation product created when two strong signals mix. Here the job is to identify exactly what it is and where it lives, then either remove it, retune around it, or filter it out.
The second question: internal or external?
Where the source sits decides who fixes it. An internal source — your own equipment, or a neighbour's on the same site — is something you can usually switch off, move, filter or replace. An external source bleeding in from beyond your boundary, or interference landing on a licensed radio service, is a different matter and may be an Ofcom issue rather than a private one.
The usual suspects
- Switch-mode supplies and chargers — cheap or failing ones are a classic broadband source.
- Variable-frequency drives and motors — strong, often intermittent, tied to machinery cycles.
- LED and lighting drivers — surprisingly noisy across HF and VHF.
- Faulty or arcing equipment — intermittent and broadband, often weather- or load-dependent.
- Nearby transmitters — producing narrowband spikes, harmonics or intermod where they weren't expected.
How an RF engineer finds it
The method is the same whatever the source. First a spectrum survey to characterise the interference — is it broadband or narrowband, continuous or bursty, modulated or a bare carrier, and how does it move? That description alone usually rules half the suspects out. Then direction finding to walk it back to the physical source. Only once it's identified and located does the fix get chosen — remove it, filter it, shield it, bond it out, or escalate it — because choosing a remedy before you know the cause is how money gets wasted.
One observation that narrows it fast
Try to correlate the interference with equipment switching on and off. If it appears the moment a drive, charger or machine starts and vanishes when it stops, you've very likely got an internal broadband source — and you've handed the engineer half the diagnosis before they arrive.
See RF source location →When it's an Ofcom matter
Worth knowing the boundary: interference to your own private equipment — your wireless, sensors, comms — is yours to investigate and resolve, and that's exactly this kind of work. But interference to a licensed radio service, or a source clearly originating outside your premises, is reported to Ofcom. Part of a good investigation is telling you honestly which side of that line you're on.
When to get someone on site
If the interference is recurring, if it's hitting something you can't afford to lose, or if you've tried the obvious and it keeps coming back, that's the point for a spectrum survey and direction finding. Guessing at radio interference is slow and expensive; measuring it is neither.