What a spectrum analyser shows
A spectrum analyser displays RF energy as a function of frequency. For bug detection and lawful counter-surveillance work, it can reveal fixed carriers, intermittent transmissions, wideband signals and changes in the local RF environment.
Why one sweep is not enough
Modern radio devices may transmit intermittently or use changing frequencies. A brief sweep can therefore miss activity that is only present for a fraction of the observation period. Persistence displays and spectrograms add a time dimension and are useful for identifying short-duration or repeating emissions.
Bandwidth matters
Resolution bandwidth affects how closely spaced signals appear and influences displayed noise level. Very narrow settings provide frequency detail but sweep more slowly, while wider settings improve time coverage. Professional analysis chooses settings according to the type of signal being investigated.
Strong signals are not automatically suspicious
Buildings contain Wi-Fi, cellular, Bluetooth, broadcast, telemetry and many other legitimate transmitters. The objective is to identify signals that do not fit the known environment and then correlate them with location, timing and physical inspection.
Near-field localisation
When a suspicious emission is identified, controlled attenuation, directional or near-field probes can help determine where the energy is strongest. This is a localisation technique rather than proof of a listening device; final assessment normally includes physical inspection and other TSCM methods.
Spectrum analysis has limits
A non-transmitting recorder or a device that is completely inactive during the search may not create a useful RF signature. Professional searches therefore combine RF analysis with physical inspection and, where appropriate, non-linear junction detection and other methods.