There is no single “bug frequency”
Listening devices can operate across many parts of the radio spectrum. The frequency is selected around antenna size, propagation, available components, bandwidth, regulatory constraints and the intended radio architecture.
Why lower frequencies are attractive
For a given environment, lower frequencies can penetrate or diffract differently from microwave signals. The penalty is antenna size: an electrically short antenna is less efficient and becomes more sensitive to its surroundings.
UHF and compact equipment
UHF is attractive for compact equipment because practical antenna dimensions become smaller while readily available RF components support efficient transmitters and receivers. Building construction and local interference still have a large effect on actual range.
Cellular and Wi-Fi bands
Devices that use cellular or Wi-Fi inherit the operating bands and protocols of those networks. They should therefore be considered networked radios rather than simply conventional single-frequency audio transmitters.
Bandwidth and audio quality
The occupied RF bandwidth depends on modulation and data rate, not just audio bandwidth. Digital systems also need capacity for framing, coding, synchronisation and sometimes encryption overhead.
Spectrum analysis
A spectrum analyser shows energy versus frequency and is one of the core tools for examining an RF environment. Interpretation is crucial because legitimate Wi-Fi, Bluetooth, cellular, telemetry and other signals can vastly outnumber signals of interest.
Related technical resources
Professional listening devices · Professional bugging devices · Miniature listening devices · Long-range listening devices · Listening-device detection