The signal path
A typical RF audio system contains a microphone and low-noise audio stage, signal processing or modulation, an RF transmitter, matching network and antenna. At the receiving end, the reverse process recovers the audio. Each stage contributes to the overall signal-to-noise ratio and usable range.
Frequency and propagation
Lower frequencies can offer useful propagation and less path loss in some environments, but require physically larger efficient antennas. Higher frequencies permit smaller antennas but can be more affected by building materials and line-of-sight conditions. Installation environment matters as much as the nominal frequency.
Why power is not the whole story
Range is a link-budget problem. Transmitter power is only one term alongside antenna efficiency, path loss, receiver noise figure, bandwidth and required signal-to-noise ratio. Increasing power produces diminishing range improvement because free-space power density falls rapidly with distance.
Analogue versus digital RF
Analogue FM is straightforward and can provide good audio, but a compatible receiver can demodulate an unencrypted signal. Digital RF permits encryption and coding, at the cost of additional processing and design complexity.
Frequency hopping
A frequency-hopping radio changes channel according to a shared sequence. The receiver remains synchronised with the transmitter. This can provide resilience to narrowband interference and creates a different spectrum signature from a continuously occupied single channel.
Engineering the antenna
Antenna performance can dominate a miniature transmitter. Nearby metal, wiring, the enclosure and the object in which a device is installed can detune an antenna. Practical RF design therefore includes testing in representative installation conditions, not only on an open bench.
Related technical resources
Professional listening devices · Professional bugging devices · Miniature listening devices · Long-range listening devices · Listening-device detection
Further RF engineering guides
RF range & link budget · Antenna engineering · RF propagation · Spectrum-analyser detection