Clandestine use of FM transmitters

FM transmitters have been used to construct miniature wireless microphones
for espionage and surveillance purposes (covert listening devices or
so-called "bugs"); the advantage to using the FM broadcast band for such
operations is that the receiving equipment would not be considered
particularly suspect. Common practice is to tune the bug's transmitter off
the ends of the broadcast band, into what in the United States would be TV
channel 6 (<87.9 MHz) or aviation navigation frequencies (>107.9); most FM
radios with analog tuners have sufficient overcoverage to pick up these
beyond-outermost frequencies, although many digitally-tuned radios do not.

Constructing a "bug" is a common early project for electronics hobbyists,
and project kits to do so are available from a wide variety of sources. The
devices constructed, however, are often too large and poorly shielded for
use in clandestine activity.

In addition, much pirate radio activity is broadcast in the FM range, due to
the band's greater clarity and listenership