A covert listening device on a person (also known as a "wire") is often used to gain evidence for criminal prosecutions.
The first bugs were designed by Leon Theremin.
Surveillance devices require a communications channel. The idea of a 'bug' usually involves a radio transmitter, but there are many other options for carrying a signal; you can send radio frequencies through the main wiring of a building and pick them up outside, you can pick up the transmissions from a cordless phones, and you can pick up the data from poorly configured wireless computer networks or tune in to the radio emissions of a computer monitor.
Bugs come in all shapes and sizes. The original purpose of bugs was to relay sound. Today the miniaturisation of electronics has progressed so far then even TV pictures can be broadcast via bugs that incorporate miniature video cameras (something made popular recently during TV coverage sports events, etc.).
Older bugs used the VHF radio band. Modern bugs, thanks to the developments in electronics for mobile phones, work in the UHF and microwave bands. The use of digital rather than analogue technology means that the most professional bugs can encrypt the output signal, and change the frequency of operation in a pseudo-random pattern to make finding them harder. The range of these bugs varies from a few hundred yards to a few miles. Some of the state's bugging devices are even linked to satellite systems. There is a growing commercial market in surveillance devices such as audio and CCTV bugs, mainly for observing people in the workplace. Officially very little of this equipment is used for spying on the activities of pressure groups - but the potential is there.
Amateur bugs are usually the size of a cigarette packet. Professional bugs can fit into pens, calculators and other commonplace items. Some are only the size of small shirt buttons - but the power and operation life of the smallest bugs is very short.
The devices used by persons or organisations without the funding to buy professional equipment are crude. These devices can be bought from electronics magazines, and designs to build them are available on the Internet. They tend to broadcast in or around the VHF frequency band. They are also fairly bulky because they are made from ordinary electrical components and need a conventional battery power supply. However a well-made amateur bug can be just as effective as a professional one for conducting surveillance.
Another great problem with modern technology is the development of 'wireless' appliances. To be 'wireless' a device must transmit information, either by radio waves or infra-red light, and this potentially makes all the information sent via that link available to others. Radio waves are the worst option, but even infra-red can be picked up through a window. Some wireless devices, such as wireless computer networks, do encrypt transmissions, but the standard forms of encryption are weak.
Wireless devices, be it a wireless keyboard or a wireless telephone, should not be used in any environment where sensitive information is handled.
Bugs emit radio waves. The standard counter-measure for bugs is therefore to 'sweep' for them with a receiver, looking for the radio emissions. Professional sweeping devices are very expensive. There are low-tech sweeping devices available, through amateur electrical magazines, or that can be built from circuit designs on the Internet. But sweeping is not fool proof. Advanced bugs can be remotely operated to switch on and off, and some even rapidly switch frequencies according to a pre-determined pattern in order to make location with sweepers more difficult. You may also be bugged, but you don't detect it when you sweep because it's run out of power.
The other problem are those bugs that do not emit radio waves - they are very difficult to detect. Bugs are a technical solution to a problem - remotely listening to people's conversations. A simpler option is simply to record the conversation on a normal recording machine. There are a number of options for this:
Bugging devices in EU headquarters
Electronic bugging devices were found in March 2003 at offices used by French and German delegations at European Union headquarters in Brussels. Devices were also discovered at offices used by other delegations. The discovery of the telephone tapping systems was first reported by Le Figaro newspaper, which blamed the US.