There are four main categories of bugs (hard wire, radio frequency, optical and acoustic). Sometimes a combination of these are used to form a hybrid bug. 'Acoustic' bugging is the direct listening without any Rf or hardwire transmission system, such as, stethoscopes, wall contact microphones or even a glass on the wall, shotgun microphone or parabolic reflector. Another variation of this is the sound picked up is converted to a frequency outside the human hearing range in the ultrasonic spectrum then amplified.This can be listened to with a suitable ultrasonic listening device from a distance away.
Optical devices normally convert audio signals into transmitted light pulses and this is converted back to audio signals when received. The main use for this system is the laser bounce principal, which relies on the propagation of sound waves, causing vibrations on objects such as windows. The laser beam is projected onto these, and is modulated by the small vibrations which, when received, can be converted back to audio signals by a similar principal to a CD player. These systems are very expensive, awkward to use and easily detected.
Radio Frequency Transmitters Moving further up the scale and very similar in operation is the crystal controlled VHF transmitter. These devices tend to operate slightly higher up the frequency band than the free oscillating transmitter and are tuned to a fixed frequency. These require a dedicated receiver, or if you prefer a narrow band scanner may be used, the advantages are greater stability and less chance of casual interception of the signal.
UHF Transmitters Microwave VLF Specialised RF Transmitter in Brief Hardwire Bugs
Probably the most commonly and widely used of all the devices is the RF transmitter bug. At the lower end of the scale you will find the free oscillating VHF transmitter, transmitting on the commercial FM band 88-108 MHZ or the VHF airband 108-140MHZ. These devices are inexpensive easy to use and require no specialised receiving equipment. Due to their low cost, there is very little incentive to recover these once planted. This is the type of bug most often used by private investigators and individuals, and the chance of finding the person who plants it, is low.
It was a very similar bug to this which was used in Watergate.
Again these transmitters are crystal locked to a particular frequency, usually between 350 MHZ - 1GHZ and require a dedicated receiver, but offer greater penetrating power and privacy over the VHF types, these transmitters are very expensive and are often confined to professional use.
All the transmitters listed so far can be detected reasonably easily with basic equipment and an understanding of basic principals. Once you enter the realm of microwaves and VLF transmitters you can range from as low as 5KHZ to over 1000 GHZ and a specialised knowledge of radio and surveillance
techniques are required to use or detect these devices, perhaps the Rolls Royce of transmitter bugs is the compressed data type, which records all the activity for 3 to 4 hrs. and then transmits all the compressed data in a few seconds, making it virtually impossible to detect by normal RF detector, unless you happen to be in the right place at the right time when the transmission burst takes place. Other types of devices which can be equally difficult to find are the type that superimpose the transmission on the top of a legitimate transmission, such as commercial radio station, other variations of rf transmitters may use frequency inversion cicuits. The detection of such devices is possible, but most only by specialised counter surveillance companies and certainly not by many private investigation companies, claiming to be specialists in this field.
Often referred to as a wire tap, this form of surveillance is the most reliable and gives high quality results. The wiretap can be installed onto existing
wiring ie: telephone or alarm systems. If you are reading this document on the WWW then you have got the perfect hard wire bug in your premises ie: 'the telephone'. Many think that telephones are only a security risk when being used but a simple modification to the handset needing only a small capacitor can leave the microphone connected, even when the phone is not in use, enabling the operation to connect a lead with an amplifier and headphones to the line and listen in high quality audio to all activity in the room (BEWARE). The hardwire bug in its simplest form is a microphone (this may be as small as 6mm in diameter) a pair of thin wires or a track of conductive paint leading back to a listening post and connected to a high gain amplifier or recording device. The only drawback to this system is the concealment of the wires and the fact that if they are discovered they can be tracked back to the listening post. Often hardwire bugs are used from outside the premises, either by placing a miniature microphone
into an air vent, or any other opening or by just locating the microphone near an opening, as often all the conversation in the room can be overheard
if you use a quality audio booster, such as the one we supply. Other variations of the wiretap are transmitters or recorders being attached to the wiring
enabling the listening post to be outside the area, allowing the operator to just recover the recordings.