Transmitter Detector for FM Bug Surveillance

Figure 1 

The circuit was constructed using a few components that is powered by a 9 V battery for sensing the presence of bugs transmitting within the frequency modulation range.

• Frequency Modulation (FM) – transmits its signal or information over a carrier wave by changing its frequency but it can also be taken into account as a special case of phase modulation where the carrier phase modulation is the time integral of the FM modulating signal
• Transmitter – an electronic device that can produce or amplify a carrier wave signal, modulates it with a significant signal, and radiates the resulting signal from an antenna which are being utilized in television, telecommunications, and radio
• Radio Frequency (RF) – operates within the range of 3 Hz to 300 GHz
• Field Effect Transistor (FET) – used for amplifying weak signals by controlling the current and the shape of an electric field where the flow of current or the conductivity of material is only through a single type of semiconductor material

Bugs are also known as listening devices, wireless microphones, transmitters and telephone gadgets which are easy to use and very effective. The FM bugs are typically designed to work in the commercial range of 87 MHz to 108 MHz. The bug detectors will pickup almost anything that transmits in this frequency band. In some case, when a bug made of sensitive microphone is broadcasting in close proximity, it may produce a feedback whistle in the detector.
The inductor used in the circuit has a value of 0.389 uH molded as an RF coil. It is tuned for broadband and possesses a quality factor at about 170. The coil picks up the signal and is rectified by the OA91 gold bonded germanium diode. It has low power consumption and very low noise level. The minimal DC voltage will give the deflection on the meter, having 250 uA full scale deflection, while reversing the bias on the FET. For higher sensitivity, the meter to be used should contain a full scale deflection of 50 uA or 100 uA. The FET used in this circuit can be the 2N3819 or MPF102 which offers good performance at mid to high frequencies and a low cost type. It has low noise and leakage, very high system sensitivity, high speed switching capability, wideband high gain at 100 MHz, high low-level signal amplification and high quality of amplification.
The use of the detector is applied in a sweeping motion while being carried around a room, and when a bug is nearby, a feedback whistle will be heard. Even a small battery transmitter can cause deflection the meter from a distance away from it. During the construction of an FM transmitter, this detector will provide a valuable part of the equipment.
Bugs or listening devices can be utilized in several forms of equipments such as phone line bug, credit card UHF bug, pen style UHF bug, long range UHF bug, calculator style UHF bug, phone line FM bug, powerboard bug, wall contact microphone, super directional listening set, inline microphone, digital voice recorder, monocular scope and wrist watch walkie talkie. Bugs can also come in the form of radio transmitters which transmits sounds over the radio frequency; in the form of lasers where it aims a laser that has sine waves on an object and analyzing the returned laser beam; in the form of contact microphones and speakers which both contains a diaphragm to interact with the air, a power source, and 2 coils of fine wire; in the form of hardwired devices like stereo speakers, power lines, wired intercoms, phone lines and LAN cabling; in the form of visible light transmitters which functions by modifying the voltage flowing in a light source; and in the form of infrared transmitters which utilizes infrared lights to transmit sound rather than radio waves.
There are quite a lot of detection tools that detect wiretaps, wireless video transmitters, radio interference, electronic eavesdropping, and bugs. In testing the telephone trunks for wiretaps, a device known as time domain reflector (TDR) is used which acts as radar for wires that gives a waveform display in its scope unit. Another tool for testing telephone taps is telephone sequencer which can separate out all fifty wires in a standard telephone wall connection and it comes in the form of a patch bay. There are detectors known as non-linear junction detectors which consists of a headphone and meter that gives sound and visual feedback in the presence of a semiconductor material. In radio transmission wire tap bugs, the radio spectrum analyzer and field strength meter is used. The field strength meter uses a slope detector by having an audio listening circuit. The radio frequency spectrum analyzer that shows the waveform of the carrier that is tuned on using the built-in scope.

Source:www.zen22142.zen.co.uk/Circuits/rf/bugdetector.htm

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