Radio frequency eavesdropper finder. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Radio frequency eavesdropper finder. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Security devices and object signaling

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Today, anyone can purchase or assemble their own radio microphone, as well as a telephone radio listening device. If you are in business, then sometimes you need to be sure that your conversation in your apartment or office is not being tapped. Indeed, the success of a business often depends on the observance of trade secrets.

Typically, eavesdropping devices ("bugs") emit a small power (up to 30 mW) at one frequency in the range of 500 ... 5 MHz. In some cases, such devices operate in standby mode: they are switched on for transmission in the presence of noise in the room (which ensures the economy of energy consumption of batteries) or when the handset is picked up. "Bugs" can have a constant power supply from the 220 V network - in this case they are located inside sockets or adapter tees.

The services of specialists in the search for such bookmarks are quite expensive. Disassembling and inspecting all electrical appliances on your own will take a very long time and does not guarantee success (you cannot make out an electric light bulb, but it may contain a radio microphone).

Fig. 3.41

The simplest device that can help you detect listening devices is shown in Fig. 3.41.

The circuit is a broadband bridge RF voltage detector that covers the frequency range of 1 ... 200 MHz (when using microwave diodes as VD1 ... VD6, the operating band can be expanded) and allows you to detect "bugs" at a distance of about 0,5. ..1 m (it depends on the power of the transmitter).

It is known that the measurement of RF voltages with a level of less than 0,5 V is difficult because already at 0,2...0,3 V all semiconductor diodes become ineffective during detection due to the peculiarity of their current-voltage characteristics.

In this scheme, a well-known method for measuring small alternating voltages using a balanced diode-resistive bridge is used. A small current flowing through the diodes VD3, VD4 improves the detection conditions (increases sensitivity) and allows you to move the lower limit of the measured voltage level to 20 mV with a uniform amplitude-frequency characteristic.

Diodes VD5, VD6 form the second arm of the bridge and provide thermal stabilization of the circuit. Three-level comparators are assembled on the elements of the D1.2 ... D1.4 chip, to the outputs of which LED indicators HL1 ... HL3 are connected.

Diodes VD1, VD2 are used as 1,4 V voltage regulators, which is necessary for stable operation of the circuit in a wide range of supply voltages.

The use of the device requires certain skills, since the circuit is quite sensitive and capable of capturing any radio emission close to it, for example, the operation of a receiver or TV local oscillator, as well as secondary re-emission by conductive surfaces.

To facilitate the search for a "bug", replaceable antenna pins with different lengths are used (Fig. 3.42), which can reduce the sensitivity of the circuit.

RF eavesdropper finder
Fig. 3.42

When using the device, after turning it on, it is necessary to achieve the glow of the HL2 indicator with the resistor R3. This sets the initial sensitivity level relative to the background. When bringing the antenna to the source of radio emission, the HL2 and HL1 LEDs should begin to glow as the amplitude of the received signal increases.

The adjustment of the circuit with a tuning resistor R9 is performed once (during the initial setup of the device, the level of sensitivity thresholds of the comparators depends on it).

The circuit is powered by a 7D-0.125D battery or a Krona battery and remains operational when the power changes from 6 to 10V.

The circuit uses: variable resistors R2 of type SPZ-36 (multi-turn), R9 of type SPZ-19a, the remaining resistors are of type C2-23; capacitors C1 ... C4 type K10-17; socket X1 type G4,0, switch S1 type PD-9-2.

LEDs can be replaced with any of the instrumentation series (with a low current consumption, they glow quite brightly).

The design of the circuit can be anything, for example, in the form of a notebook (when using flat batteries).

Publication: cxem.net

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