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, more and more often you can encounter the use of radio microphones and telephone radio listening devices for various purposes. Sometimes you need to be sure that the conversation in the apartment or office is not tapped.

Typically, eavesdropping devices ("bugs") emit a small power (up to 30 mW) at one frequency in the range of 500 ... 5 MHz. Sometimes such devices operate in standby mode: they turn on when there is noise in the room (which ensures the economy of energy consumption of batteries) or when the handset is lifted. The simplest device that can help detect listening devices is shown in the figure.

(click to enlarge)

The circuit is a broadband bridge RF voltage detector that covers the frequency range 1 ... 200 MHz (when using microwave diodes as D01 ... D06, 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 V is difficult because already at 5 ... 0,2 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 DЗ, D0,3 improves the detection conditions (increases sensitivity) and allows you to move the lower limit of the measured voltage level to 4 mV with a uniform amplitude-frequency characteristic. Diodes D20, D5 form the second arm of the bridge and provide thermal stabilization of the circuit. On the elements of the chip U6 ... U1.2. 1 three-level comparators are assembled, to the outputs of which LED indicators HL4 ... HL1 are connected. Diodes D3, D1 are used as voltage regulators of 2 V, 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, which reduce the sensitivity of the circuit. For example, it is possible to use interchangeable pins with a length of 400 - 700 - 1200 (mm). When using the device, after turning it on, it is necessary to use the resistor R2 to make the indicator HL3 glow. This sets the initial sensitivity level relative to the background. When bringing the antenna to the source of radio emission, the LEDs НL2 and HL1 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 remains operational when the power changes from 6 to 10V.

Publication: cxem.net

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