Anti-radio spy. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Anti-radio spy (spectrum analyzer in the frequency range 65-410 MHz). 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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The device is designed to detect electronic bookmarks in the frequency range of 65-410 MHz, determine their operating frequency and listen to a standard and FM receiver (88-108 MHz).

Typically, radio bug control devices have a very wide control band of hundreds of megahertz, which reduces their sensitivity and detection range to several tens of centimeters. The proposed device has a relatively narrow frequency response scanning over the frequency range, which increases its sensitivity by an order of magnitude compared to broadband devices, and has the ability to listen at "suspicious" frequencies.

The principle of operation is based on the indication of the level of electric field strength of the bookmarks when the detection device moves away or approaches them. The radiation level of radio bookmarks is estimated by the built-in indicator.

The detection device consists of two amplifiers, one of which, together with the filter, provides control in the first sub-band 65-155 MHz, and the other from 155 MHz to 410 MHz in the second sub-band. The first amplifier is assembled on a transistor VT1 with an input filter C1, L1, C2 (Fig. 1), which provides filtering of mirror frequencies above 155 MHz. Both amplifiers are connected to a mixer on a VT2 transistor and can only work in turn - each with its own local oscillator. The local oscillator working with the first amplifier is assembled on a VT8 transistor. It covers the frequency range from 155 to 245 MHz.

(click to enlarge)

The second local oscillator is assembled on a transistor VT7. It works as a joint amplifier on the VT6 transistor and covers the range from 345 to 500 MHz.

The mixer on the transistor VT2 is loaded on the bandpass filter C12, L2, C13, L3 with an intermediate frequency (90 ± 5) MHz. Thus, due to the first local oscillator, control is provided in the first frequency subrange from 65 to 155 MHz, and the second - in the second subrange from 155 to 410 MHz. The sensitivity of the device is regulated by resistor R15. The attenuator on the VT4 transistor provides level control by more than 25 dB. The signal detected by the detector VD1, VD2 is fed to the operational amplifier DA1 and then to the level indicator. The antenna input of the receiver is connected through a resistor R24, which reduces sensitivity, to the collector VT5.

The scale of the variable resistor R23 is calibrated for both the first and second subranges according to the high-frequency generator.

All parts of the device are small. Coil L1 is frameless, with an inner diameter of 6 mm, has 5 turns of PEV wire 0,7 mm. Coils L2, L3 are wound on polystyrene frames with tuned cores made of 100NN ferrite with a diameter of 2,8 mm and a length of 14 mm, each have 10 turns of PELSHO wire 0,15 mm. Coils L4, L5 are frameless, with an inner diameter of 6 mm and a winding pitch of 1 mm, have: L4 - 4 turns and L5 - 6 turns of PEV wire 0,7 mm. Coil L6 is wound over L5, has 2 turns of 0,5 mm PEV wire.

The device is configured using a device for measuring frequency response, such as X1-42. Initially, the bandpass filter C12, L2, C13, L3 is tuned to a center frequency of 90 MHz with a bandwidth of approximately 5 MHz. If there are radio stations in your area that have a significant signal level in this range, then it is better to select a different center frequency in the range of 88-108 MHz or narrow the filter band.

Next, the first and second local oscillators are tuned. By changing the parameters of the coils and the modes of operation of the transistors for direct current, they ensure that the frequency response of the filter moves smoothly in the first range from 65 MHz to 155 MHz, and in the second range from 155 MHz to 410 MHz. The last filter C1, L1, C2 is adjusted so that the signal level of the mirror channel is 15 - 20 dB less than the main channel.

Working with the device is as follows. Initially, resistor R15 sets the maximum sensitivity threshold of the "anti-spy". Further, by smoothly rotating the knob of the resistor R23, the signal level is controlled by the indicator PA1, first in the first and then in the second subband. If the radio station or "bug" works, then the arrow of the indicator will deviate. Further, changing the position of the device in the room, monitor the change in the readings of the level indicator. If the radio bookmark is working, then the readings will depend significantly on the spatial orientation. More precise control is carried out by the receiver, which adjusts within the intermediate frequency of the device (90 ± 2,5) MHz.

Publication: cxem.net

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