Almost every novice radio amateur has tried to assemble a radio bug. There are quite a few circuits on our website, many of which contain only one transistor, a coil and a harness - several resistors and capacitors. But even such a simple scheme will not be easy to configure correctly without special equipment. We won’t talk about the wave meter and HF frequency meter - as a rule, beginning radio amateurs have not yet acquired such complex and expensive devices, but assembling a simple HF detector is not just necessary, but absolutely necessary.

Below are the details for it.

This detector allows you to determine whether there is high-frequency radiation, that is, whether the transmitter generates any signal. Of course, it will not show the frequency, but for this you can use a regular FM radio receiver.

The design of the RF detector can be any: wall-mounted or a small plastic box in which a dial indicator and other parts will fit, and the antenna (a piece of thick wire 5-10 cm) will be brought out. Capacitors can be used of any type; deviations in part ratings are permissible within a very wide range.

RF Radiation Detector Parts:

- Resistor 1-5 kilo-ohms;
- Capacitor 0.01-0.1 microfarad;
- Capacitor 30-100 picofarads;
- Diode D9, KD503 or GD504.
- Pointer microammeter for 50-100 microamps.

The indicator itself can be anything, even if it is for high current or voltage (voltmeter), just open the case and remove the shunt inside the device, turning it into a microammeter.

If you do not know the characteristics of the indicator, then to find out what current it is at, simply connect it to an ohmmeter first at a known current (where the marking is indicated) and remember the percentage of scale deviation.

And then connect an unknown pointer device and by the deflection of the pointer it will become clear what current it is designed for. If a 50 µA indicator gives a complete deviation, and an unknown device at the same voltage gives a half deviation, then it is 100 µA.

For clarity, I assembled a surface-mounted RF signal detector and measured the radiation from a freshly assembled FM radio microphone.

When the transmitter circuit is powered from 2V (severely shrunken crown), the detector needle deviates by 10% of the scale. And with a fresh 9V battery - almost half.

In accordance with SanPiN 2.2.4.1191-03, for measuring EMF levels in the frequency range ≥ 300 MHz - 300 GHz, instruments are used that are designed to estimate average values ​​of energy flux density with an acceptable relative error: no more than ± 40% in the range ≥ 300 MHz - 2 GHz and no more than ± 30% in the range above 2 GHz.

The means for measuring PES are given in Table 7.4.

Table 7.4 – Energy flux density meters

	Frequency range, GHz	Measurement limits, μW/cm 2
		0,32 – 100000
		0,32 – 100000
		20,0 – 100000
		20,0 – 100000

The energy flux density meters shown in Table 7.4 are designed to measure average PES values ​​of the electromagnetic field in a wide frequency range. They are used to assess the degree of biological hazard of microwave radiation in continuous generation and pulse modulation modes in free space and limited volumes near powerful radiation sources.

Devices of type P3, measuring PES, consist of antenna-converters and an indicator. The transducer antenna includes a system of series-connected resistive thin-film thermocouple transducers that are placed on a conical surface. During measurements, EMF energy is absorbed by thermocouple elements. A thermo-emf proportional to the PES occurs at each thermocouple. The thermocouple meter sums and amplifies the constant emf of thermocouples according to the logarithmic law. The EMF intensity reading is displayed on a digital display in decibels relative to the lower measurement limit of the used antenna-converter. Among the means of measuring PES, there are instruments that can also determine the radiation dose - the total PES over a period of time.

Currently, the following devices are widely used to determine the microwave radiation flux density: P3-33, P3-33M, P3-40, P3-41 and IPM-101M.

The P3-33 (P3-33M) microwave radiation flux density meter is shown in Figure 7.1.

Figure 7.1 – Microwave radiation flux meter P3-33 (P3-33M).

Many instruments designed to measure EMR allow one to determine not only PES, but also the strength of electric and magnetic fields and operate accordingly in different frequency ranges. This type of device includes the portable measuring device P3-40 (Figure 7.2), the EMI intensity meter P3-41, the small-sized microprocessor field strength meter IPM-101M, etc.

Figure 7.2 – Portable measuring device P3-40.

1. Description of the laboratory setup

The appearance of the laboratory installation is shown in Figure 7.3.

The stand is a table made in the form of a welded frame with a tabletop 1, under which replaceable screens 2 are placed, used to study the shielding properties of various materials. On the tabletop 1 there is a microwave oven 3 (radiation source) and a coordinate device 4.

Coordinate device 4 records the movement of the microwave field sensor 5 along the “X” and “Y” axes. The “Z” coordinate is determined by a scale marked on the measuring stand 6, along which the sensor 5 can move freely. This makes it possible to study the distribution of microwave radiation in space from the front panel of the microwave oven (elements of the most intense radiation).

Sensor 5 is made in the form of a half-wave vibrator, designed for a frequency of 2.45 GHz and consisting of a dielectric housing, vibrators and a microwave diode.

The coordinate device 4 is made in the form of a tablet on which a coordinate grid is applied. The tablet is glued directly to tabletop 1. Stand 6 is made of dielectric material (organic glass) to eliminate distortion of the microwave field distribution.

Refractory fireclay bricks are used as a load in a microwave oven.

The signal from sensor 5 is sent to multimeter 7, located on the free part of tabletop 1 (outside the coordinate grid).

Figure 7.3 – Laboratory setup.

The work uses an electronic digital multimeter DT-830D, which can operate in the position of a voltmeter, ammeter and ohmmeter (see Figure 7.4). To measure the radiation intensity of a microwave oven, the multimeter is turned on to the “A 2000 µ” position. In this position, the multimeter operates as a DC milliammeter and is used to measure small currents up to 2000 μA with a measurement accuracy of ± 1% ± 2 count units.

On the table top 1 there are slots for installing replaceable protective screens 2 made of the following materials:

mesh made of galvanized steel with 50 mm cells;

mesh made of galvanized steel with 10 mm cells;

aluminum sheet;

polystyrene;

Figure 7.4 – Multimeter DT-830D.

Well, in general, everything is as always. I needed a microwave radiation detector. The Internet is not rich in schemes. And they are so old and obscene. Nothing suited me... But I had to make something portable and economical so that the circuit would work from at least 3 V, for example from a mobile phone battery.

In addition, in the “technical specifications” I set the following conditions:

the device can detect modern microwave “bugs” (radio bugs);

will help in setting up security systems (radio beam sensors);

can check medical equipment operating on microwaves;
will help detect leaks in the waveguides of your microwave equipment;

can become part of a security system.

It will also help you check if your microwave is working, for example. Or detect a microwave field around it. Check the autonomously working handsets of your home telephone. Well, and other, standard, or invented by you, areas of application.

There is not much to say about the principles of operation. The detector is like a detector, only for ultra-high frequencies. The waveguide allows this detector to set (indicate) the direction of radiation. If it is used as a control receiver or detector of the presence of radiation, then the waveguide may not be used at all....

Fig.1

I strive for maximum simplicity in my devices (just like in military equipment).

The diagram (Fig. 1) uses the most common parts. Not SMD. Although there is nothing easier than implementing the circuit in the SMD version. But to do this, you need to independently wire the board for these elements.

In such designs, it is usually recommended to use Soviet diodes for the 3 cm range with the highest conversion efficiency, type 2A203A. Then comes 2A202A..., but the D405 is already outdated and has low parameters, especially since it is a mixer. It will work though. And it's easier to get. This link also contains data on D405 diodes, in the mixing section http://www.npptez.ru/en/production/micr... 59-41.html.

Diode D405 or similar should be handled very carefully!!! I'm terribly afraid of static! Be sure to ground yourself, ground the tool you use to remove the diode from the package. The waveguide must be of such a design that the diode does not need to be soldered! These diodes do not solder!!! (Accordingly, the walls of the waveguide with which the diode leads are in contact must be insulated from each other).

I used the transistor KT6113. You can use any other one that makes less noise, for example, KT3102E (D), etc.

The MC34119 microcircuit, I think, is known to everyone. The construction and installation work shows And link to datasheet.

The speaker is a simple 32 ohm headphone. My headphone jack is wired in such a way that the headphone coils are connected in series.

The entire design fit on a breadboard smaller than a matchbox.

Any waveguide for the D405 microwave diode will do. From any old design. But you can make it yourself - it’s just a box for a microwave diode, made of foil PCB. Although it can be made of tin or aluminum with a flat, smooth surface of the walls. Approximate dimensions (precision is not important here): height = 20 mm, width = 22 mm, length = 30 mm.

Fig.2

In this design, the waveguide is made without a horn. In the photo (Fig. 2) it is shown with a microwave diode behind glass, which introduces large losses. Instead of glass, it is best to place a thin fluoroplastic plate on superglue or hot melt glue, or one made of dense, finely porous foam. Even better is an antenna, like a “dielectric carrot” made of fluoroplastic, tightly inserted into the waveguide.

The device is powered from 2.5 - 4 V, and consumes 4 mA in this version.

Well, there is nothing complicated in the design of a microwave detector. No setup required.It turned out that it receives frequencies (this is only approximately!!!) from 4 to at least 12 GHz.

Kirill Sotnikov,

Novosibirsk city

Let's consider the principle of operation of the detector.

The simplest receiver, as is known, is a detector. And such microwave receivers, consisting of a receiving antenna and a diode, find their application for measuring microwave power.

The most significant disadvantage is the low sensitivity of such receivers. In order to reliably detect a change in diode current under the influence of a microwave field, a microwave amplitude on the diode of several tens of millivolts is required. This is a very low sensitivity, corresponding to detecting a 10 mW transmitter at a distance of only a few meters.

In order to dramatically increase the sensitivity of the detector without complicating the microwave head (i.e., without amplifiers, converters, etc.), a circuit of a detector microwave receiver with a modulated rear wall of the waveguide was developed.

Microwave field detector with horn antenna

At the same time, the microwave head was almost not complicated; only the modulation diode VD2 was added, and VD1 remained a detector one.

Let's consider the detection process.

The microwave signal received by the horn (or dielectric) antenna enters the waveguide. Since the back wall of the waveguide is short-circuited, a standing wave regime is established in the waveguide. Moreover, if the detector diode is located at a distance of half a wave from the rear wall, it will be at a node (i.e., minimum) of the field, and if at a distance of a quarter of a wave, then at the antinode (maximum). That is, if we electrically move the back wall of the waveguide by a quarter wave (applying a modulating voltage with a frequency of 3 kHz to VD2), then on VD1, due to its movement with a frequency of 3 kHz from the node to the antinode of the microwave field, a low-frequency signal with a frequency of 3 will be released kHz, which can be amplified and highlighted by conventional ULF.

Thus, if a rectangular modulating voltage is applied to VD2, then when the microwave field drops, a detected signal of the same frequency will be removed from VD1. This signal will be out of phase with the modulating one (which will be successfully used in the future to isolate the useful signal from interference) and have a very small amplitude.

That is, all signal processing will be carried out at low frequencies, without the scarce microwave parts. Using microwave technology, you will need to make a head according to the drawings, which does not require any adjustment.

Let's consider the working design of the microwave field detector "Radar Anti" as an example.

Waveguide and horn

The waveguide and horn are made of thin copper or tinned sheet metal. You can also use foil fiberglass, having previously polished the foil and coated it with alcohol rosin flux (so that it does not oxidize).

Particular care must be taken when handling microwave diodes. They are afraid of electrostatic electricity and during a breakdown, the sensitivity to the microwave field drops by an order of magnitude or more. When checked by a tester, an electrostatically damaged diode behaves exactly the same as a working one. Therefore, when working with microwave diodes, you must take the same precautions as when working with MOS transistors.

Schematic diagram of the electronic filling of a microwave field detector.

Electronic circuit diagram of a microwave field detector

This reference guide provides information on using different types of caches. The book discusses possible options for hiding places, methods for creating them and the necessary tools, describes the devices and materials for their construction. Recommendations are given for arranging hiding places at home, in cars, on a personal plot, etc.

Particular attention is paid to methods and methods of control and protection of information. A description of the special industrial equipment used in this case is given, as well as devices available for repetition by trained radio amateurs.

The book provides a detailed description of the work and recommendations for the installation and configuration of more than 50 devices and devices necessary for the manufacture of caches, as well as those intended for their detection and safety.

The book is intended for a wide range of readers, for everyone who wishes to become acquainted with this specific area of ​​​​the creation of human hands.

Industrial devices for detecting radio tags, briefly discussed in the previous section, are quite expensive (800-1500 USD) and may not be affordable for you. In principle, the use of special means is justified only when the specifics of your activity can attract the attention of competitors or criminal groups, and information leakage can lead to fatal consequences for your business and even health. In all other cases, there is no need to be afraid of industrial espionage professionals and there is no need to spend huge amounts of money on special equipment. Most situations can come down to banal eavesdropping on conversations of a boss, an unfaithful spouse or a neighbor at the dacha.

In this case, as a rule, handicraft radio markers are used, which can be detected by simpler means - radio emission indicators. You can easily make these devices yourself. Unlike scanners, radio emission indicators record the strength of the electromagnetic field in a specific wavelength range. Their sensitivity is low, so they can detect a source of radio emission only in close proximity to it. The low sensitivity of field strength indicators also has its positive aspects - the influence of powerful broadcasting and other industrial signals on the quality of detection is significantly reduced. Below we will look at several simple indicators of the electromagnetic field strength of the HF, VHF and microwave ranges.

The simplest indicators of electromagnetic field strength

Let's consider the simplest indicator of electromagnetic field strength in the 27 MHz range. The schematic diagram of the device is shown in Fig. 5.17.

Rice. 5.17. The simplest field strength indicator for the 27 MHz range

It consists of an antenna, an oscillating circuit L1C1, a diode VD1, a capacitor C2 and a measuring device.

The device works as follows. HF oscillations enter the oscillating circuit through the antenna. The circuit filters out 27 MHz oscillations from the frequency mixture. The selected HF oscillations are detected by the diode VD1, due to which only positive half-waves of the received frequencies pass to the diode output. The envelope of these frequencies represents low frequency vibrations. The remaining HF oscillations are filtered by capacitor C2. In this case, a current will flow through the measuring device, which contains alternating and direct components. The direct current measured by the device is approximately proportional to the field strength acting at the receiving site. This detector can be made as an attachment to any tester.

Coil L1 with a diameter of 7 mm with a tuning core has 10 turns of PEV-1 0.5 mm wire. The antenna is made of steel wire 50 cm long.

The sensitivity of the device can be significantly increased if an RF amplifier is installed in front of the detector. A schematic diagram of such a device is shown in Fig. 5.18.

Rice. 5.18. Indicator with RF amplifier

This scheme, compared to the previous one, has a higher transmitter sensitivity. Now the radiation can be detected at a distance of several meters.

High-frequency transistor VT1 is connected according to a common base circuit and works as a selective amplifier. The oscillatory circuit L1C2 is included in its collector circuit. The circuit is connected to the detector through a tap from coil L1. Capacitor SZ filters out high-frequency components. Resistor R3 and capacitor C4 serve as a low-pass filter.

Coil L1 is wound on a frame with a tuning core with a diameter of 7 mm using PEV-1 0.5 mm wire. The antenna is made of steel wire about 1 m long.

For the high frequency range of 430 MHz, a very simple field strength indicator design can also be assembled. A schematic diagram of such a device is shown in Fig. 5.19, a. The indicator, the diagram of which is shown in Fig. 5.19b, allows you to determine the direction to the radiation source.

Rice. 5.19. 430 MHz band indicators

Field strength indicator range 1..200 MHz

You can check a room for the presence of listening devices with a radio transmitter using a simple broadband field strength indicator with a sound generator. The fact is that some complex “bugs” with a radio transmitter start transmitting only when sound signals are heard in the room. Such devices are difficult to detect using a conventional voltage indicator; you need to constantly talk or turn on a tape recorder. The detector in question has its own sound signal source.

The schematic diagram of the indicator is shown in Fig. 5.20.

Rice. 5.20. Field strength indicator 1…200 MHz range

Volumetric coil L1 was used as a search element. Its advantage, compared to a conventional whip antenna, is a more accurate indication of the location of the transmitter. The signal induced in this coil is amplified by a two-stage high-frequency amplifier using transistors VT1, VT2 and rectified by diodes VD1, VD2. By the presence of constant voltage and its value on capacitor C4 (the M476-P1 microammeter operates in millivoltmeter mode), you can determine the presence of a transmitter and its location.

A set of removable L1 coils allows you to find transmitters of various powers and frequencies in the range from 1 to 200 MHz.

The sound generator consists of two multivibrators. The first, tuned to 10 Hz, controls the second, tuned to 600 Hz. As a result, bursts of pulses are formed, following with a frequency of 10 Hz. These packets of pulses are supplied to the transistor switch VT3, in the collector circuit of which the dynamic head B1 is included, located in a directional box (a plastic pipe 200 mm long and 60 mm in diameter).

For more successful searches, it is advisable to have several L1 coils. For a range of up to 10 MHz, coil L1 must be wound with 0.31 mm PEV wire on a hollow mandrel made of plastic or cardboard with a diameter of 60 mm, a total of 10 turns; for the range of 10-100 MHz the frame is not needed, the coil is wound with PEV wire 0.6...1 mm, the diameter of the volumetric winding is about 100 mm; number of turns - 3...5; for the 100–200 MHz range, the coil design is the same, but it has only one turn.

To work with powerful transmitters, smaller diameter coils can be used.

By replacing transistors VT1, VT2 with higher frequency ones, for example KT368 or KT3101, you can raise the upper limit of the detector detection frequency range to 500 MHz.

Field strength indicator for the range 0.95…1.7 GHz

Recently, ultra-high frequency (microwave) transmitting devices have been increasingly used as part of radio launchers. This is due to the fact that waves in this range pass well through brick and concrete walls, and the antenna of the transmitting device is small in size but highly efficient in its use. To detect microwave radiation from a radio transmitting device installed in your apartment, you can use the device whose diagram is shown in Fig. 5.21.

Rice. 5.21. Field strength indicator for the range 0.95…1.7 GHz

Main characteristics of the indicator:

Operating frequency range, GHz…………….0.95-1.7

Input signal level, mV…………….0.1–0.5

Microwave signal gain, dB…30 - 36

Input impedance, Ohm………………75

Current consumption no more than, mL………….50

Supply voltage, V………………….+9 - 20 V

The output microwave signal from the antenna is supplied to the input connector XW1 of the detector and is amplified by a microwave amplifier using transistors VT1 - VT4 to a level of 3...7 mV. The amplifier consists of four identical stages made of transistors connected according to a common emitter circuit with resonant connections. Lines L1 - L4 serve as collector loads of the transistors and have an inductive reactance of 75 Ohms at a frequency of 1.25 GHz. The coupling capacitors SZ, C7, C11 have a capacitance of 75 Ohms at a frequency of 1.25 GHz.

This design of the amplifier makes it possible to achieve maximum gain of the cascades, however, the unevenness of the gain in the operating frequency band reaches 12 dB. An amplitude detector based on a VD5 diode with a filter R18C17 is connected to the collector of transistor VT4. The detected signal is amplified by a DC amplifier at op-amp DA1. Its voltage gain is 100. A dial indicator is connected to the output of the op-amp, indicating the level of the output signal. An adjusted resistor R26 is used to balance the op-amp so as to compensate for the initial bias voltage of the op-amp itself and the inherent noise of the microwave amplifier.

A voltage converter for powering the op-amp is assembled on the DD1 chip, transistors VT5, VT6 and diodes VD3, VD4. A master oscillator is made on elements DD1.1, DD1.2, producing rectangular pulses with a repetition frequency of about 4 kHz. Transistors VT5 and VT6 provide power amplification of these pulses. A voltage multiplier is assembled using diodes VD3, VD4 and capacitors C13, C14. As a result, a negative voltage of 12 V is formed on capacitor C14 at a microwave amplifier supply voltage of +15 V. The op-amp supply voltages are stabilized at 6.8 V by zener diodes VD2 and VD6.

The indicator elements are placed on a printed circuit board made of double-sided foil fiberglass 1.5 mm thick. The board is enclosed in a brass screen, to which it is soldered along the perimeter. The elements are located on the side of the printed conductors, the second, foil side of the board serves as a common wire.

Lines L1 - L4 are pieces of silver-plated copper wire 13 mm long and 0.6 mm in diameter. which are soldered into the side wall of the brass screen at a height of 2.5 mm above the board. All chokes are frameless with an internal diameter of 2 mm, wound with 0.2 mm PEL wire. The wire pieces for winding are 80 mm long. The XW1 input connector is a C GS cable (75 ohm) connector.

The device uses fixed resistors MLT and half-string resistors SP5-1VA, capacitors KD1 (C4, C5, C8-C10, C12, C15, C16) with a diameter of 5 mm with sealed leads and KM, KT (the rest). Oxide capacitors - K53. Electromagnetic indicator with a total deviation current of 0.5...1 mA - from any tape recorder.

The K561LA7 microcircuit can be replaced with K176LA7, K1561LA7, K553UD2 - with K153UD2 or KR140UD6, KR140UD7. Zener diodes - any silicon with a stabilization voltage of 5.6...6.8 V (KS156G, KS168A). The VD5 2A201A diode can be replaced with DK-4V, 2A202A or GI401A, GI401B.

Setting up the device begins with checking the power circuits. Resistors R9 and R21 are temporarily unsoldered. After applying a positive supply voltage of +12 V, measure the voltage on capacitor C14, which must be at least -10 V. Otherwise, use an oscilloscope to verify the presence of alternating voltage at pins 4 and 10 (11) of the DD1 microcircuit.

If there is no voltage, make sure that the microcircuit is in working order and installed correctly. If alternating voltage is present, check the serviceability of transistors VT5, VT6, diodes VD3, VD4 and capacitors C13, C14.

After setting up the voltage converter, solder resistors R9, R21 and check the voltage at the op-amp output and set the zero level by adjusting the resistance of resistor R26.

After this, a signal with a voltage of 100 μV and a frequency of 1.25 GHz from a microwave generator is supplied to the input of the device. Resistor R24 ​​achieves complete deflection of the indicator arrow PA1.

Microwave radiation indicator

The device is designed to search for microwave radiation and detect low-power microwave transmitters made, for example, using Gunn diodes. It covers the range 8...12 GHz.

Let's consider the principle of operation of the indicator. The simplest receiver, as is known, is a detector. And such microwave receivers, consisting of a receiving antenna and a diode, find their application for measuring microwave power. The most significant disadvantage is the low sensitivity of such receivers. To dramatically increase the sensitivity of the detector without complicating the microwave head, a microwave detector receiver circuit with a modulated rear wall of the waveguide is used (Fig. 5.22).

Rice. 5.22. Microwave receiver with modulated waveguide rear wall

At the same time, the microwave head was almost not complicated; only the modulation diode VD2 was added, and VD1 remained a detector one.

Let's consider the detection process. The microwave signal received by the horn (or any other, in our case, dielectric) antenna enters the waveguide. Since the rear wall of the waveguide is short-circuited, a standing will mode is established in the waveguide. Moreover, if the detector diode is located at a distance of half a wave from the rear wall, it will be at a node (i.e., minimum) of the field, and if at a distance of a quarter of a wave, then at the antinode (maximum). That is, if we electrically move the back wall of the waveguide by a quarter wave (applying a modulating voltage with a frequency of 3 kHz to VD2), then on VD1, due to its movement with a frequency of 3 kHz from the node to the antinode of the microwave field, a low-frequency signal with a frequency of 3 will be released kHz, which can be amplified and highlighted by a conventional low-frequency amplifier.

Thus, if a rectangular modulating voltage is applied to VD2, then when it enters the microwave field, a detected signal of the same frequency will be removed from VD1. This signal will be out of phase with the modulating one (this property will be successfully used in the future to isolate the useful signal from interference) and have a very small amplitude.

That is, all signal processing will be carried out at low frequencies, without the scarce microwave parts.

The processing scheme is shown in Fig. 5.23. The circuit is powered by a 12 V source and consumes a current of about 10 mA.

Rice. 5.23. Microwave signal processing circuit

Resistor R3 provides the initial bias of the detector diode VD1.

The signal received by diode VD1 is amplified by a three-stage amplifier using transistors VT1 - VT3. To eliminate interference, the input circuits are powered through a voltage stabilizer on transistor VT4.

But remember that the useful signal (from the microwave field) from diode VD1 and the modulating voltage on diode VD2 are out of phase. That is why the R11 engine can be installed in a position in which interference will be suppressed.

Connect an oscilloscope to the output of op-amp DA2 and, by rotating the slider of resistor R11, you will see how compensation occurs.

From the output of the pre-amplifier VT1-VT3, the signal goes to the output amplifier on the DA2 chip. Please note that between the VT3 collector and the DA2 input there is an RC switch R17C3 (or C4 depending on the state of the DD1 keys) with a bandwidth of only 20 Hz (!). This is the so-called digital correlation filter. We know that we must receive a square wave signal with a frequency of 3 kHz, exactly equal to the modulating signal, and out of phase with the modulating signal. The digital filter uses this knowledge precisely - when a high level of the useful signal is to be received, capacitor C3 is connected, and when it is low, C4 is connected. Thus, at SZ and C4, the upper and lower values ​​of the useful signal are accumulated over several periods, while noise with a random phase is filtered out. The digital filter improves the signal-to-noise ratio several times, correspondingly increasing the overall sensitivity of the detector. It becomes possible to reliably detect signals below the noise level (this is a general property of correlation techniques).

From the DA2 output, the signal through another digital filter R5C6 (or C8 depending on the state of the DD1 keys) is supplied to the integrator-comparator DA1, the output voltage of which, in the presence of a useful signal at the input (VD1), becomes approximately equal to the supply voltage. This signal turns on the HL2 “Alarm” LED and the BA1 head. The intermittent tonal sound of the BA1 head and the blinking of the HL2 LED is ensured by the operation of two multivibrators with frequencies of about 1 and 2 kHz, made on the DD2 chip, and by transistor VT5, which shunts the VT6 base with the operating frequency of the multivibrators.

Structurally, the device consists of a microwave head and a processing board, which can be placed either next to the head or separately.