NONLINEAR RADAR AND FEATURES OF THEIR APPLICATION FOR SEARCHING FOR BUGS.

LOBASHEV Aleksey Konstantinovich, Candidate of Technical Sciences, Associate Professor

NONLINEAR RADAR AND FEATURES OF THEIR APPLICATION FOR SEARCHING FOR BUGS

Source: Magazine «Special Equipment» No. 6 2006

As is known, the operating principle of the NRL is close to the operating principle of radar stations, widely used for observation of various objects. During operation, the NRL emits a high-frequency signal, which easily penetrates many materials, furniture, can pass (with attenuation) through internal partitions of premises, concrete walls and floors, is reflected from the surface under study and is received by the NRL receiver. A significant difference is that if the receiver of a radar station receives an echo signal reflected from an object at the frequency of the emitted signal, then the NRL receiver receives multiple harmonics of the reflected signal (2f, 3f). The appearance of these harmonics in the reflected signal is due to the nonlinearity of the characteristics of the PP, which are part of the ZU. As a result of nonlinear transformation of the electrical signal induced in the elements of the ZU circuit by the high-frequency field of the locator, a signal is formed, in the spectrum of which, in addition to the fundamental frequency, there are its multiple harmonics with frequencies of 2f, 3f, etc. Since the amplitude of the harmonics decreases sharply with an increase in its number, the 2nd and 3rd harmonics are used when operating the NRL. In this case, the amplitudes of the harmonics largely depend on the nature of the nonlinearity of the electronic radio elements included in the ZU, and the power of the radiated electromagnetic field.

where I is the current, Is is the breakdown current, q is the electron charge, V is the voltage, K is the Boltzmann constant, T is the temperature.

Despite the great difference among PP devices, they all have fairly «clean», predictable characteristics. For PP connections, the above formula is described by a curve (Fig. 1). For MOM diodes, this curve is not predictable and cannot be described by a strict formula. Nevertheless, with a certain degree of error, the current-voltage characteristic of MOM diodes can be considered fairly symmetrical in most cases (Fig. 2).

Fig. 1. Characteristic of a PP connection

Fig. 2. Characteristics of the MOM diode

The level of «purity» of the connection, and therefore the selection of signals from the PP and MOM diodes, is manifested in the difference in the values ​​of the harmonic signals received by the NRL. Thus, when the NRL is irradiated by the PP connection, a strong signal of the 2nd and a weak signal of the 3rd harmonic arise. The MOM diode behaves differently, has a weak signal at the 2nd and a strong signal at the 3rd harmonic. Thus, a comparative analysis of the levels of the 2nd and 3rd harmonics allows the operator to select their sources. The use of the NRL ensures a high percentage of detection of the ZU placed in various shelters (for example, in reinforced concrete walls the probability of detecting the ZU is 90 — 95%).

The main advantage of NRL is the ability to detect electronic circuits both in the on and off state, the disadvantage is a relatively large number of false detections of natural nonlinear reflectors of the MOM type.

The main tactical and technical characteristics of nonlinear radars and their features

Let's consider the main technical characteristics of NRL and their features, from the point of view of the tactics of using locators. The main parameters of NRL are:

— transmitter probing radiation power and frequency;
— operating mode;
— receiver sensitivity;
— directional properties of the antenna system;
— accuracy of indication devices;
— ergonomic characteristics of the devices.

Of great importance for the operation of the NRL is the depth of the material being studied, which depends on the penetrating power of the radiating wave, which in turn depends on the frequency and power of the NRL. There are two conditions for choosing the NRL frequency. On the one hand, due to the increase in attenuation of the electromagnetic wave in the propagation medium with increasing frequency, the power level of the converted reflected signal is higher, the lower the NRL frequency. But, on the other hand, for radiation with a lower frequency, the NRL's capabilities for accurately localizing the location of the ZU are deteriorated, since with acceptable dimensions of its antenna, the directional pattern of the NRL antenna expands. Let us note another NRL parameter on which its frequency characteristics depend. Namely: the higher the radiation frequency of the device, the smaller the geometric dimensions of the NRL antenna system, the more convenient it is to work with the device.

Let's consider the influence of power on the operation of the NRL. It is obvious that the higher the power of the locator's radiation, the deeper the electromagnetic wave penetrates and the greater the probability and range of detecting a bookmark placed in a shelter. But, on the other hand, given that the NRL operates in the area of ​​ultra-high frequencies — microwaves (the devices currently offered operate in the frequency range of 680 — 1000 MHz), high radiation power has a harmful effect on the operator.

Also, the radiation power of the NRL largely determines the conversion factor (Kp) of the probing signal energy into the energy of higher harmonics. With an increase in the radiation power, the value of Kp increases. At the same time, as noted earlier, an increase in power improves the characteristics of the NRL, but at the same time leads to an increase in the hazardous impact on the operator. Therefore, to solve the problems of increasing the range of the NRL and the effective implementation of the device for localizing the ZU, modern types of NRL use modes with continuous and pulsed radiation. Thus, the average power of the NRL of continuous radiation is from 0.3 to 3 W. The peak power of pulsed NRLs is from 150 to 400 W, i.e. almost 30 dB higher than the power of continuous radiation devices. Considering that the conversion efficiency (Kp) is determined not by the average radiation power, but by its peak value, the range of locators operating in pulse mode is higher than that of devices operating with continuous radiation (all other things being equal).

In general, the transmitting devices of locators generating the probing signal are characterized by the following main tactical and technical characteristics:

— operating mode (continuous or pulsed);
— output power control limits (dB);
— continuous emission frequency;
— radio pulse repetition rate and duration (μs).

The receiver sensitivity largely determines the maximum range of the NRL. For modern devices, this figure ranges from -110 to -145 dB. In general, the quality of the receiving device that records re-radiated signals is reflected by the following indicators:

— tuning frequencies (MHz) for the recorded harmonics (2 and 3);
— actual sensitivity at a certain signal-to-noise ratio (dBW);
— sensitivity control limits (dB).

The main parameters of the antenna system emitting probing signals and receiving reflected radiation at the frequencies of higher harmonics are:

— directivity coefficient (DCC);
— width of the main lobe of the radiation pattern at half power level (deg.);
— level of suppression of the rear lobes of the radiation pattern (dB);
— ellipticity coefficient (for antennas with circular polarization).

The tactical and technical indicators of the NRL are largely determined by the quality of the devices indicating the operating modes and signal parameters. Most modern NRLs are equipped with multi-segment (display) LED indicators and variable-tone sound alarms. To improve the accuracy of object identification, the NRL provides for reception modes at frequencies of the 2nd and 3rd harmonics of the probing radiation, as well as listening to signals transmitted by the ZU outside the surveyed premises.

Ergonomic characteristics include, first of all, the weight and size indicators and ease of use of the NRL.

Features of tactics of working with the locator

Knowledge of the tactical and tactic characteristics largely determines the effectiveness of the NRL. When performing search activities using the NRL, the operator must consistently perform three main functions: detection, location determination and identification of the PP.

The detection characteristic of the NRL is standardized only for free space. Moreover, in the conditions of searching for a ZU, it is not so much about the range, but about the maximum depth of detection of objects in a masking environment. The assessment of this indicator is carried out according to the response level, increasing as you approach the object, which allows you to determine the location of the ZU.

When working in open areas or large unequipped rooms, pulse locators can provide a detection range several times greater than continuous ones, which reduces the inspection time. At the same time, when working in offices, the maximum range of both types of locators is practically not used due to the saturation of the allocated and neighboring rooms with electronic equipment and contact interference objects. The actual required range in these cases is approximately 0.5 m for locators of any type. It is adjusted by the operator taking into account the interference environment by reducing the transmitter power or «roughening» the sensitivity of the receiver to the limit that allows you to distinguish from which object the response came. The range depends on the type of the detected device (for example, a bug with a longer antenna is usually detected at a greater distance) and the conditions of its placement (in furniture, behind barriers made of wood, brick, concrete, etc.).

Let us consider the previously presented basic functions of the NRL and the algorithm for their implementation. Thus, to solve the first stage of search activities — detection of the ZU, the operator must perform the following operations:

— having turned on the NRL, detect and, if possible, eliminate sources of interfering signals;

— set the maximum sensitivity level of the receiving device and the maximum power level of the transmitter of the probing signal;

— check the premises for the presence of powerful interference objects, both corrosive and electronic (mainly electronic office equipment and radio equipment) by scanning the enclosing structures and interior items from a distance of approximately 1 m; in this case, the purpose of the objects must be precisely established and they must either be removed from the premises or not taken into account in further search; it should be taken into account that these interference objects may be in adjacent rooms and on other floors, which, if possible, should be inspected;

— after removing sources of powerful interference from the room, repeat the inspection of the walls, ceilings, furniture and devices from a distance of 20 cm or less; during the inspection, mark suspicious areas.

The second stage of search activities — determining the location of the ZU is carried out by assessing the level and bearing of the response signal. Bearing is understood as the direction corresponding to the maximum level of the received signal. It should be taken into account that the probing and reflected signals are reflected by nearby objects. Effective reflectors are mirrors, metal plates, mesh, reinforcement, etc. When irradiated, they can record reflected signals from non-linear reflectors, including those located behind the operator. To determine the exact location of the ZU, it is necessary to:

— reduce the level of radiated power and receiver sensitivity;

— by moving the antenna near suspicious areas, analyze the readings of the light indicator and the frequency of the tone signal in the headphones;

— determine the direction of arrival of the reflected signal of the maximum level, take a bearing by the orientation of the antenna;

— having determined the exact location, proceed to identify the object.

To eliminate errors when comparing the indicator readings, it is necessary to reduce the receiver sensitivity or reduce the transmitter power as any of the LED columns reaches its maximum height so that the illuminated trail does not reach the scale limit by 1 — 3 segments.

Finally, to solve the third stage of search activities — identification of MOM diodes and PP, there are a number of methods that allow achieving a high practical effect.

In devices that receive response signals simultaneously at the 2nd and 3rd harmonics of the probing signal, the first (and main) step in identifying an object is to compare the signal levels at the outputs of both receiving paths. When a PP connection is irradiated, a strong re-reflection occurs at the frequency of the 2nd harmonic and a weak one at the frequency of the 3rd. The MOM diode behaves differently, creating a strong re-reflection at the 3rd and a weak one at the 2nd harmonic.

As a second step in identifying MOM diodes and PPs, a number of NRLs provide the ability to “listen” to demodulated harmonic signals, which allows identifying an object using the effect of changing the noise level. As the NRL approaches the PP, a significant decrease in the noise level is observed, reaching a minimum directly above the object. When MOM diodes are irradiated, this effect is practically not observed (Fig. 3).

Fig. 3. Change in the noise level at the location of the object

However, there are false connections that also reduce the noise level as the PP. Therefore, as a third step in identifying the PP (MOM diode), it is recommended to perform a mechanical action on the suspicious place. In practice, mechanical action is carried out by the «tapping» method. Any mechanical action leads to a change in the geometry of the MOM diode and its converting properties. In this case, the tapping frequency is clearly heard in the converted signal. The signal level during tapping can be minimal, so lightly tapping with your hand on the surface being examined is enough. This operation allows for more accurate identification of the object.

Some models of pulse NRLs have a fourth identification method — the «20K» mode, which involves identifying the envelope of the re-radiated signal, named after the repetition rate of the probing pulses, equal to 20 kHz. In this case, the sound signal obtained during detection of re-radiations from the PP is beyond the perception of the human ear. With an unstable MOM contact, not all probing pulses are re-reflected, i.e. the envelope corresponding to a lower frequency heard in the headphones is identified.

Main trends in improving non-linear radars

A study of the opinions of NRL users shows that for successful work with NRL, not the least important thing is to study the main trends in improving NRL and the correct choice of equipment for work. Modern types of NRL include many new engineering solutions that make the search more accurate and efficient. Let's name the main trends in improving NRL:

— the use of automatic power control of the NRL transmitting device, which can provide dynamic control of the output signal level, preventing overload of the receiving path;

— the use of digital NRL transceivers with synthesizers, which ensures frequency stability and automatic search for «clean» operating frequencies in a wide range of operating frequencies;

— the use of a circularly polarized antenna design in NRL, which eliminates the need for multiple passes over the surface being studied and reduces the risk of missing a ZU due to incorrect antenna orientation;

— use of a single ergonomic lightweight NRL design, including a transmitting and receiving device, antenna and display, combined with an extendable boom; it is desirable that the design be easily folded, ensuring its compactness and mobility.

— transmission of all transmit, receive and digital display control signals via a single cable with the cable being laid inside the NRL design, which eliminates the need for any additional connections of the NRL components before starting work;

— use of lightweight and compact batteries and fast chargers;

— use of wireless headphones in the NRL, providing listening to received signals without a wired connection to the NRL.

This article does not claim to fully cover all the problems associated with the use of NRL, but only focuses on issues that, from the author's point of view, are the most important for increasing the efficiency of searching for memory devices.

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