Use of component analysis of the electromagnetic field to solve search problems.

Electromagnetic field indicators are evidently the most affordable and simplest devices for radio transmitters searching. Indicators are successfully used not only as a means of rapid analysis, but also as a supplement to more complex means of radio monitoring.

The classic field indicator is an all-wave meter of the power of the electromagnetic field directed to its receiving antenna. The "full spectrum" of the indicator is ensured by the absence of narrow-band selective elements in the input circuits. In fact, the bandwidth of the input circuits is the certified value of the operating frequency range of the instrument. Since the indicator has no frequency selection, the power measured by the indicator is the sum of the signal of the required radio transmitter and of the background consisting of signals of radio, television, communications, etc. If the target source has little power, and the distance to it is large, its contribution to the meter reading will be negligible, and the operator will only see the background. During the survey in the process of moving the indicator on the inspected object by the operator, the convergence of the indicator with the required radio transmitting device occurs at some point. At that time, the received power of the signal of the required source increases and becomes greater than the total power of all background signals: the operator will record the fact of detection.

This space-power method of detecting and localizing radio signals allows efficiently searching for signals of radio transmitting devices existing in the air, regardless of their frequency (within the operating range of the instrument), bandwidth, and type of modulation of their radiation. The main thing is that the signal energy should be sufficiently superior to the field energy. The independence of the characteristics of detection of field indicators on the type of signal is relevant due to the impossibility to ensure coherent reception of the required signals, especially in view of the prevalence of complex digital radio broadcasts.

The main problems when dealing with field indicators are associated with the considerable power dispersion of background sources and the complexity of spatial distribution of these fields, which results in large fluctuations in background power in different points of the subject volume. Thus, the background power measured by the indicator may change by the value of 30-40 dB in different reception conditions, so the range of detection of the same required source changes in the process of search. In addition, the signals of background sources, after being reflected many times from the surrounding structures, create a complex interference field characterized by a sharp, by an order of magnitude or more (10-20 dB), change in the background power in different parts of the room [1], which often leads to numerous false alarms.

In order to provide the operator with additional information about the detected signal, many models of indicators are complemented with some functions, which are undoubtedly useful: frequency meter, acoustic correlator (acoustic feedback scheme), ability to identify type standard signals (GSM, DECT, Wi-Fi), etc.

Comparatively recently, a relatively new component analysis method has started to be used in the search practice, allowing the operator to distinguish the required radio transmitter signal from the local maxima of the background fields.

As is well known [2], the electromagnetic field is a combination of two components: electric (vector Е) and magnetic (vector ­Н), which are mutually orthogonal. In the far zone from the source, both these vectors change in time synchronously, i.e., the phase difference between them is zero degrees. But the ratio of the amplitudes of these vectors away from the source tends to a free space characteristic impedance of 377 ohms. When approaching the radio transmitter, its field structure is changed, and the near zone fields range in quadratures, i.e., the phase difference between the electric and magnetic components approaches 90 degrees. At that, the amplitude ratios take extreme values significantly different from 377 ohms.

The mathematical description of the fields allows calculating the radius of the near-field of the source as a function of the emitted wavelength λ and of the geometric dimensions D of the radiating antenna. The approximate dependence of the phase difference between the electric and magnetic components Е, Н and the amplitude ratio are shown in Fig. 1. A numerical estimate of the radius of the near zone as the biggest of the radiation wavelength and the geometric size of the antenna is applicable for most real sources.

Fig. 1.

The simplest technical way to implement separate reception for the electric and magnetic field components is by using a small magnetic frame as antennas for the H-component and a small electric dipole for E-component. It was that pair of antennas that underlay the special antenna devices of HF radio receivers insensitive to signals of distant sources of interference [3]. Specialists of a US company have developed a specialized radio system that measures the distance to a beacon operating at a frequency of about 1 MHz on the basis of measuring only the phase difference between the E- and H-components of the field [4]. As applied to the search problem where it is important to register only the deviation of the field characteristics from the typical values, it is more efficient to measure not only the phase difference, but also the ratio between the amplitudes of the field components.

This method [5] is implemented in the field indicator NR-D (Fig. 2), which, in addition to the classical channel of the power meter, contains a channel for analyzing the electric and magnetic components. The results of measuring the phase difference and amplitude ratio are processed and displayed on the instrument display simultaneously with the classical power scale.

Fig. 2.

The practical range of detecting the signs of the near-field of a radio receiver with a radiation frequency of 400 MHz (wavelength about 0.75 m) is about 0.4-0.5 m, and with a frequency of 900 MHz about 0.1-0.2 m.

A significant advantage of this method of component analysis is the extremely low probability of false alarm conditioned by the independence both of the angle between the vectors and of the relation between them on the level of the received signal power.

Literature

1. Shebunin S.N., Lesnaya L.L. Radio wave propagation in mobile communications. - Ekaterinburg: Ural State Technical University, 2009.

2. Markov G.T., Sazonov D.M. Antennas. - М.: Energiya, 1975.

3. Grechikhin A.I. Component selection // Radio. No. 3, 1984.

4. Near Field Phase Behavior Hans Gregory Schantz (h.schantz@q-track.com) Q-Track Corporation; IEEE APS Conference July 2005.

5. Patent RU 2349927 Component indicator of the near field.