The paper presents results of numerical simulation and experimental testing of a microwave sensor for non-invasive glucose monitoring. The sensor represents a conical horn with a conical conductor inside expanding toward the horn aperture. Such a sensor has a significantly wider passband in comparison with sensors of other designs. It is essential that the sensor geometry provides formation of an extended near-field zone with high electric field strength near the sensor aperture. A clear relationship between the dielectric permittivity of the phantom biological tissue and the frequency dependence of the parameter S11 of the sensor is observed at frequencies in the range from 1.4 to 1.7 GHz. This circumstance can be used to develop a procedure for measuring the glucose level in blood that correlates with the parameter S11 of the sensor. From the viewpoint of monitoring of the glucose content in blood, the most convenient body sensor location is on the hands or feet, in particular, wrists.

The problem of active control for both the magnitude and spatial distribution of individual components of the interference component of the Poynting vector within the near zone of a system of radiators is studied. The characteristic size of this zone is on the order of the wavelength and is characterized by the presence of evanescent (nonpropagating) fields, which are formed due to the interference interaction of radiators. Using multipole expansions for fields and special summation formulas for such expansions allows one to obtain concise expressions convenient in carrying out numerical calculations. The results of calculations confirm the feasibility of the above-mentioned control in principle in solving problems of medium and object sensing.At the present time, the application area of nearfield microscopy, especially with the use of optical and infrared radiation, has been considerably expanded [1]. The possibility of breaking the diffraction limit was clearly experimentally shown for the first time in the microwave band [2]. This possibility was efficiently used, in particular, in problems of near-field microwave diagnostics of biological media [3]. At the same time, the results of some works (e.g., [4]) testify to the appropriateness of applying the technology in problems of radio tomography [5] with the use of evanescent (attenuating) fields of systems of identical radiators.The dynamics of total and interference energy flows in near fields of such systems was studied in part in [6]. Simultaneously, the presence of a nonradiative energy transfer between radiators and an additional increase or decrease in the radiation power were revealed. However, an important aspect related to the meaningful control in the near zone of a system of radiators by individual components of the real part of the interference component of the complex Poynting vector has not been studied. This topic is discussed in this work.The simplest model of a system of identical radiators is a pair of collinear electric dipoles with moments and (Fig. 1a).In a system of spherical coordinates connected in a conventional manner with Cartesian coordinates , components of their fields can be represented [7] in the form of the following expressions (under the assumption that the time dependence has the form ):(1)where is the imaginary unit, is the wavenumber, is the wave resistance of the ambient medium, are spherical Bessel and Hankel funcint S 1 p 2 p , , r θ ϕ , , x y z exp( ) i t ω ( ) ( ) ( ) ( ) ( ) ( ) ( ) [ ] ( ) ( ) ( ) (1) 1 0

It is shown theoretically and experimentally that in the field of an electromagnetic radiator located in an absorbing medium, there exists a virtual surface that encompasses the near-field zone and is referred to as causal in the present study at which the field undergoes a second order phase transition. This transition is characterized by a rapidly changing phase and conversion of the energy of the quasistatic/reactive field into the energy of the field in the state of radiation. Behind the causal surface, the law of phase change sufficiently quickly acquires a linear character, and the field strength decreases with an increase in the distance following either an inverse-square law or an exponential function depending on the absorption coefficient of the medium, which is manifested through the formation of an intensively absorbing layer. Within the near zone, exponential attenuation is not observed. The size of the near zone depends on the frequency and the refractive index of the medium. Based on the studies performed, a new approach to the problem of sensing of absorbing media is suggested.

The article presents the design of the near-field probe, which is a combined emitter (a combination of a symmetric dipole and an annular frame). The design of the probe allows forming a prolonged zone of the near-field. This effect can be used for in-depth penetration of the field in media with high absorption, without loss of information. Particular attention in this article is given to a detailed study of the interaction of the field created by this probe on plane-layered biological media. A theoretical analysis of the interaction of the electromagnetic field was carried out in a wide frequency band with a model plane-layer biological medium containing blood vessels of shallow depth using the proposed probe design. Conclusions are drawn about the depth of penetration of a useful signal into different media-analogs of biological tissue. This study is necessary to consider the possibility of using this probe for non-invasive measurements of blood glucose concentration. The studies were carried out using numerical simulation in the CST (Computer Simulation Technology) Microwave Studio environment. All biological tissues were simulated over a wide frequency range from 10 MHz to 10 GHz.