The new near-field approach for microwave tomography of absorbing media

Abstract

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.