Diffraction of the quasi-static antenna field by a sphere in near-field microwave probing

The theory of near-field (NF) location of a sphere of an arbitrary diameter dsph is developed. This theory is used to calculate the impedance of an antenna of small wave diameter D in the case when a spherical contrast object is located at distance h in the antenna near-field zone. It is shown that excitation of multipole modes in the sphere perturbs the impedance, which determines the response of a locator to the studied object. The contributions from radiations of electric and magnetic multipoles of different orders to the impedance is analyzed as a function of parameters dsph/D and h/D. It is found that, for fixed parameters of the antenna-sphere system, number n* of multipoles that should be taken into account is finite and functions n*(dsph/D) and n*(h/D) are determined. The limits of applicability of the electric-dipole (Rayleigh) approximation in calculation of near-field diffraction are determined. The conditions of resonant excitation of multipole modes, which is accompanied by a sharp increase in the real part of the antenna impedance, are found. The developed theory is used to analyze the prospects for application of the NF locator for detection of a cancerous tumor inside the human body. It is shown that, in the case of optimal selection of the device parameters, excitation of higher-order multipoles increases the detection range, which is ∼1.3 cm for a tumor size of 1–1.5 cm.