Near-field microwave probing of a spherical object

We have developed a theory for near-field (NF) sounding of a sphere located in a homogeneous medium. We have considered the NF probe as an antenna with small aperture size D≪λ, where λ is the wavelength. The developed algorithm provides calculation of antenna impedance as a function of sphere diameter d_s and antenna-to-object separation h. Our theory is based on the obtained solution of the diffraction problem for a sphere irradiated by the antenna near field. We have investigated quantitatively the excitation of electrical and magnetic multipoles in the sphere. Governing parameters of the electrodynamic system are d_s/D and h/D. We have found the number of multipoles that should be taken into account for system characterization with given accuracy. The exact condition has been obtained for the applicability of the small-particle (Rayleigh) approximation. The excitation of multipole resonances has been revealed. We have used the developed theory to study the detectability of tumors in a human body. It has been shown that tumors as small as 1-1.5 cm located at a depth up to 1.3 cm can be detected.