Increments of Admittance Parameters in Microwave Imaging

We present a simple derivation of the increments of the admittance parameters of an antenna array caused by scattering from a target in an inhomogeneous reciprocal medium. The basis of the derivation is the reciprocity theorem, adapted for the excitation by ideal voltage generators. In this way, we extend previous results on this topic, which comprised formulas for the increments of impedance and scattering parameters in a nonmagnetic medium. The additional value of this derivation is a relationship between the increments of the different network parameters. The derivations are general, but they reach simple forms in the case of weak coupling. The validity of the obtained expressions is confirmed through a formal mathematical derivation.