Characteristic impedance matrix of a suspended microstrip line

This paper studies the characteristic impedance matrix of a single suspended microstrip. This peculiar network modeling quantity, completely different from the conventional knowledge about the characteristic impedance matrix of a system of parallel transmission lines supporting quasi-TEM modes, is the result of the coupling of the bound mode and the space-wave-type leaky mode propagating simultaneously on the same guiding structure. Theoretical data show that the two modes are not orthogonal and they couple significantly. The formulation and the codes are validated by checking against the known data. The power-current definition for defining the impedance is adopted. This characteristic impedance matrix is found to be complex and satisfies the necessary and sufficient condition for passivity.