Resonant singularities in microstrip lines caused by periodical bonding of lateral ground strips

The problem of periodic bonding of lateral ground strips in microstrip line configurations is examined under the assumption that the lossy conducting strips are equally characterized by a given surface impedance. The resulting propagation parameters are evaluated and compared with those obtained for uniform bonding. A symmetrical three-conductor microstrip line is analyzed, and the wave characteristics of the central strip propagation mode are shown to exhibit resonant singularities whenever the bonding period approaches a multiple of one half wavelength. A method based on the evaluation of the immittance matrix of a six-port network describing the elemental cell of the periodic structure is used to derive the wave parameters of the external central strip mode