Part II: A Conductor-Backed Slot Antenna Element Surrounded by a Shorting-Post Cavity to Suppress Parallel-Plate Mode Excitation—Design Analysis and Experiment

We investigate the basic physical characteristics of a conductor-backed slot element, surrounded by conducting shorting posts, designed to suppress the undesired parallel-plate mode. The shorting posts are assumed to be thin and are placed periodically along a circular boundary in the form of a “shorting-post cavity.” For analytical simplicity, thin cylindrical conducting posts are modeled as conducting strips of suitable equivalent width. The scattering matrix of the structure is modeled using cylindrical wave functions, by enforcing periodic boundary conditions on the conducting strips. The scattering matrix is then used, together with the general analysis presented in Part-I of the paper, to model the impedance and radiation characteristics of the total structure. Basic design trends of the shorting-post structure are investigated. Design data are presented, relating desired levels of parallel-plate mode suppression with the physical size, the number and the placement of the shorting posts. Reactive loadings associated with the designs are examined. Selected results are validated by measurements and independent computations.