An Approximate Multiconductor Transmission Line Model for an Antenna Loaded by High-Impedance Unit Cells

We present a transmission line model for a microstrip line or antenna above a row of high-impedance unit cells, such as those used in a high-impedance surface (HIS) ground plane. The model is built on a multiconductor transmission line circuit description of the joint microstrip and HIS cell structure as stacked microstrip lines. Discontinuities in the HIS cell layer are modeled as circuit elements using well-known closed-form expressions. S-parameters predicted by the transmission line model agree with full-wave simulations from low frequencies to above the parallel resonance of the HIS unit cells. The model also provides an approximate forward prediction of the impedance behavior of antennas above HIS ground planes, which cannot be obtained using the reflection phase method.