Broadband Unit-Cell Design for Highly Anisotropic Impedance Surfaces

A unit-cell design for highly anisotropic impedance surfaces is simulated and experimentally demonstrated. The unit cell consists of a grounded dielectric substrate with a metal patch and plated metal via. The design has a ring of metal removed from the patch so that the resonant effect of the via is reduced. The reduced resonance prevents backward-wave modes which are difficult to excite experimentally. On grounded dielectric substrates, the transverse-magnetic mode is fundamental and the transverse-electric (TE) mode is bound above a cutoff frequency. It is shown that highly anisotropic surface impedance can only be achieved below the TE cutoff frequency. The proposed unit cell is constructed using printed-circuit-board fabrication technology, and the result is a unit cell that obtains highly anisotropic impedance over a broader frequency range than traditional patch or mushroom unit cells.