A Broadband Monopole Antenna Enabled by an Ultrathin Anisotropic Metamaterial Coating

A new type of compact flexible anisotropic metamaterial (MM) coating is proposed, which greatly enhances the impedance bandwidth of a quarter-wave monopole to over an octave. The MM coating has a high effective permittivity for the tensor component oriented along the direction of the monopole. By properly choosing the radius and tensor parameter of the MM coating, another resonance at a higher frequency can be efficiently excited without affecting the fundamental mode of the monopole. Additionally, the similar current distributions on the monopole at both resonances make stable radiation patterns possible over the entire band. To experimentally verify the concept, an S-band MM coated monopole was designed, fabricated, and characterized, exhibiting a 2.14:1 bandwidth (2.15-4.6 GHz) with a VSWR of less than 2:1. The demonstrated MM coating has a radius of only λ/24 and extremely light weight, which renders it attractive for use in applications such as broadband arrays and portable wireless devices.