Design and Experiments of Bandwidth-Controllable Broadband Monopole Antennas With Conformal Anisotropic Impedance Surface Coatings

In this paper, we report an ultrathin and lightweight coating approach for enhancing the impedance bandwidth of a conventional monopole antenna by using a single-layer dispersive anisotropic impedance surface (AIS). The design is facilitated by a rigorous and efficient modal-expansion analysis of a monopole coated by a homogeneous AIS, which is developed by approximating the AIS as a slab of anisotropic medium with a deep subwavelength thickness and introducing an auxiliary perfectly matched boundary above the AIS-coated monopole. This method is further utilized to efficiently optimize the dispersive properties of the AIS, which has a deep subwavelength radius, for tuning the impedance bandwidth of a monopole, thereby achieving a greatly broadened yet controllable bandwidth. Three AIS-coated antenna examples with different ratio bandwidths are illustrated, with their corresponding discrete realistic structures designed, fabricated, and characterized. The measured results agree strongly with simulation predictions, experimentally exhibiting a VSWR < 2 ratio bandwidth of 1.91: 1, 2.79: 1, and 3.81: 1, respectively. The proposed antennas possess a stable omnidirectional pattern with a low cross-polarization level of less than -20 dB. The demonstrated AISs can serve as redeployable conformal coatings for obtaining broadband and ultrawideband antennas with a controllable bandwidth, which will be useful for various wireless systems, including the vehicle-to-X network.