Antenna Close to Tissue: Avoiding Radiation Pattern Minima With an Anisotropic Substrate

Wireless body area networks have been a focus of telecommunication research in recent years. An efficient wearable antenna design requires an understanding of wave propagation in the proximity of the human body. Because of the large bandwidth in an ultrawideband (UWB) system from 3.1 up to 10.6 GHz, there are always harmful significant minima (i.e., nulls) in antenna patterns for some frequencies. To address this challenge, the purpose of this letter is to show an approach for designing an anisotropic substrate for a wearable UWB antenna. The proposed substrate improves the gain performance and pattern shape of the broadband antenna by preventing the appearance of harmful pattern minima. The anisotropic substrate is formed by assembling sheets with relative permittivities ε_e ≈ 1 and ε_i ≈ 9 (subscripts: for environment and inclusion materials) as a stack in order to effectively enable different permittivities ε_x´ ≈ 5.0 and ε_y´ ≈ 1.8 for wave components propagating in between an antenna and a body. This is concluded to modify the sum polarization so that smoother patterns over the bandwidth are achieved.