A Wideband Circularly Polarized Antenna With Metasurface Plane for Biomedical Telemetry

This letter presents a metasurface (MS)-based wideband wearable circularly polarized (CP) antenna to communicate with in-body devices. The antenna consists of a crossed dipole, an MS plane and a CP feeding network. The feed network generates a quasi-90° phase delay between each adjacent arm of the crossed dipole. This produces CP radiation and reduces polarization mismatch between the telemetry antenna and the implantable antenna. The MS, consisting of a 5 × 5 unit cell array, functions as a reflector for the crossed dipole to improve forward radiation while maintaining a low-profile structure. This is by designing the unit cells with a near-zero phase reflection over the operating band. To enable conformity with on-body usage, the antenna is designed fully using flexible materials. Textile conductive elements are embedded into a PDMS substrate. It is sized at 106 × 106 × 5 mm³ (1.24λ × 1.24λ × 0.05λ at 3.5 GHz). Results indicated a −10 dB impedance bandwidth from from 2.25 to 6 GHz (90%). The 3 dB axial ratio bandwidth is from 2.9 to 4.95 GHz (52%), with a maximum on-body gain of −10.5 dBi. Finally, the specific absorption rate (SAR) assessment of the antenna indicated values far lower than the limits of the FCC.

I. Introduction

Biomedical telemetry between implanted and wearable devices has been an important research topic within the context of wireless body area networks (WBANs). Sensing, diagnostics, and communications between them require electronic modules and antennas to receive/transmit signals from/to implantable antennas in the human body from an external on-body antenna [1], [2]. While most antennas introduced to date for this purpose have focused on linearly polarized (LP) antennas, communication with implanted sensors can be more complex due to their potentially dynamic location and orientation when they are operating in the body. To overcome this, circular polarization (CP) is effective in minimizing polarization loss, fading and multipath [3], [4].