Author:

Abstract:

A set of circularly-polarized textile antennas is featured in a recently reported Smart Wearable Textile Array System (SWTAS). This combined direction of arrival (DoA) estimation and beamforming system is developed for biomedical telemetry applications, which enables effective and continuous patient monitoring when combined with one or more health sensors. Information about the subject's health condition is to be received adaptively to guarantee link reliability. The telemetry operation is facilitated by a receiver front-end, a digital baseband beamforming (DBBF) network, and a pair of textile antennas. Their arbitrarily location on a cylindrically-shaped body phantom ensure a wide spatial DoA estimation capability. To suit on-body placement, the antennas are characterized in this work to enable an optimal system performance. Evaluations were performed using a commercial body-emulating liquid, and by placing the antennas at a realistic distance from the body, considering user clothing. Evaluation of the microstrip antenna made from felt substrate and ShieldIt conductive textile indicated satisfactory operation in the 2.45 GHz band, with up to 270 MHz of 10-dB impedance bandwidth and 100 MHz of 3-dB axial ratio bandwidth in free space. Due to the existence of the ground plane, its good performance was maintained when placed on body. Antenna evaluation operating in both planar and curved configurations when placed at ±70o on the cylindrical phantom's forward plane also indicated minimal degradation in terms of reflection, radiation and axial ratio. Next, a combined assessment of the antenna was performed by evaluating their system spatial amplitude. This is done by rotating the system from -170o < θ < +170o while measuring the complex received signal at each antenna port, with its adjacent port terminated using a matched load. Measurement repeatability is clearly obtained for both amplitude and phase evaluations. System evaluation also indicated a good performance in the setup ± 80o forward plane with a DoA accuracy of 3o. Finally, the antennas' specific absorption rate (SAR) was determined both through simulations using a Hugo body model in CST and measurements in a certified facility available at IMST GmbH. Simulated and measured SAR yielded values of 0.018 W/kg and 0.013 W/kg, respectively, well below the 2 W/kg European safety limit.