Quarter-Wave Plates to Improve Rotational Misalignment Robustness in Medical Telemetry.

Abstract

A major challenge in developing robust wireless links to implanted/ingestible antennas is the potential for rotational misalignment. In this paper, we present an artificially anisotropic quarter-wave plate (QWP) capable of developing a circularly polarized wave from a linearly polarized wave. Without loss of generality, our QWP is composed of plastic and hydrogel, while the linearly polarized wave is developed by a bio-matched antenna-a high gain, broadband antenna with a dielectric engineered to match to biological tissues. Using a basic implanted patch antenna, we demonstrate a 1.00\u2009dB (1.26) variance in transmission coefficient over a 90° variance, with a remarkable average measured transmission coefficient of -34.4\u2009dB (3.63\u2009×\u200910-4 ) at 2.4\u2009GHz. Without the QWP, the rotational variance is 12.52\u2009dB (17.9). Notably, the QWP increases the maximum input power to comply with specific absorption rate limitations. In our case, this allows for -15.0\u2009dBm (31.6\u2009µW) of power to be received by the implant, which is comparable to the -15.7\u2009dBm (26.9\u2009µW) received without the QWP. Additionally, we demonstrate that with the QWP, the standard deviation from the mean transmission for rotational misalignments remains below 3\u2009dB (2.00) from 2 to 3.62\u2009GHz, resulting in a simulated 57.7% fractional bandwidth. © 2021 Bioelectromagnetics Society.