An Ultracompact Triple-Band Circularly Polarized MIMO Model for Wireless Capsule Endoscopy and Midfield Power Transfer

This article presents a miniaturized, symmetric, triple-band circularly polarized (CP) implantable MIMO antenna for wireless capsule endoscopy (WCE) systems. The antenna operates within the Medical Implant Communication Service (MICS) band (402–406 MHz), the Industrial, Scientific, and Medical (ISM) bands (433.05–434.79 MHz and 902–928 MHz), and the midfield wireless power transfer (WPT) band (1315–1371 MHz). The two-port MIMO structure rests in a compact volume of 9.6 × 5 × 0.254 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>, realized using spiral meandered-line resonators without vias. Electromagnetic performance was optimized using a realistic gastrointestinal (GI) tract phantom in SIM4Life. The antenna is presented with impedance bandwidths of 24.7%, 11.57%, and 10.51% at center frequencies of 0.433 GHz, 0.915 GHz, and 1.345 GHz, respectively. Respective peak gains are −47.21 dBi, –38.13 dBi, and –33.28 dBi, indicating tissue absorption effect with the lowering frequencies. The antenna is circularly polarized in the right-hand direction in all bands, which facilitates reliable in-body–to–on-body communication. Furthermore, the structure meets IEEE-specific absorption rate (SAR) requirements, which ensures safe operation for power transmission and biotelemetry systems.