915 MHz Miniaturized Loop Conformal Antenna for Capsule Endoscope

High-performance miniaturized antennas are one of the goals that have been pursued in the design of wireless communication systems for implantable medical devices. In this article, a 915 MHz novel-shape loop antenna conforming to the inner wall of the capsule is developed by comprehensively adopting meander and symmetrical structure patch, folding technique, and vector current reversal technique. Through electromagnetic simulation, testing, and experimentation, the antenna shows good characteristics such as small size, ultra-wideband, circular polarization, strong robustness, and safety. Specific performances include the following: 1) the size of the developed miniaturized loop conformal antenna is only 30.5 mm $\times $ 4.6 mm $\times $ 0.05 mm and the maximum gain is -21.5 dBi at 915 MHz, which solves the problem of low gain due to the small antenna radiation area; 2) the antenna has a 4.5 GHz ultra-wideband impedance matching bandwidth ( $\vert S_{11}\vert < -10$ dB) from 0.5 to 5 GHz. Even if the parameters such as the implantation depth and the thickness of the capsule shell change, or there exists a wireless power transfer system, the antenna can still cover the required frequency band with strong robustness; and 3) the specific absorption rate (SAR) of human tissues radiated by the antenna is far lower than the safety limit in IEEE standards and ICNIRP guidelines. The experimental platform using pork to simulate human tissues was built for antenna performance testing and communication testing. The measurement results verified the good resonance performance, radiation characteristics, and omnidirectionality of the developed antenna. The antenna link loss model is also given, which has been used in wireless communication and localization.