Comparative Study of Dual-Polarized and Circularly-Polarized Antennas at 2.45 GHz for Ingestible Capsules

Circularly-polarized (CP) antennas are frequently proposed for ingestible capsules. However, to-date, the way the CP characteristics of these antennas affect the communication link (CL) between the internal antenna (capsule antenna) and the external antenna has not been investigated in-depth. In this paper, we first show that when CP waves illuminate a human body, they effectively become depolarized as they penetrate the body. Next, we show that the performance of the CP antenna varies significantly with the location and orientation when located in-body. Following this, we carry out a series of simulations to demonstrate the fact that-contrary to the widely-held belief-a CP antenna does not provide, in ingestible applications, a more robust CL with the external antenna in comparison to either a dual-polarized or a linearly-polarized (LP) antenna. These simulations are carried out at 2.45 GHz and the results are confirmed by experiments. Given this background, we propose a simple and effective communication scheme, which is based on the use of an omnidirectional LP internal antenna and a CP external antenna, and we demonstrate that this combination provides a CL that is more robust than those realized by using the CP antennas alone.