Highly Isolated Multiple-Input Multiple-Output Antenna System for Ingestible Implants

In this paper, a dual-antenna multiple-input multiple-output (MIMO) system is designed for deep-tissue indigestible capsules. The system operates at 915 MHz and 2450 MHz frequency bands, covering the desired ISM band. The 10-dB bandwidths are 210 MHz (840–1050 MHz) and 220 MHz (2350–2570 MHz). The antenna’s overall volume is 12.34 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>, with a radius of 5.5 mm and a height of 0.13 mm. Compactness is achieved through the use of a high permittivity substrate, shorting pins, and multiple slots in the patch. A common ground plane with a rectangular slot in the middle is incorporated to reduce electromagnetic (EM) coupling between the antenna elements. Additionally, two inductors are placed between the antenna patches to further minimize the EM coupling. Inductor-I (8.5 nH) combined with a slot capacitance generates a transmission zero (TZ) at the lower frequency band, while Inductor-II (1 nH) combined with a slot capacitance generates a TZ at the higher frequency band. This design results in very low coupling values of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>34.6 dB at 915 MHz and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>38.92 dB at 2450 MHz. The antenna achieves peak realized gains of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>31.5 dBi at 915 MHz and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>22.3 dBi at 2450 MHz. With 1 W incident power, SAR values of 43.1 W/kg at 915 MHz and 46.3 W/kg at 2450 MHz are observed. The envelope correlation coefficient is less than 0.1 in both bands, making this antenna suitable for high-speed communication in ingestible implants.