A miniaturized quad-band circular polarized implantable antenna for intracranial pressure monitoring devices

This letter presents a compact quad-band circularly polarized implantable antenna, which adopts a “umbrella” structure together with the technique of loading meander slots and shorting pin. The size is reduced to 0.0263λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.0263λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.0027λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the free-space wavelength corresponding to the lowest operating frequency). The antenna operates in the 860 MHz and 1400 MHz bands for telemetry, the 1900 MHz mid-field band for wireless power transmission, and the 2450 MHz band for control signal. The effective axial bandwidths of the antenna at the frequencies of 860 MHz, 1400 MHz, 1900 MHz, and 2450 MHz are 11.3%, 5.1%, 5.3%, and 6.5%, and the corresponding −10 dB impedance bandwidths are 97 MHz, 78 MHz, 117 MHz, and 160 MHz, respectively. The peak gains are −32.81 dBi, −26.23 dBi, −26.15 dBi, and −25.47 dBi, respectively. The fabricated prototype underwent testing, and its experimental results matched the simulations. The combined link budget and specific absorption rate (SAR) analysis results show that the antenna has the potential for intracranial pressure monitoring applications. To the authors’ knowledge, this is the first quad-band circularly polarized implantable antenna.