Considerations for the Design and Placement of Implantable Annular Slot Antennas for Intracranial Pressure Monitoring Devices

Deviation in the performance of measured implanted antennas [such as S11 measured by a vector network analyzer (VNA)], compared to that of antennas encapsulated inside an implant and fully embedded in tissue, is not fully understood. This letter presents simulation and measurement results of an annular slot antenna (ASA) for its use in implantable intracranial pressure (ICP) devices operating at 2.45 GHz. ASA is chosen for its higher radiation efficiency compared to strip antennas such as printed inverted-F antennas (PIFAs) used previously by the authors and some other groups. The performance of the antenna ( S11, gain, and radiation efficiency) with a coaxial feed passing through the device and the surrounding tissue is compared to its performance when the feed is applied from inside the device. It is demonstrated that the insulation coating of the exterior of the cable in the case of the VNA test, and proper grounding of the outer conductor of the cable in the case of the internal feed, are necessary to minimize the performance deviation. Moreover, measured gain and S21 demonstrate the superiority of the ASA over the PIFA, thereby implying higher radiation efficiency of the ASA, which is consistent with simulation results.