A Miniaturized Circularly Polarized Implantable Annular-Ring Antenna

A novel miniaturized circularly polarized (CP) annular-ring microstrip antenna with a smooth profile is presented for Industrial, Scientific, and Medical (ISM) band (2.4-2.48 GHz) implantable applications. The radiating element of the antenna consists of an annular ring and a central circular patch. The CP radiation is generated by adding a pair of open stubs in the inner boundary of the annular ring. The antenna profile is minimized by properly cutting two kinds of rectangular slots onto the annular ring. Furthermore, the Z-shaped slot on a central circular patch is implemented for improved axial ratio (AR) and better impedance matching. The volume of the proposed antenna including substrate and superstrate is 120.69 mm(3), which is smaller than the other CP implantable antennas with similar performance. Simulated and measured -10-dB impedance bandwidth are 8.3% and 11.4%, respectively. The simulated 3-dB AR bandwidth is 2.49%. In addition, the radiation and specific absorption rate performance of the antenna are also characterized. It is a capable candidate for implantable applications.