Miniaturized Circularly Polarized Loop Antenna for Biomedical Applications

A novel circularly polarized antenna is proposed at 902-928 MHz Industrial, Scientific, and Medical band for implantable applications. By properly positioning the feed and shorts, either right-hand circular polarization property or left-hand circular polarization property can be realized. Slow wave concept is utilized by loading patches to the radiated loop antenna to achieve miniaturization. Thus, a compact size of 13 mm × 13 mm × 1.27 mm is obtained. Compared to the unloaded loop antenna of the same size, the centre frequency shifts from 1.93 GHz to 882.5 MHz, which suggests a miniaturization of 54.4%. The simulated results show that a wide bandwidth of 18.2% can be realized with |S₁₁| below -10 dB and axial ratio below 3 dB. The simulated realized gain is -32 dBi at 915 MHz. The measurement is carried out in both skin-mimicking gel and pork, and a bandwidth of 27.8% and 29.4% can be achieved with |S₁₁| below -10 dB, respectively. The measurement of |S₂₁| reveals that circular polarization can be obtained for the proposed configuration.