Abstract
Designing multi-functional antennas is challenging for smartwatches enclosed by metallic frame. In this letter, we propose an efficient and practical method to realize circular polarization (CP) and multiband operation for smartwatch antennas. The method is based on the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> -port network, with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> ports initially placed along the ground clearance. By varying the impedance matrix using reactance, the total currents of the antenna and hence the far-field properties are optimized. The optimization is performed using multi-objective optimizer NSGA-II. As a result, three reactive loads, with the optimized locations and values, are applied to the smartwatch with metallic frame, providing an axial ratio (AR) of 2.2 dB at the GPS L1 band and wide bandwidths at both the GPS L1 and WLAN/BT bands in the arm-worn scenario. Antenna prototype was fabricated, with the measured results agreeing well with the simulated ones. The simple and practical implementation is quite attractive for commercial smartwatch antennas.
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