A Bandwidth-Enhanced Sub-GHz Wristwatch Antenna Using an Optimized Feed Structure

In this letter, the design and modeling of a bandwidth-enhanced 915 MHz wristwatch antenna is presented. This letter extends previous work by the authors, with a key focus on significantly improving the impedance bandwidth of the antenna. The antenna proposed here achieves a measured 107% bandwidth improvement compared to the previously reported work. This is achieved by optimizing the feed structure to excite two closely located resonant modes that can be independently tuned to maximize the impedance bandwidth. Conceptual equivalent circuit models for the previous and proposed antennas are derived to explain the bandwidth enhancement. In addition, an equivalent transmission line model is, then, presented in order to accurately explain and analyze the impedance characteristics of the antenna. On-body measurements of the prototype antenna show a -10 dB impedance bandwidth of 62 MHz, a peak realized gain of -4.6 dBi, and a simulated radiation efficiency of 17.8% at 915 MHz. This antenna exhibits a low specific absorption rate value of 0.02 W/kg with 11 dBm input power. The previous performance makes the antenna a suitable candidate for 915 MHz wristwatch-based wireless applications.