In this study, a tri-band wearable antenna with a metal frame of 36×36×6.6 mm3 is designed, fabricated, and measured based on the characteristic mode theory. By analyzing the current and electric field distribution of the characteristic mode, the antenna is determined to be fed by a T-coupled structure. Moreover, a circular ring ground structure is added to the initial elliptical model structure to generate a new resonance in the n78 band. On the other hand, the current's path is changed by etching a rectangular slot, allowing the high-frequency resonance mode to be shifted to the right. Simulated and measured results show that the proposed antenna covers Bluetooth/Wi-Fi (2.4G, 5.8G) and N78 frequency bands, which can be respectively used for connecting a watch to a mobile phone, accessing the Internet and making phone calls. Furthermore, the antenna has a maximum peak gain of 4.11 dBi in free space and 6.9 dBi when being placed on the wrist, with a Specific Absorption Rate (SAR) lower than international standards, making it suitable for wearable devices.
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