A Cavity-Backed Annular Slot Antenna With High Efficiency for Smartwatches With Metallic Housing

A cavity-backed annular slot antenna designed for the use in smartwatches in the 2.4-GHz Wi-Fi band is proposed. The cavity is metallic and has a cylindrical shape with a volume of pi x 21(2) x 10 mm(3). An annular slot is cut along the edge of the top surface (as the smartwatch screen) of the cavity. The circumference of the annular slot is about 1 lambda, making the design small enough for the smartwatch applications. The resonant modes, current distribution, and transverse mode of the cavity-backed annular slot antenna are studied using simulations and the results are used to design a smartwatch antenna. The proposed smartwatch is studied in free space, on a computer hand model, and on a phantom hand using simulations and measurements. Measured results show that the antenna has an efficiency of 57%-66% on a phantom hand. To study the antenna in a more realistic environment, the electronic components inside the smartwatch are modeled as a metallic block. For health risk concerns related to exposure to an electromagnetic radiation, the specific absorption rate for the smartwatch in the wrist-worn and next-to-mouth conditions is also studied using simulations.