A Low-Profile Wearable Antenna Using a Miniature High Impedance Surface for Smartwatch Applications

A novel antenna structure enabled by a miniature high impedance surface (HIS) is proposed for smartwatch applications. The smartwatch antenna must be low-profile, highly directive, low specified absorption rate (SAR), and robust to the loading effect due to a human body. HISs are particularly suitable to cope with these design goals. However, an HIS is usually too electrically large to fit into the design space of smartwatch applications. Furthermore, the characterization of HISs is determined by observing the reflection phase of a unit cell, but this method becomes improper for finite-size and miniaturized HISs. In this letter, a new design method is presented. By using fractional factorial designs (FFD), the antenna performances are significantly enhanced even though the size of the HIS is only 0.3 lambda(0) x 0.3 lambda(0) Accordingly, the dimensions of the proposed antenna are 38 x 38 x 3 mm(3) (at 2.4 GHz), which is the most compact HIS structure to date. The directivity of the antenna is 6.3 dBi, and the maximum 1 g averaged SAR value is only 0.29 W/kg for an input power of 100 mW; moreover, the radiation efficiency and impedance matching are very robust against the loading effect due to wrist tissue.