This letter proposes an orthogonal decoupling method for co-frequency MIMO antennas with unbroken metal rim, operating in Wi-Fi 2.4GHz. The decoupling technique just requires two lumped elements between two ports. Two orthogonal surface currents and radiation pattern diversity are obtained. By adding a capacitive and an inductor, with insensitive of matching circuits, it is easy and significant to improve isolation with near 50dB for smart watch antennas. Moreover, forearm causes lower isolated frequency, and has a little effect on impedance matching with the radiation pattern orthogonal to the forearm.

Introduction: Smart watches become an important component of the Internet of Things (IoT). Many wireless techniques are applied to user scenarios, such as LTE cellular communications, Wi-Fi, near field communication (NFC), or ultrawide band (UWB). At present, 2.4-GHz wireless local-area network (WLAN), Bluetooth and GPS are the most common wireless communication protocols adopted in commercially available smartwatch products [1]. However, there are many challenges in designing multiple and multiband antennas on smart watches. Firstly, severe mutual coupling of antennas with unbroken metal rim lowers total efficiency and deteriorates channel capacity. For example, Wi-Fi 2.4GHz and LTE B40(2300MHz-2400MHz) have strong coupling, often operating in different antennas because of more than 2dB insertion loss of extractors. Forearm also has important effects on isolations and efficiency degradation. A limited space and industry design have no chance to use bulk decoupling methods, such as neutralization line or special ground structure [2,3,4]. Secondly, curved full display and other components decrease the clearance, or no ground region, to reduce system efficiency. Thirdly, mixed higher order modes inevitably introduce surface current cancellation and more nulls in the radiation patterns [5].

This article discusses a simple and effective decoupling method for two Wi-Fi 2.4GHz MIMO antennas. The technique introduces two orthogonal modes with 90-degree rotation of surface current and radiation pattern by adding two lumped components. The loadings are insensitive to antenna matching network and their values are determined by isolated frequency and their placements. Also, forearm effect on the isolation is demonstrated with GaN back cover.

Decoupling Formula: Network of four ports can be expressed by its impendence matrix. In Fig.1, Port1 and Port2 mean the feed ports of Antenna1 and Antenna2, respectively. Port3 and Port4 are respectively the decoupling loads, \(Z_{L1}\)and \(Z_{L2}\).Substituting\(Z_{L1}I_{3}=V_{3},Z_{L2}I_{4}=V_{4}\) to the impendence matrix, a quadratic equation of unknown \(Z_{L1}\)and \(Z_{L2}\) results in two possible solutions [6]: