An Orthogonal Decoupling Method of Co-frequency Antennas for Smart Watch
Applications
Rui Lv,1 ,✉Peiyu
Liu2
1 Shenzhen Institute of Information Technology, Shenzhen, China
2 Shenzhen Polytechnic, Shenzhen, China
✉Email: lvrui@sziit.edu.cn
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]: