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Efficient NFC Coil Antennas for Fully Enclosed Metallic-Framed Wearable Devices

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IET Microwaves, Antennas & Propagation
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Jaehyun Choi
Jaehyun Choi
Jaehyun Choi
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Sangho Lim
Sangho Lim
Sangho Lim
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Wonbin Hong
Wonbin Hong
Wonbin Hong
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Abstract and Figures

This study reports methods of alleviating the effect on inductive near‐field communication (NFC) coils caused by fully enclosed metallic‐frames for wearable devices such as smartwatches. Two techniques that, respectively, involve creating two short‐circuit passages to modify the distribution of the out‐of‐phase current on the metallic‐frame and reducing the out‐of‐phase current amplitude by devising an asymmetric coil topology is presented and studied in detail. Extensive simulation, prototyping and measurement are carried out for 13.56 MHz NFC applications. The presented techniques can, respectively, improve the induced magnetic coupling by 43 and 30% under identical conditions in the presence of the metallic frame. Furthermore, both methods comply with the NFC forum certification and do not require the need to increase the drive current and power consumption of the NFC circuitry.
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IET Microwaves, Antennas & Propagation
Research Article
Efficient NFC coil antennas for fully enclosed
metallic-framed wearable devices
ISSN 1751-8725
Received on 22nd May 2019
Revised 30th September 2019
Accepted on 29th November 2019
E-First on 18th December 2019
doi: 10.1049/iet-map.2019.0451
www.ietdl.org
Jaehyun Choi1, Sangho Lim2, Wonbin Hong3
1School of Mechanical Engineering, Pohang University of Science and Technology, Pohang, Republic of Korea
2Institute of Agency for Defense Development, Daejeon, Republic of Korea
3School of Electrical Engineering, Pohang University of Science and Technology, Pohang, Republic of Korea
E-mail: whong@postech.ac.kr
Abstract: This study reports methods of alleviating the effect on inductive near-field communication (NFC) coils caused by fully
enclosed metallic-frames for wearable devices such as smartwatches. Two techniques that, respectively, involve creating two
short-circuit passages to modify the distribution of the out-of-phase current on the metallic-frame and reducing the out-of-phase
current amplitude by devising an asymmetric coil topology is presented and studied in detail. Extensive simulation, prototyping
and measurement are carried out for 13.56 MHz NFC applications. The presented techniques can, respectively, improve the
induced magnetic coupling by 43 and 30% under identical conditions in the presence of the metallic frame. Furthermore, both
methods comply with the NFC forum certification and do not require the need to increase the drive current and power
consumption of the NFC circuitry.
1 Introduction
Near-field communication (NFC) operating at 13.56 MHz is
instrumental in uniting various proprietary standards and
technologies used for ranging from contactless payment, security,
identification and so on. Due to its relatively simple circuitry and
low power consumption, NFC has been the predominant
methodology amid the recent explosive proliferation of mobile
payment services for smartphones and wearable devices.
Furthermore, with the advent of the Internet of Things (IoT) NFC
can enable the connection settings between adjacent devices with
relative ease. Among consumer electronics devices, wearable
devices such as smartwatches are one of the most evident
benefactors of NFC technology. Users wearing NFC-equipped
smartwatches can conveniently exploit cashless/cardless
merchandise and service payments, facility and computer access
control and event ticketing [1–3] omitting wallets and smartphones.
However, the aesthetic appeal of wireless smartwatches remains
paramount and this is reinforced by the recent surge in adaptation
of metallic frames and chassis to better resemble the look and feel
of traditional analogue watches.
Conventional NFC antennas within wearable devices are
realised by devising a planar and occasionally multilayer two-
terminal loop with inductive and resistive properties optimised to
the impedance of the NFC transceiver IC [4]. In [5], inkjet-printing
technology is applied with two parallel coils to improve the
coupling between transmitter and receiver. In general, ferrite sheets
exhibiting high permeability properties are widely implemented in
conjunction with NFC antennas to mitigate undesired eddy currents
[6] caused by metallic components situated above or under the loop
antenna. In [7], a modified NFC loop antenna with a parasitic loop
is integrated with a ferrite-polymer composite to enhance the
overall physical flexibility and cost effectiveness. However, this
approach becomes ineffective in situations where the loop structure
is fully enclosed by a metallic frame along the horizontal plane as
it is the case for majority of smartwatches. In [8–11], NFC
antennas for metal-cover smartphone are studied. While
implementing a small aperture on the metallic frame would enable
induced magnetic coupling, such approaches are prohibitive for
smartwatches with water-proof requirements. In [12], an invisible
antenna for smartwatches is devised for the first time.
This paper reports unprecedented methodology of devising
highly efficient NFC antennas, which are horizontally surrounded
by a metallic frame in close proximity. The topologies are
demonstrated on a commercially available smartwatch device and
its effectiveness are measured and verified based on the NFC
Forum technical specification and certification procedure. The
presented approaches can be extended to various metallic wearable
and IoT devices.
2 Proposed methods and topologies
The challenge associated with NFC coil antennas enclosed in
metallic surroundings is illustrated in Fig. 1a. Let us consider a
wearable device such as a smartwatch that consists of a cover glass
window, touch sensor panel, and display panel such as a liquid
crystal display or organic light emitting diode display along the
vertical axis as illustrated in Fig. 1a. Typically, the NFC coil
structure is placed beneath the display panel encompassed by a thin
ferrite sheet to alleviate the conductive characteristic of the display
panel. Configuring the diameter of the NFC coil D = 38 mm, the
magnetic fields of the conventional coil structure are computed at
(x, y, z) = (0, 0, 25) (unit: mm) using finite-element based
numerical simulator for three discrete scenarios. At a vertical
position 25 mm above the cover glass, the horizontal magnetic
field distributions are plotted, respectively, for each case as
presented in Fig. 1b. It can be observed that the induced magnetic
fields of the NFC coil are approximately halved as we sequentially
add the display panel and the metallic frame structure. When the
metallic frame is added, 180° out-of-phase electric current is
induced on its surface which negates the electric current
distribution situated on the outer boundaries of the NFC coil.
Smartwatch manufacturers typically compensate the degradations
associated with the display panel by increasing the drive current.
However, trade-offs such as reduced battery expectancy and
overheating limit its applicability for scenarios involving metallic
frame designs. Two novel loop topologies which drastically reduce
the effect of the fully-enclosed metallic frame are proposed.
2.1 Short-circuited NFC coil structure
As illustrated in Fig. 2, the dominant loss factor of conventional
NFC coil designs is attributed to the out-of-phase current
IET Microw. Antennas Propag., 2020, Vol. 14 Iss. 3, pp. 211-214
© The Institution of Engineering and Technology 2019
211
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