Compact BLE Antenna with a Modified PIFA Configuration for

Wearable EMG Monitor

1
Sanghoek Kim
1
Kyung Hee University

October 30, 2023

Posted on 19 Apr 2020 — CC-BY 4.0 — https://doi.org/10.36227/techrxiv.22647865.v1 — e-Prints posted on TechRxiv are preliminary reports that are not peer reviewed. They should not b...

Abstract
While the BLE is very popular for indoor communication, the antenna for the BLE in wearable sensor systems is lack investi-
gation. This work proposes the BLE antenna for a wearable sensor, which is (i) small enough to fit in a compact device and
(ii) tolerant against the nearby medium change.

1
 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, XXXX 2017 1

Compact BLE Antenna with a Modified PIFA

Configuration for Wearable EMG Monitor
Ashwini Kumar Arya, Taein Kim, Hyunjong Kim, Jongkap Oh,Jeongmin Kim, Donghyeok Moon, and
Sanghoek Kim∗ Member IEEE

Abstract— An antenna mounted underneath the top cover of a

wearable device, fed by an ultra-low-profile connector, is proposed
for Bluetooth-Low-Energy (BLE) communication at 2.4 GHz. As the 90ŪU.FL plug to 90Ū
U.FL plug cable
antenna is to be used for a wearable device, it is essential that it Device top cover
should be compact in size and tolerant against the nearby medium
Antenna position
change. The modified planar inverted-F antenna (PIFA) that we Air gap

12 mm
propose consists of a main patch and a parasitic element to broaden PCB circuit area
the antenna bandwidth. The end of the parasitic element is shorted Battery area
to miniaturize the antenna to 25 by 10.8 millimeters, fitting well Device bottom cover
inside a watch-type wearable device. Also, the complete ground
layer of the antenna makes it radiate well in the outward direction, 50 mm
and minimally interact with the back-side medium. This results in
the reflection coefficient being insensitive to the medium change on the backside. The proposed antenna has a peak
gain of 3.62 dBi along with 20% efficiency and the impedance bandwidth of 80 MHz (2.4 ∼2.48 GHz). To examine
the communicative operation of the antenna in practice, the received signal strength indicator (RSSI) of the complete
prototype device with the antenna is measured in various postures and orientations, demonstrating reliable connectivity
within a typical indoor distance of 10 meters. Lastly, the antenna is embedded in a wearable device, demonstrating
electromyography’s wireless monitoring.
Index Terms— Antenna input impedance, reliability testing, wearable device, BLE communication

I. I NTRODUCTION foldable dipole antennas were reported in the literature for

Bluetooth operation [6], [8]. The complete sizes of the anten-

T HE Bluetooth Low Energy (BLE) is a wireless com-

munication scheme designed especially for short-range
communications. This scheme operates in the same frequency
nas including the ground plane are less than 3 cm in length.
In this topology, however, the antenna is assumed to operate
in an open space, rather than in the presence of tissue. A
spectrum, 2.4∼2.4835-GHz ISM band, as the classic Bluetooth
wearable magneto-electric dipole antenna for WBAN/WLAN
one, but uses a different set of channels for the communica-
application was also reported [9]. While the antenna has the
tion [1]. This work reports the antenna design for the BLE
complete ground plane on the bottom to suppress the radiation
communication to be embedded in a watch-type wearable
toward the back side, the size of the antenna is prohibitively
device as shown in Fig. 1(a). As the antenna for the wearable
large, 51.5×63 mm2 , to be adopted in a watch-type device.
device, it should be compact, low-profile in size and insensitive
Another antenna for on-body communication was reported
to the presence of a body nearby.
for the operation in the ISM band [10]. The antenna has a
Several antenna designs for the related fields have been re- complete ground plane to reduce the back radiation. However,
ported in the literature [2]–[11], but are not directly applicable the antenna is a multilayered structure and the size is 37×30
for the wearable device of our purpose. For example, printed mm2 , making the antenna expensive for fabrication and bur-
densome to be embedded inside a watch-type device. Recently,
This work was supported in part by the Technology Innovation
Program (20008801) funded from the Ministry of Trade, Industry & metamaterials and metasurfaces are also actively adopted to
Energy (MOTIE, Korea), in part by National Research Foundation of realize compact antennas with high performances [12]–[18].
Korea (NRF-2018R1A6A1A03025708, NRF-2023R1A2C2004236), and They are the artificial structures composed of subwavelength
in part by MSIT (Ministry of Science and ICT), Korea, under the ITRC
(Information Technology Research Center) support program (IITP-2023- macro cells with the advantage that their effective medium
2021-0-02046) supervised by the IITP (Institute for Information and properties can be manipulated. Nevertheless, compact antenna
Communications Technology Planning and Evaluation) design with high reliability for wearable devices lacks attention
Ashwini Kumar Arya, Taein Kim, and Sanghoek Kim are with the
Department of Electronics Engineering, the Department of Electronics in the literature yet.
& Information Convergence Engineering, and the Institute of Wearable This paper presents an antenna design for the watch-typed
Convergence Electronics, Kyung Hee University, Yongin 17104, South wearable device for the BLE communication. Fig. 1(b) shows
Korea. (∗ Corresponding author’s e-mail: sanghoek@khu.ac.kr).
Hyunjong Kim, Jongkap Oh, and Jeongmin Kim, and Donghyeok the cross-section of the wearable device, presenting various
Moon are with SMDsolutions Co., Ltd, Seoul 08826, South Korea. electronic components, such as a battery, electronic circuitry,
2 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, XXXX 2017

90Ū
U.FL plug to 90Ū
U.FL plug cable
Device top cover
Antenna position W
Air gap 8 via
feeding

12 mm
PCB circuit area
Battery area Top layer
Device bottom cover
50 mm
(a) (b) via 6

Fig. 1: (a) Antenna for a watch-type wearable device that this

work proposes. (b) Cross-section view of the wearable device Bottom layer
including the position of the antenna. (a)

and the antenna mounted underneath the top cover of the via

device. All the components are enclosed inside the gray- G

colored frame, which consists of top and bottom covers, with

its total size as compact as 50×40×12 mm3 . The proposed Top layer Bottom layer
antenna fits well within such a small area and is fed by a feeding
commercial, ultra-small connector to ease the assembly with (b)
the main circuitry board. The antenna has a complete ground
Fig. 2: Schematics of antennas. (a) Reference antenna, (b)
layer that ensures the antenna to operate whether the medium
Proposed antenna.
under the device is air or body tissue, making the antenna
suitable for wearable purpose.
TABLE I: Dimension for the reference antenna (units in mm)
To confirm the operation of the antenna for the communi-
cation, the antenna is included in a complete prototype device Variable Value Variable Value Variable Value
LG 18.5 L4 17.1 L9 8.9
and the RSSI is measured to check the connectivity [19], WG 4.1 L5 1.8 L10 12.0
[20]. We show the RSSI levels higher than a typical receiver L1 14.8 L6 3.9 L11 0.9
sensitivity of -95 dBm for various postures and orientations L2 4.3 L7 1.0 L12 17.9
L3 11.7 L8 2.7 L13 11.8
of the device within a typical indoor distance of 10 m. w1 0.3 w3 0.2 w5 0.5
The antenna is integrated with a wireless electromyogram w2 0.4 w4 0.9 w6 5.2
(EMG) monitoring device [21], [22]. It is demonstrated that
the activity of the forearm muscle can be wirelessly monitored
over the BLE communication. device, it largely interacts with and is affected by a medium
The paper is organized as follows. The studies of antenna that may be placed under the wearable device.
design and its characterizations with versus without tissue In contrast, the proposed antenna has a complete ground
are discussed in Section II. With the communication module plane on the bottom layer as shown in Fig. 2(b). The same
equipped with the proposed antenna, the wireless connectivity substrate material with the same thickness as the reference
for various environments is discussed in Section III and IV. antenna has been used for the proposed one. The proposed
Finally, the paper is concluded in Section V. . antenna is designed to be minimally affected by the medium
behind as shall be demonstrated by simulations and measure-
II. A NTENNA D ESIGN S TUDIES AND ments.
C HARACTERIZATIONS
To design the antenna for wearable devices, the size consid- A. Antenna Design Procedure
eration of the antenna to fit in a compact device is an essential The antenna design follows the concept of the Planar
part of the investigation. Also, the antenna needs to radiate Inverted-F antenna (PIFA), which reduces the antenna size by
well in the surrounding directions except for the direction adopting a shorting pin near the feeding pin [23]. The proposed
toward the body tissue to minimize the medium dependency. design of modified PIFA is considered to be installed on the
Fig. 2 shows the two antennas designed and investigated inner side of the device top cover making its effective radiation
for this work. The first antenna in Fig. 2(a) shows a folded toward the outside of the device.
meander dipole antenna as a reference one with a size of The available size and location for the antenna underneath
18.5×4.1 mm2 . Over the substrate of FR4 with a relative per- the device top cover can be seen in Fig. 3(a), denoted by the
mittivity of 4.4 and 0.2-mm thickness, the detailed dimensions orange-colored borderline. The allowable size on the bottom
of the reference antenna are tabulated in Table I. Due to its was approximately 26.5×12 mm2 . Here the proposed antenna
small size (∼ 0.15λ0 ), this antenna structure is widely adopted is fed with the U.FL connector, a type of ultra-small connector
for miniaturized mobile devices [2]–[4]. One of the key factors produced by the Hirose Electronics, so that the antenna can be
to reduce the size is that one arm of the antenna is folded on easily connected to the device circuits with a low profile [24].
the other side of the other arm [7]. The radiation pattern is The footprint for the U.FL adaptor followed the guideline pro-
almost omnidirectional as that of a regular dipole antenna. As vided from the product (U.FL-R-SMT-1) shown in Fig. 3(b).
a result, when the reference antenna is located in the wearable The connection of the antenna feed via the regulated footprint
AUTHOR et al.: PREPARATION OF PAPERS FOR IEEE TRANSACTIONS AND JOURNALS (FEBRUARY 2017) 3

(a) (b)

(a) Step I
Step II
Step III

(c) (d)

Fig. 4: Antenna design procedure. (a) Step I, (b) Step II, (c)
Step III, (d) S-parameters.

becomes longer to decrease the resonant frequency, which is

(b) clearly seen from Fig. 5(b).
4) Step IV: The elaboration of the feed point design to
Fig. 3: Substrate size and location of antennas and connector.
connect the U.FL port is made. Fig. 6(a) shows the top view of
(a) Available space on the top cover, (b) 3-D and top view of
the complete antenna on a substrate with 100% transparency
the U.FL connector (left) and the footprint for the connector
to show both the upper patch and the pattern on the ground
(right) [24].
plane for the U.FL connection. The signal pad is connected to
the feed line with the via connection. The feed line is extended
from the edge toward inside of the main patch by creating a
eases the stable soldering and assures the robust operation of slot to make an inset feed configuration. As the depth of the
the antenna. Considering the footprint, the area of 5×4.7 mm2 inset goes deeper than the initial position of the feed point
was additionally reserved for the port connection at the top-left in Step III, the resonance behavior of the antenna changes as
corner of the antenna as seen in Fig. 3(a). Fig. 6(b). It is observed that the position of the feed point
Upon the size specification allowed for it, the antenna is effectively changes the current path length and can be used as
designed with a step-by-step procedure considering the plastic a knob to shift the resonant frequency.
cover of 0.5-mm thickness and a conducting layer in the PCB
circuitry.
1) Step I: A rectangular patch was made on one side of
the substrate and another side was used as a complete ground
plane. The initial dimensions of the patch are 23×7 mm2 .
Based on the well-known transmission-line model for the
rectangular patch antenna design [25], the feeding point was
moved along the center line as can be seen in Fig. 4 to have the
impedance matching at 3.32 GHz, which is near the desired
operating frequency (2.45 GHz). (a) (b)

2) Step II: To improve the impedance matching and band- Fig. 5: Antenna design procedure Step III. (a) Shorting pin at
width, a parasitic element is added as seen in Fig. 4(b). When various locations, (b) S-parameters study.
the structure is simulated using a commercial electromagnetic
solver (the CST Microwave Studio), this choice yields a 5) Step V: After the antenna configuration is determined,
better impedance matching with a small frequency shift of the the parametric study is carried out to finalize the antenna
resonant frequency to 3.2 GHz. The bandwidth enhancement geometry. For this purpose, the feeding slot width Sw , the
is due to the two similar current paths produced by the patch feed line width wf , the gap between the main patch and the
itself and by the added parasitic element [26]–[30]. parasitic patch wn2 , and the shorting pin position Sn shown
3) Step III: To reduce the size of the antenna the shorting in Fig. 7 are chosen as the parameters to tune the resonance
pin method is used [31]. The shorting pin is made at the end frequency and the bandwidth. When those parameters vary, the
of the parasitic patch in Fig. 4(c). The S-parameters for the S-parameter results are shown in Fig. 8(a-d). As the feeding
three steps are plotted in Fig. 4(d). It can be seen that the slot width Sw and feeding line width wf increases from 1.3
resonance frequency appears close to the target frequency. mm to 1.7 mm and 0.3 mm to 0.7 mm, the resonant frequency
The effect of shorting pin location on the resonance behavior rarely changes but the impedance matching improves at certain
of the antenna is investigated in Fig. 5(a). As the shorting cases in Fig. 8(a-b). A large frequency shift can be observed
pin moves from the point A to F , the length of current path when the gap variation wn2 changes from 0.2 mm to 0.6 mm
4 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, XXXX 2017

(a) (b)
(a) (b)
Fig. 6: Antenna design procedure Step IV. (a) U.FL connection
Fig. 9: Tissue effect study. (a) Simulation model (antenna
details, (b) resonance behavior on the depth of inset feed.
inside the device, over the tissue), (b) resonance behavior.

TABLE II: Dimensions for the proposed antenna (units in mm)

Variable Value Variable Value Variable Value

Lng 25.0 Ln4 19.3 wn1 0.5
W mg 15.5 Ln5 8.5 wn2 1.1
Ln1 7.8 Ln6 10.8 wn3 1.0
Ln2 23.8 Ln7 4.8 wn4 1.1
Ln3 6.1 Ln8 1.9 wn5 2.4

Fig. 7: Tuned parameters for the antenna optimization

and also when the shorting pin position Sn does from 1.0 mm Top Layer
to 4.0 mm. The above study helps to fine tune the resonance
Top Layer
behavior of the antenna at the required frequency band.

5 mm Bottom Layer 5 mm Bottom Layer

(a) (b)

Fig. 10: Images of fabricated antennas. (a) Top and bottom

layers of the reference antenna. (b) Top and bottom layers of
the proposed antenna

(a) (b)
B. Measurement Results of the Proposed Antenna
The reference and the proposed antenna are fabricated
as shown in Fig. 10(a) and (b), respectively. The reference
antenna is vertically placed at the edge of the wearable device
as shown in Fig. 11(a) and (b), while the proposed antenna
is attached to the bottom surface of the top cover as initially
intended (Fig. 11(c) and (d)). The U.FL cable allows one to
(c) (d) place the antenna at a preferred position considering other
requirements of the device design, such as the position of a
Fig. 8: Antenna design procedure Step V. (a) Feeding slot visual interface for the device. Most importantly, the complete
width variations, (b) Feed line width variations, (c) Gap ground plane on the other side of the two-layered antenna can
between the main patch and parasitic patch variations, (d) effectively block the back radiation, and hence the antenna
Shorting pin position variation (along the arrow direction in performance is less affected by the medium beneath the
Fig. 7). wearable device.
To evaluate and compare the effect of a nearby medium for
The simulation model to consider the tissue medium below two antennas, the antennas are installed inside the wearable
the wearable device embedding the antenna is studied after the device (Fig. 11) and the S-parameters are measured on and
antenna optimization as shown in Fig. 9(a). It is observed that off the body. A vector network analyzer (VNA; Anritsu
due to the shielding by the ground plane of the antenna itself MS46122A) was used to measure the S-parameters. To mimic
and the conducting sheet of the PCB circuitry, the resonance the on-body situation, a pork meat piece of about 20-mm
behavior remains the same in Fig. 9(b). The final dimensions thickness with the same size of the device was placed below
of the proposed antenna are summarized in Table II. the wearable device. Fig. 12 shows the S-parameter results
AUTHOR et al.: PREPARATION OF PAPERS FOR IEEE TRANSACTIONS AND JOURNALS (FEBRUARY 2017) 5

U.FL cable Antenna Antenna location TABLE III: Comparison with previous antennas for smart-
watch, body-area networks, and tele-medicine applications at
2.4 GHz
Ref. Size Feeding Configuration
LED [37] 45 × 19 mm2 CPW PIFA
[38] 60 × 20 mm2 Inset Patch
[39] 81 × 81 mm2 CPW Slot antenna
[40] 38 × 38 mm2 Contact Inverted-L Antenna
Device inside view Device top view [41] 66 × 66 mm2 CPW Rectangular Patch
(a) (b) [42] 66 × 42 mm2 MS line Rectangular patch
U.FL cable Antenna location This work 25 × 10.8 mm2 U.FL Modified PIFA

Antenna
that the S-parameter of the reference antenna shifts when the
device is placed on the tissue. The shift of resonant frequency
can affect the performance of the antenna depending on the
LED user environment, which is not desirable. In contrast, the
Top cover proposed antenna shows a stable S-parameter regardless of
Device inside view Device top view
whether the device is on or off the tissue in Fig. 12(b). It
(c) (d)
can be inferred that the performance of the proposed antenna
Fig. 11: Images of the prototype wearable device including is hardly affected by the user environment. Based on the 3:1
antennas. (a) Inside view of the wearable device when the VSWR (S11 ≤ −6 dB [33]–[36]) impedance matching for the
reference antenna is installed, (b) Top view of the wearable mobile antenna requirement, the impedance bandwidth was
device when the top cover is closed, (c) Inside view of the measured to be 80 MHz from 2.4∼2.48 GHz with the proposed
wearable device when the proposed antenna is installed, (d) antenna on tissue.
Top view of the device when the top cover is closed. Besides, the radiation patterns were measured in an anechoic
chamber along with the tissue (pork meat) and shown in
Fig. 13. Fig. 13(a) shows the principle planes (xy, xz and yz)
0
of radiation from the device when the device is placed with its
normal direction along the z-axis. As conventionally defined in
S-Parameter (dB)

-10
the spherical coordinate [2], the angle θ and φ refer to the polar
-20 and the azimuthal angle, respectively. It can be observed that
the peak radiation is observed in broadside directions around
-30 w/o tissue
+50◦ for yz plane, around +90◦ in xz plane. For the xy plane,
it is almost uniform and nulls are observed at ±90◦ .
with tissue
-40
2.1 2.4 2.7 3.0

Frequency (GHz) The proposed antenna is compared with the reported an-
tennas utilized for the smartwatch, body-area network and
(a) (b)
tele-medicine applications at 2.4 GHz, in terms on the size,
Fig. 12: Measured S-parameters of (a) reference antenna and feeding techniques and the design configuration in Table III.
(b) proposed antenna. For each antenna, the S-parameter was Compared to the existing solutions, the proposed antenna
measured with and without tissue behind the wearable device exhibits the smallest size by the techniques of modified PIFA
in which the antenna is placed. with the shorting pin.
Since the antenna design is compact in size and demon-
Antenna location
strates little medium dependency, the proposed antenna is
chosen for the watch-type wearable device.

III. C ONNECTIVITY FOR VARIOUS C ONDITIONS

Since the antenna targets a wearable application, we exam-
LED
ine the reliability of connectivity depending on the various
postures of the user and the orientations of the device in this
xy- xz- yz- section. It is inconvenient to measure the S-parameter of the
(a) (b) antenna for various scenarios using the VNA. Therefore, the
connectivity is evaluated instead by measuring the RSSI, with
Fig. 13: Antenna radiation pattern. (a) Wearable device with the antenna assembled within a prototype device in Fig. 11.
the definitions of radiation planes, (b) Radiation patterns per The measurement of the RSSI also better reflects the actual
each radiation plane. operating condition of the device since it does not require
a coaxial cable from the VNA which prevents the complete
closure of the device cover. The RSSI can be measured with a
for both antennas without and with tissue. Fig. 12(a) shows Bluetooth-enabled tablet in which an in-house developed BLE
6 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, XXXX 2017

communication code is executed. While the wearable device is -60

set to transmit power of 6 dBm level, the tablet as an external kŒŠŒG–•Gž™š›

RSSI (dBm)
reader records the strength of the received signal at a fixed -70 free space

position.
-80
The measurement scenarios for the wearable device to work
on the human body are shown in Fig. 14. The external reader -90
2 4 6 8 10
is placed on a computer table with the height of 70 cm. In Distance Dx (m)

the first scenario, the device is worn on the wrist facing the
(a) (b)
external reader and the distance Dx between transceivers is
varied up to 10 m (Fig. 14(a)). The RSSI measured by the -60
external reader is presented in Fig. 14(b). It can be observed

RSSI (dBm)
that the device can operate well up to the range of 10 m, where -70

the RSSI value drops to -90 dBm. Considering the typical

BLE receiver sensitivity of about -95 dB [20] and the typical -80
wrist

distance between indoor transceivers, the measurement results pocket

reveal that the device is suitable for indoor operation. -90

0 90 180 270 360
Further measurements are performed in various scenarios Rotation angle ( )

in which the device is wore on the wrist, inserted in the (c) (d)
front pocket, or attached on the back side of upper waist
as shown in Fig. 14(c), (e), and (g). The second scenario -60

is shown in Fig. 14(c) where the user is standing at 1 m

RSSI (dBm)
distance from the reader and turning in the clockwise direction ɂ -70

as shown in the inset of the figure. In this scenario, the ~™š›

-80
device is worn on a wrist like a watch or inserted in the wrist

front pocket. The orientation of the device can be informed pocket

-90
by considering the illustration of the coordinate space shown 0 90 180 270 360

in Fig. 13(a) and Fig. 14(c). For both situations, the RSSI is w–Š’Œ›
w–Š’’Œ› Rotation angle ( )

recorded and plotted in Fig. 14(d). The different orientation of (e) (f)
the device inside the pocket makes the maximum RSSI appear
at a different rotation angle from the case when the device is -60

worn on the wrist. One can also observe that the RSSI level

RSSI (dBm)
gets lower when the user is rotated by around 180◦ obviously -70

because the signal is severely blocked by the body of the user.

-80
Nevertheless, the RSSI does not drop below -90 dB, which waist (P1)

indicates the BLE communication is remained connected. waist (P2)

-90
The third scenario is shown in Fig. 14(e). In this scenario, 0 90 180 270 360
Rotation angle ( )
the user is in the attention position, where the device is again
worn on a wrist or inserted in the front pocket. The user (g) (h)
rotates in the same way as in Fig. 14(c) and the signal strength Fig. 14: The setup and the measurement results of the RSSI.
is measured as Fig. 14(f). For the wrist case, the rotation (a) Measurement setup for the first scenario in which distance
angle with the maximum strength shifts to 90◦ because the Dx between the transceivers varies. (b) Signal strength on
orientation of the device has changed from Fig. 14(c). In other distance Dx variations. For (c, e, g), the distance between the
words, when the user is rotated by 90◦ , the reader is in the transceivers is fixed to be 1.2 m and the inset figures defines
broadside direction of the antenna. The front-pocket case has the angle of rotation θ. For (c, e), the device is either on
the similar trend as the front-pocket case of Fig. 14(c) because a wrist or inside the pocket. (c) Posture of the user for the
the orientation of the device has not changed. second scenario. (e) Posture of the user for the third scenario.
The last scenario is when the device is attached on the back (g) Device on back side of the upper waist with two different
side at the upper waist as presented in Fig. 14(g). The device orientations. (d, f, h) presents the measured signal strength for
is placed with two different orientations as shown by the illus- the rotation θ given in (c, e, g), respectively.
trations of the coordinate space in (g). When the user makes
the rotation, the signal strengths are plotted in Fig. 14(h). It
is observed that in the case of the waist measurements, the
IV. S YSTEM I NTEGRATION FOR EMG M ONITORING
maximum RSSI occurs at θ = 180◦ because at this angle, the
device and reader are facing each other. In all the experiments The proposed antenna is integrated with a wearable elec-
of the above scenarios, the RSSI remains in the affordable tromyography (EMG) monitoring device. The EMG is a
range that the device is connected, which indicates the reliable common clinical test used to assess the function of muscles
operation of indoor communication as a wearable device. and the nerves that control them. The EMG can be detected
either directly by inserting electrodes or indirectly with surface
AUTHOR et al.: PREPARATION OF PAPERS FOR IEEE TRANSACTIONS AND JOURNALS (FEBRUARY 2017) 7

the wireless recording of the EMG signal at a distance of 10 m.

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