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Effects of a phantom hand on a non-resonant element for a 2.5 GHz smartwatch antenna
Accepted Manuscript Regular paper Effects of a Phantom Hand on a Non-Resonant Element for a 2.5 GHz Smartwatch Antenna Aurora Andújar, Yolanda Cobo, Jaume Anguera PII: DOI: Reference:
S1434-8411(18)30089-X https://doi.org/10.1016/j.aeue.2018.10.013 AEUE 52540
To appear in:
International Journal of Electronics and Communications
Received Date: Accepted Date:
11 January 2018 13 October 2018
Please cite this article as: A. Andújar, Y. Cobo, J. Anguera, Effects of a Phantom Hand on a Non-Resonant Element for a 2.5 GHz Smartwatch Antenna, International Journal of Electronics and Communications (2018), doi: https:// doi.org/10.1016/j.aeue.2018.10.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of a Phantom Hand on a Non-Resonant Element for a 2.5 GHz Smartwatch Antenna Aurora Andújar1, Yolanda Cobo1, Jaume Anguera1,2 1) Fractus Antennas, Barcelona, Spain 2) Electronics and Communications Dept., Universitat Ramon Llull, Barcelona, Spain. [email protected] Abstract Smartwatches with wireless connectivity are becoming widely used and therefore, antenna engineering efforts are addressed to obtain small and efficient antenna systems. Since the space in a smartwatch is considerably small compared to other wireless devices, a non-resonant element of 12.0 mm x 3.0 mm x 2.4 mm for 2.4 GHz-2.5 GHz antenna system operation is proposed herein on a 30 mm x 30 mm ground plane. The ground clearance area is 14 mm x 5 mm, which leaves more space for facilitating the integration of other electronic components in the reduced ground plane area available for this kind of applications. Since the element is non-resonant, a matching network is used to match the antenna system when regarding the presence of a phantom hand. Several distances between the ground plane and the phantom hand are analyzed, from touching position to 6 mm distance. For each case, a new matching has been designed without the need of modifying the geometry of the antenna element. Measured total efficiency is in average (2.4 GHz-2.5 GHz) 15.1% for the worst case (touching position) and increases up to 37.0% for the 6mm case. These results are of practical interest to design smartwatch devices. Keywords Smartwatch, small antennas, phantom hand 1. Introduction
With the rapid growth of the wireless industry, more and more wireless applications are being incorporated into wireless devices. Besides the well-known smartphone, other devices are gaining relevance. This is the case of smartwatches. In 2013, the number of units sold was 2 million and the forecast for 2020 rises to 87 million devices. If still nowadays, the integration of antenna elements inside handset devices supposes a challenging effort for antenna engineers, it exacerbates when talking about smartwatches mainly due to the significant reduced space available to integrate electronic components. It is therefore interesting to research how small antennas can be integrated into such small devices. Human hand interaction adds further challenges to this integration, since antenna engineers do not have only to deal with the small available space but also with the power absorption losses that the presence of the human hand introduces into the radiating system. Several antenna strategies have already appeared in the literature. In [1]-[2], a monopole antenna on a ground clearance of 38 mm x 10 mm is proposed for 2.4 GHz-2.5 GHz operation. In order to mitigate the effect of power absorption due to the hand, a highimpedance surface is analyzed. Simulated total efficiency of 40% is reported taking into account a phantom hand. In [3], a loop antenna for 2.4 GHz-2.5 GHz is proposed taking advantage of a front metal frame achieving a total efficiency of 26% with the phantom hand included. In [4]-[5], a slot antenna formed by the ground plane and the metal housing is analyzed. Efficiency at the 2.4 GHz-2.5 GHz reaches 65% in hand conditions. In [6], a conformal antenna of 25 mm x 4 mm above the ground plane features a total efficiency of 30% when the ground plane is touching the phantom hand. In [7], a multiband antenna for GPS/GSM1800, Bluetooth, and WiFi is proposed. However, the solution uses two antennas of considerable size having each one 2 mm x 35 mm x 5 mm. Other examples including cellular communications, MIMO and GPS
have also been investigated [8]-[11]. Besides efficiency considering the hand, SAR is also an important parameter to consider for assessing the quantity of power absorbed by the user [12]. In this regard, the paper proposes a small non-resonant element (12 mm x 3 mm x 2.4 mm) for an antenna system that facilitates its integration into the small space available in a smartwatch. The proposed clearance area has been reduced as much as possible in order to leave more space for other electronic components. Firstly, the antenna is evaluated in free-space (Section 2). Secondly the effect of the distance between the ground plane and a phantom hand is evaluated to determine some practical rules for design purposes (Section 3). Then, a technique to improve the efficiency in hand conditions is proposed (Section 4). Finally, conclusions are drawn in Section 5. 2. Free-Space design Since the available space in a smartwatch is reduced, the technique proposed herein to reduce the antenna size is the use of a non-resonant element and a matching network with low insertion losses (Fig. 1) [13]-[20]. The size of the non-resonant element is a 3D conductive brass-made parallelepiped L x W x H (12.0 mm x 3.0 mm x 2.4 mm) which represents an electric length of 0.098 at 2.45 GHz which is 2.5 less than a quarter-wavelength monopole. The ground clearance has been reduced as much as possible (14 mm x 5 mm) so more grounding is left to host the circuitry of the smartwatch. The reason why the size has been chosen is based on a previous research where the non-resonant element was proposed for multiband operation in a platform similar to a smartphone [15], [19]. The objective here is to use the same geometry but in this case for operation in the 2.4 GHz-2.5 GHz band changing only the matching
network, not the geometry. This scalability property is interesting since the same element can be used across different devices For feeding the structure, a 50 Ω Coplanar Waveguide (CPW) transmission line is used with a SMA connector. The PCB (Printed Circuit Board) is made out of FR4 (r=4.15 and tan=0.014) having ground plane on both top and bottom layers. It should be pointed out that in order to mitigate the impact of the coaxial transmission line when testing the antenna with the network analyze or anechoic chamber, said coaxial cable includes ferrites. In this manner, the possible current flowing out the coaxial cable is minimized minimizing the impact in SWR and efficiency. 14mm
2mm
30mm
25mm
Matching network
50Ω CPW line
30mm
Fig. 1 Non-resonant element is a 3D brass made structure of 12 mm x 3 mm x 2.4 mm (H) and it is placed on a grounding clearance area of 14 mm x 5 mm. A matching network is used with SMD 0402 type lumped components.
As the non-resonant element features an impedance of Zin=11.3-27.2jΩ at 2.45 GHz, a matching network using lumped components is designed to match the antenna to 50 Ω. Since the non-resonant element is smaller than a quarter wave-length monopole, the
impedance is capacitive (Fig. 2). In this sense, a possible matching network comprises a two element matching network, one shunt and one series inductor. For this case, Murata
0.8
Swp Max 2.6GHz
0.
6
S(1,1) Without matching
1.0
high-Q tight tolerance SMD 0402 type components are used.
2.
0
S(2,2) With matching 0.
3.
0
4
4.
0.2 10.0
5.0
4.0
3.0
2.0
1.0
0.8
0.6
0.4
0
10.0
Z2
-10.0
[S]
0.2
0 .0
Z1
-1.0
-0.8
-0
.6
-2
.0
.0
.
-3
-0
-4
2.45 GHz r 11.3 Ohm x -27.2 Ohm 4
-5.
0.2
2.45 GHz r 50 Ohm x -0.00738 Ohm
-
0
5.0
Swp Min 2.3GHz
Fig. 2 Input impedance of the non-resonant element without matching and with a matching network. Z1=3nH, Z2=2.3nH To give a physical insight, the current distribution is computed at 2.45GHz (Fig. 3). It is observed that the current is mainly concentrated along the edge close to the feeding point. Since the ground plane is comparable to the wavelength (0.245 @ 2.45GHz), it is a big player in the radiation process.
Fig. 3 Current distribution at 2.45GHz
In a free-space scenario, the measured Voltage Standing Wave Ration (SWR) presents values less than 2 for the 2.4-2.5 GHz frequency range and a total efficiency between 70% and 80% (Fig. 4). The total efficiency (t) takes into account the radiation efficiency (r) and mismatching as t=r·(1-|S11|2). Antenna efficiency is measured
6,0
100
5,5
90
5,0
80
4,5
70
Z2
SWR
4,0
60
[S]
3,5
50
Z1
3,0
40
Measured SWR 2,5
Total Efficiency (%)
using 3D pattern integration with the Satimo Startgate-32 anechoic chamber.
30
Simulated SWR 2,0
1,5
1,0 2,30
20
Measured Total Efficiency Simulated Total Efficiency 2,35
10
2,40
2,45
2,50
2,55
0 2,60
Frequency (GHz)
Fig. 4 Simulated and measured SWR and total efficiency in free-space. Z1=2.3 nH (LQW15AN2N2N3B80 from Murata) and Z2=3.3nH (LQW15AN3N3B80). Total efficiency takes into account both radiation efficiency and mismatching. It is measured using 3D pattern integration with Satimo Stargate-32 anechoic chamber When comparing total efficiency from 2.4GHz to 2.5GHz, the average values are 78.5% and 88.9% for the simulation and measurements which is an acceptable difference of 0.5dB. Since the smartwatch is thought to operate very close the human hand, its effect must be taken into account. In particular, next section analyzes the effect in efficiency when the distance between the ground plane and a phantom hand varies.
3. Human-hand effect The effect over total efficiency as a function of the distance between the ground and the phantom hand from Indexsar (r=25.7, =1.32 S/m) is analyzed for three cases: case a which is the touching position (the bottom ground layer directly touches the phantom hand); case b and c where the distance between the bottom ground layer is 3 mm and 6 mm respectively (Fig. 5). The objective is to study how critical the distance is in order to determine design recommendations for this kind of applications. Porexpan spacers have been used to separate the PCB from the phantom hand. Case a PCB 0 mm from the phantom hand Z1=1.5 nH Z2=0Ω (PCB: Printed Circuit Board) Case b PCB 3 mm from the phantom hand Z1=1.8 nH Z2=3.3 nH Case c PCB 6 mm from the phantom hand Z1=1.8 nH Z2=3.4 nH Fig. 5 Different scenarios to evaluate the impact of the phantom hand as a function of the distance between the PCB and the phantom: a) 0 mm; b) 3 mm; and c) 6 mm.
Distance is measured between the bottom ground layer of the PCB and the surface of the phantom hand. 0 mm means than the ground is in contact with the phantom hand Since the proximity of the human hand detunes the antenna, a new antenna matching has been designed for each one of the three cases under study to minimize mismatch losses (Fig. 5). At this point it is interesting that for each case, the geometry of the nonresonant element is maintained; only the matching network values changes from one case to the other. In this sense, no customized antennas are required. 6.0
100
SWR
5.0
SWR 0mm
SWR 3mm
SWR 6mm
Efficiency 0mm
Efficiency 3mm
Efficiency 6mm
90
80
4.5
70
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50
3.0
40
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30
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1.0 2.30
2.33
2.35
2.38
2.40
2.43
2.45
2.48
2.50
2.53
2.55
2.58
Total Efficiency (%)
5.5
0 2.60
Frequency (GHz)
Fig. 6 Measured SWR and total efficiency for 0 mm, 3 mm, and 6 mm distance case. Total efficiency is measured using 3D pattern integration with the Satimo Stargate-32 anechoic chamber The results reflect the critical effect of the hand into the total efficiency (Fig. 6, Table 1). In particular, the average efficiency across the 2.4 GHz-2.5 GHz frequency range is 15.1% in touching position. It increases up to 25.0% and 37.0% for the 3 mm and 6 mm distance, respectively. In summary, the average efficiency may be increased about 3dB from the worst case (touching position) to the case where the ground plane is spaced apart 3 mm to 6 mm. Since in real situations, the ground is not touching the hand,
efficiency can always be improved if the distance of the ground plane is kept apart from the hand as long as the aesthetic and mechanical aspects of the smartwatch allows doing that. It is interesting to observe that the 0mm case has a very good SWR. This is due to the losses introduced by the human hand. However, this improvement in SWR due to losses degrades efficiency when compared to 3mm and 6mm case. Absorption due to the phantom hand is computed as the ration in dB between the average total efficiency in free-space and the same taking into account the phantom hand (Table 1). Since, for the three cases, antennas are well matched (SWR<2.5), such ratio mainly reflects how much power is absorbed by the hand. As general numbers: approximately 7dB are absorbed in touching position and 3dB for the 6 mm case. Absorption & Mismatching (dB)
SWR
Measured total efficiency across 2.4-2.5 GHz (%)
Distance
2.4-2.5 GHz
ɳa (2.4GHz)
ɳa (2.5GHz)
ɳa (min)
ɳa (max)
ɳa (average)
FreeSpace
< 2:1
76.0
75.5
74.5
81.2
78.6
-
0 mm
< 2:1
13.8
16.1
13.8
16.1
15.1
7.1dB
3 mm
< 2.5:1
21.6
24.4
21.6
26.9
25.0
4.9dB
6 mm
< 2.5:1
32.1
33.6
32.1
39.5
37.0
3.2dB
Table 1 Summary of the measured results in free-space and taking into account the hand effect Although the arm is not considered in this analysis, the trend should be similar to the one shown in this paper. Absorption might increase since not only the hand is considered. Radiation patterns taking into account the phantom hand are measured for the three scenarios: touching, 3 mm, and 6 mm apart from the phantom hand (Fig. 7). Although
the phantom hand introduces losses, there are no big nulls in the direction pointing to the hand. However, some attenuation effect is observed. For example, the radiation at =0º is above 5dB to 10dB with respected the radiation at =180º. At the end, radiation patterns are quite pseudo-isotropic which it is a desired feature for smartwatch applications.
Measurement System Set-Up
= 90º Plane XY at 2.45 GHz at 0mm,
PCB in Plane XY
3mm and 6mm
= 0º Plane XZ at 2.45 GHz at 0mm,
= 90º Plane YZ at 2.45 GHz at 0mm,
3mm and 6mm
3mm and 6mm
Fig. 7 Measured radiation pattern (total normalized gain in dB is shown) 4. Improvements by adding ground layers
In order to minimize the impact of power absorption due to the hand, this section proposed a simple technique to reduce its influence. Since the space for electronics in a smartwatch is limited, it is reasonable to reuse the space by using two stacked PCB. For this case, the original case having the PCB 6 mm above the hand is modified including another PCB with full ground plane underneath. To have both PCB electrically connected, a conductive strip at the edge is proposed (Fig. 8 left). For this new situation, the matching network is redesigned to minimize mismatch losses. The values for the matching network components are Z1=1.8 nH and Z2=1.8 nH following a different topology of Fig. 4. In this case, Z1 is a series component connected to the non-resonant element, and Z2 is a shunt component. This particular configuration enhances the measured total efficiency from the 37.0% (averaged from 2.4 GHz to 2.5 GHz) to 49.1% (Fig. 8 right) being a substantial efficiency increment. This result is close to reported values in [4] where total efficiency with the phantom hand is 65% at the same operating bandwidth but using a total height of 10mm and a conductive wrist which is not always possible. Therefore, results presented here make the solution attractive for smartwatches. 100 90
6mm Single layer
80
6mm Two layers
Total Efficicency (%)
70 60 50
40 30 20 10 0 2.30
2.35
2.40
2.45
2.50
2.55
2.60
Frequency (GHz)
Fig. 8 Left) PCB 6 mm above the phantom hand, an extra PCB having a ground layer is connected to the top ground through the edge (total gap is maintained to 6mm); right)
total efficiency measured for the PCB 6 mm above the hand and for the same case having the configuration shown in the left figure. Porexpan spacers are used.
It is worth analyzing the current distribution of the two layer solution compared to the single layer (Fig. 3, Fig. 9). It is observed that for the two layers solution, currents are aligned along the ground plane. Since the ground plane is larger and as it is a relevant part to the radiation process, antenna efficiency increases. Furthermore, computed directivity for the single layer solution is 5.1dB whereas is 3.0dB for the two-layers solution which is preferable in order to radiate in a similar way in all space directions.
Fig. 9 Simulated current distribution for the two layers solution at 2.45GHz. Both layers are connected with a rectangular conductive strip at the edge
At this point, it is interesting to compare, the proposed solution with other solutions existing in the prior-art (Table 2). The solutions shown in Table 2 disclose several antenna solutions operating at the 2.4 GHz-2.5 GHz band. It is interesting to observe that the best antenna in terms of antenna efficiency with the hand and at the same time with less antenna volume is the one proposed in [6]. If now, said solution is compared with the one proposed here, one can conclude that the antenna solution has similar antenna efficiency values with the phantom hand but with less volume. However, when looking with more detail the antenna solution at [6], the antenna height is 4mm in
comparison with the only 2.4mm of the present solution which is advantageously for slim smartwatches. Also, the antenna solution in [6] is bent in three parts, thus making the design more complex that the proposed in this paper with a chip antenna component being more attractive in terms of integration into a smartwatch device.
Ref. [3]
Ground clearance (mm2) 344
Antenna Efficiency Metal volume(mm3) With Hand (%) housing 360.0
26 (meas)
Supporting Material
no
FR4 Metal[4]-[5] 0 16619.0 68 (meas) yes housing [6] 0 59.2 50 (sim) no FR4 [7] 25.4 39.9 no FR4 [11] 160 20.0 no Copper Proposed 70 86.4 49 (meas) no FR4 Table 2 Summary of different smart-watch antenna designs for 2.4 GHz band
5. Conclusions A small non-resonant of 12.0 mm x 3.0 mm x 2.4 mm with a small ground clearance area of just 14 mm x 5 mm operating at the 2.4 GHz-2.5 GHz band has been proposed for being integrated in a smartwatch platform. Its performance as a function of the distance between the ground plane and the phantom hand has been analyzed. The human hand mainly introduces detuning effects and efficiency decrements. The closer the hand, the lower the antenna efficiency. A technique to mitigate the hand impact has been proposed consisting on adding an extra ground plane layer underneath the antenna board. When both layers are connected, measured total efficiency is increased from 37.0% (averaged from 2.4 GHz to 2.5 GHz) to 49.1% which is a substantial increment of 1.2dB.
The non-resonant element allows making a small antenna capable of being integrated in this kind of applications featured by significant space constraints. Its versatile design allows retuning the antenna easily through its matching network components rather than through its geometry, thus simplifying its integration process and reducing the workload that antenna customization requires. This makes the non-resonant element attractive for being integrated in multiple devices and applications since the same geometry can be used for each device, only the matching network should be customized. In terms of antenna efficiency, when the grounding is touching the phantom hand, the average efficiency across the 2.4 GHz-2.5 GHz is 15.1% and it increases up to 25.0% and 37.0% just increasing the distance up to 3 mm and 6 mm, respectively. Acknowledgements This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement Nº 674491.
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[3] Saou-Wen Su and Yi-Ting Hsieh, “Integrated Metal-Frame Antenna for Smartwatch Wearable Device”, IEEE Transactions on Antennas and Propagation, vol.63, no. 7, pp.3301-3305, July 2015. [4] Wu, D., S. W. Cheung, Q. L. Li, and T. I. Yuk, “Slot antenna for all-metal smartwatch applications,” European Conf. Antennas Propagat., 1144–1147, Davos, Switzerland, 2016. [5] D. Wu and S. W. Cheung, “A Cavity-Backed Annular Slot Antenna with High Efficiency for Smartwatches with Metallic Housing”, IEEE Transactions on Antennas and Propagation, vol.65, no. 7, pp.3756-3761, May 2017. [6] Chih-Hsien Wu, Kin-Lu Wong, Yuan-Chih Lin, and Saou-Wen Su, “Conformal Bluetooth Antenna for the Watch-Type Wireless Communication Device Application”, 2007 IEEE Antennas and Propagation Society International Symposium, Honolulu, HI, 2007, pp. 4156-4159. [7] Hong-Sheng Huang, Hsin-Lung Su, Sung-Lung Chen, "Multiband Antennas for GPS/GSM1800/Bluetooth/Wi-Fi Smart Watch Applications", 2017 IEEE International Conference on Computational Electromagnetics (ICCEM), Kumamoto, 2017, pp. 352-354. [8] Kun Zhao, Zhinong Ying, and Sailing He, “Antenna Designs of Smart Watch for Cellular Communications by using Metal Belt”, 2015 9th European Conference on Antennas and Propagation (EuCAP), Lisbon, 2015, pp. 1-5. [9] S. Woo, J. Baek, H. Park, D. Kim, and J. Choi, “Design of a Compact UWB Diversity Antenna for WBAN Wrist-Watch Applications”, 2013 Proceedings of the International Symposium on Antennas & Propagation, Nanjing, 2013, pp. 13041306.
[10]
Saou-Wen Su and Cheng-Tse Lee, “Metal-Frame GPS Antenna for Smartwatch
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Wen-Shan Chen, Chih-Kai Yang, and Wei-Syum Sin, “MIMO Antenna with
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Kun Zhao, Shuai Zhang, Chi-Yuk Chiu, Zhinong Ying, and Sailing He, “SAR
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J. Anguera, A. Andújar, C. Puente, and J. Mumbrú, “Antennaless Wireless
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[19]
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S1434-8411(18)30089-X https://doi.org/10.1016/j.aeue.2018.10.013 AEUE 52540
To appear in:
International Journal of Electronics and Communications
Received Date: Accepted Date:
11 January 2018 13 October 2018
Please cite this article as: A. Andújar, Y. Cobo, J. Anguera, Effects of a Phantom Hand on a Non-Resonant Element for a 2.5 GHz Smartwatch Antenna, International Journal of Electronics and Communications (2018), doi: https:// doi.org/10.1016/j.aeue.2018.10.013
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of a Phantom Hand on a Non-Resonant Element for a 2.5 GHz Smartwatch Antenna Aurora Andújar1, Yolanda Cobo1, Jaume Anguera1,2 1) Fractus Antennas, Barcelona, Spain 2) Electronics and Communications Dept., Universitat Ramon Llull, Barcelona, Spain. [email protected] Abstract Smartwatches with wireless connectivity are becoming widely used and therefore, antenna engineering efforts are addressed to obtain small and efficient antenna systems. Since the space in a smartwatch is considerably small compared to other wireless devices, a non-resonant element of 12.0 mm x 3.0 mm x 2.4 mm for 2.4 GHz-2.5 GHz antenna system operation is proposed herein on a 30 mm x 30 mm ground plane. The ground clearance area is 14 mm x 5 mm, which leaves more space for facilitating the integration of other electronic components in the reduced ground plane area available for this kind of applications. Since the element is non-resonant, a matching network is used to match the antenna system when regarding the presence of a phantom hand. Several distances between the ground plane and the phantom hand are analyzed, from touching position to 6 mm distance. For each case, a new matching has been designed without the need of modifying the geometry of the antenna element. Measured total efficiency is in average (2.4 GHz-2.5 GHz) 15.1% for the worst case (touching position) and increases up to 37.0% for the 6mm case. These results are of practical interest to design smartwatch devices. Keywords Smartwatch, small antennas, phantom hand 1. Introduction
With the rapid growth of the wireless industry, more and more wireless applications are being incorporated into wireless devices. Besides the well-known smartphone, other devices are gaining relevance. This is the case of smartwatches. In 2013, the number of units sold was 2 million and the forecast for 2020 rises to 87 million devices. If still nowadays, the integration of antenna elements inside handset devices supposes a challenging effort for antenna engineers, it exacerbates when talking about smartwatches mainly due to the significant reduced space available to integrate electronic components. It is therefore interesting to research how small antennas can be integrated into such small devices. Human hand interaction adds further challenges to this integration, since antenna engineers do not have only to deal with the small available space but also with the power absorption losses that the presence of the human hand introduces into the radiating system. Several antenna strategies have already appeared in the literature. In [1]-[2], a monopole antenna on a ground clearance of 38 mm x 10 mm is proposed for 2.4 GHz-2.5 GHz operation. In order to mitigate the effect of power absorption due to the hand, a highimpedance surface is analyzed. Simulated total efficiency of 40% is reported taking into account a phantom hand. In [3], a loop antenna for 2.4 GHz-2.5 GHz is proposed taking advantage of a front metal frame achieving a total efficiency of 26% with the phantom hand included. In [4]-[5], a slot antenna formed by the ground plane and the metal housing is analyzed. Efficiency at the 2.4 GHz-2.5 GHz reaches 65% in hand conditions. In [6], a conformal antenna of 25 mm x 4 mm above the ground plane features a total efficiency of 30% when the ground plane is touching the phantom hand. In [7], a multiband antenna for GPS/GSM1800, Bluetooth, and WiFi is proposed. However, the solution uses two antennas of considerable size having each one 2 mm x 35 mm x 5 mm. Other examples including cellular communications, MIMO and GPS
have also been investigated [8]-[11]. Besides efficiency considering the hand, SAR is also an important parameter to consider for assessing the quantity of power absorbed by the user [12]. In this regard, the paper proposes a small non-resonant element (12 mm x 3 mm x 2.4 mm) for an antenna system that facilitates its integration into the small space available in a smartwatch. The proposed clearance area has been reduced as much as possible in order to leave more space for other electronic components. Firstly, the antenna is evaluated in free-space (Section 2). Secondly the effect of the distance between the ground plane and a phantom hand is evaluated to determine some practical rules for design purposes (Section 3). Then, a technique to improve the efficiency in hand conditions is proposed (Section 4). Finally, conclusions are drawn in Section 5. 2. Free-Space design Since the available space in a smartwatch is reduced, the technique proposed herein to reduce the antenna size is the use of a non-resonant element and a matching network with low insertion losses (Fig. 1) [13]-[20]. The size of the non-resonant element is a 3D conductive brass-made parallelepiped L x W x H (12.0 mm x 3.0 mm x 2.4 mm) which represents an electric length of 0.098 at 2.45 GHz which is 2.5 less than a quarter-wavelength monopole. The ground clearance has been reduced as much as possible (14 mm x 5 mm) so more grounding is left to host the circuitry of the smartwatch. The reason why the size has been chosen is based on a previous research where the non-resonant element was proposed for multiband operation in a platform similar to a smartphone [15], [19]. The objective here is to use the same geometry but in this case for operation in the 2.4 GHz-2.5 GHz band changing only the matching
network, not the geometry. This scalability property is interesting since the same element can be used across different devices For feeding the structure, a 50 Ω Coplanar Waveguide (CPW) transmission line is used with a SMA connector. The PCB (Printed Circuit Board) is made out of FR4 (r=4.15 and tan=0.014) having ground plane on both top and bottom layers. It should be pointed out that in order to mitigate the impact of the coaxial transmission line when testing the antenna with the network analyze or anechoic chamber, said coaxial cable includes ferrites. In this manner, the possible current flowing out the coaxial cable is minimized minimizing the impact in SWR and efficiency. 14mm
2mm
30mm
25mm
Matching network
50Ω CPW line
30mm
Fig. 1 Non-resonant element is a 3D brass made structure of 12 mm x 3 mm x 2.4 mm (H) and it is placed on a grounding clearance area of 14 mm x 5 mm. A matching network is used with SMD 0402 type lumped components.
As the non-resonant element features an impedance of Zin=11.3-27.2jΩ at 2.45 GHz, a matching network using lumped components is designed to match the antenna to 50 Ω. Since the non-resonant element is smaller than a quarter wave-length monopole, the
impedance is capacitive (Fig. 2). In this sense, a possible matching network comprises a two element matching network, one shunt and one series inductor. For this case, Murata
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2.45 GHz r 11.3 Ohm x -27.2 Ohm 4
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2.45 GHz r 50 Ohm x -0.00738 Ohm
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0
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Swp Min 2.3GHz
Fig. 2 Input impedance of the non-resonant element without matching and with a matching network. Z1=3nH, Z2=2.3nH To give a physical insight, the current distribution is computed at 2.45GHz (Fig. 3). It is observed that the current is mainly concentrated along the edge close to the feeding point. Since the ground plane is comparable to the wavelength (0.245 @ 2.45GHz), it is a big player in the radiation process.
Fig. 3 Current distribution at 2.45GHz
In a free-space scenario, the measured Voltage Standing Wave Ration (SWR) presents values less than 2 for the 2.4-2.5 GHz frequency range and a total efficiency between 70% and 80% (Fig. 4). The total efficiency (t) takes into account the radiation efficiency (r) and mismatching as t=r·(1-|S11|2). Antenna efficiency is measured
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Fig. 4 Simulated and measured SWR and total efficiency in free-space. Z1=2.3 nH (LQW15AN2N2N3B80 from Murata) and Z2=3.3nH (LQW15AN3N3B80). Total efficiency takes into account both radiation efficiency and mismatching. It is measured using 3D pattern integration with Satimo Stargate-32 anechoic chamber When comparing total efficiency from 2.4GHz to 2.5GHz, the average values are 78.5% and 88.9% for the simulation and measurements which is an acceptable difference of 0.5dB. Since the smartwatch is thought to operate very close the human hand, its effect must be taken into account. In particular, next section analyzes the effect in efficiency when the distance between the ground plane and a phantom hand varies.
3. Human-hand effect The effect over total efficiency as a function of the distance between the ground and the phantom hand from Indexsar (r=25.7, =1.32 S/m) is analyzed for three cases: case a which is the touching position (the bottom ground layer directly touches the phantom hand); case b and c where the distance between the bottom ground layer is 3 mm and 6 mm respectively (Fig. 5). The objective is to study how critical the distance is in order to determine design recommendations for this kind of applications. Porexpan spacers have been used to separate the PCB from the phantom hand. Case a PCB 0 mm from the phantom hand Z1=1.5 nH Z2=0Ω (PCB: Printed Circuit Board) Case b PCB 3 mm from the phantom hand Z1=1.8 nH Z2=3.3 nH Case c PCB 6 mm from the phantom hand Z1=1.8 nH Z2=3.4 nH Fig. 5 Different scenarios to evaluate the impact of the phantom hand as a function of the distance between the PCB and the phantom: a) 0 mm; b) 3 mm; and c) 6 mm.
Distance is measured between the bottom ground layer of the PCB and the surface of the phantom hand. 0 mm means than the ground is in contact with the phantom hand Since the proximity of the human hand detunes the antenna, a new antenna matching has been designed for each one of the three cases under study to minimize mismatch losses (Fig. 5). At this point it is interesting that for each case, the geometry of the nonresonant element is maintained; only the matching network values changes from one case to the other. In this sense, no customized antennas are required. 6.0
100
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Fig. 6 Measured SWR and total efficiency for 0 mm, 3 mm, and 6 mm distance case. Total efficiency is measured using 3D pattern integration with the Satimo Stargate-32 anechoic chamber The results reflect the critical effect of the hand into the total efficiency (Fig. 6, Table 1). In particular, the average efficiency across the 2.4 GHz-2.5 GHz frequency range is 15.1% in touching position. It increases up to 25.0% and 37.0% for the 3 mm and 6 mm distance, respectively. In summary, the average efficiency may be increased about 3dB from the worst case (touching position) to the case where the ground plane is spaced apart 3 mm to 6 mm. Since in real situations, the ground is not touching the hand,
efficiency can always be improved if the distance of the ground plane is kept apart from the hand as long as the aesthetic and mechanical aspects of the smartwatch allows doing that. It is interesting to observe that the 0mm case has a very good SWR. This is due to the losses introduced by the human hand. However, this improvement in SWR due to losses degrades efficiency when compared to 3mm and 6mm case. Absorption due to the phantom hand is computed as the ration in dB between the average total efficiency in free-space and the same taking into account the phantom hand (Table 1). Since, for the three cases, antennas are well matched (SWR<2.5), such ratio mainly reflects how much power is absorbed by the hand. As general numbers: approximately 7dB are absorbed in touching position and 3dB for the 6 mm case. Absorption & Mismatching (dB)
SWR
Measured total efficiency across 2.4-2.5 GHz (%)
Distance
2.4-2.5 GHz
ɳa (2.4GHz)
ɳa (2.5GHz)
ɳa (min)
ɳa (max)
ɳa (average)
FreeSpace
< 2:1
76.0
75.5
74.5
81.2
78.6
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< 2:1
13.8
16.1
13.8
16.1
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24.4
21.6
26.9
25.0
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32.1
33.6
32.1
39.5
37.0
3.2dB
Table 1 Summary of the measured results in free-space and taking into account the hand effect Although the arm is not considered in this analysis, the trend should be similar to the one shown in this paper. Absorption might increase since not only the hand is considered. Radiation patterns taking into account the phantom hand are measured for the three scenarios: touching, 3 mm, and 6 mm apart from the phantom hand (Fig. 7). Although
the phantom hand introduces losses, there are no big nulls in the direction pointing to the hand. However, some attenuation effect is observed. For example, the radiation at =0º is above 5dB to 10dB with respected the radiation at =180º. At the end, radiation patterns are quite pseudo-isotropic which it is a desired feature for smartwatch applications.
Measurement System Set-Up
= 90º Plane XY at 2.45 GHz at 0mm,
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= 0º Plane XZ at 2.45 GHz at 0mm,
= 90º Plane YZ at 2.45 GHz at 0mm,
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Fig. 7 Measured radiation pattern (total normalized gain in dB is shown) 4. Improvements by adding ground layers
In order to minimize the impact of power absorption due to the hand, this section proposed a simple technique to reduce its influence. Since the space for electronics in a smartwatch is limited, it is reasonable to reuse the space by using two stacked PCB. For this case, the original case having the PCB 6 mm above the hand is modified including another PCB with full ground plane underneath. To have both PCB electrically connected, a conductive strip at the edge is proposed (Fig. 8 left). For this new situation, the matching network is redesigned to minimize mismatch losses. The values for the matching network components are Z1=1.8 nH and Z2=1.8 nH following a different topology of Fig. 4. In this case, Z1 is a series component connected to the non-resonant element, and Z2 is a shunt component. This particular configuration enhances the measured total efficiency from the 37.0% (averaged from 2.4 GHz to 2.5 GHz) to 49.1% (Fig. 8 right) being a substantial efficiency increment. This result is close to reported values in [4] where total efficiency with the phantom hand is 65% at the same operating bandwidth but using a total height of 10mm and a conductive wrist which is not always possible. Therefore, results presented here make the solution attractive for smartwatches. 100 90
6mm Single layer
80
6mm Two layers
Total Efficicency (%)
70 60 50
40 30 20 10 0 2.30
2.35
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Fig. 8 Left) PCB 6 mm above the phantom hand, an extra PCB having a ground layer is connected to the top ground through the edge (total gap is maintained to 6mm); right)
total efficiency measured for the PCB 6 mm above the hand and for the same case having the configuration shown in the left figure. Porexpan spacers are used.
It is worth analyzing the current distribution of the two layer solution compared to the single layer (Fig. 3, Fig. 9). It is observed that for the two layers solution, currents are aligned along the ground plane. Since the ground plane is larger and as it is a relevant part to the radiation process, antenna efficiency increases. Furthermore, computed directivity for the single layer solution is 5.1dB whereas is 3.0dB for the two-layers solution which is preferable in order to radiate in a similar way in all space directions.
Fig. 9 Simulated current distribution for the two layers solution at 2.45GHz. Both layers are connected with a rectangular conductive strip at the edge
At this point, it is interesting to compare, the proposed solution with other solutions existing in the prior-art (Table 2). The solutions shown in Table 2 disclose several antenna solutions operating at the 2.4 GHz-2.5 GHz band. It is interesting to observe that the best antenna in terms of antenna efficiency with the hand and at the same time with less antenna volume is the one proposed in [6]. If now, said solution is compared with the one proposed here, one can conclude that the antenna solution has similar antenna efficiency values with the phantom hand but with less volume. However, when looking with more detail the antenna solution at [6], the antenna height is 4mm in
comparison with the only 2.4mm of the present solution which is advantageously for slim smartwatches. Also, the antenna solution in [6] is bent in three parts, thus making the design more complex that the proposed in this paper with a chip antenna component being more attractive in terms of integration into a smartwatch device.
Ref. [3]
Ground clearance (mm2) 344
Antenna Efficiency Metal volume(mm3) With Hand (%) housing 360.0
26 (meas)
Supporting Material
no
FR4 Metal[4]-[5] 0 16619.0 68 (meas) yes housing [6] 0 59.2 50 (sim) no FR4 [7] 25.4 39.9 no FR4 [11] 160 20.0 no Copper Proposed 70 86.4 49 (meas) no FR4 Table 2 Summary of different smart-watch antenna designs for 2.4 GHz band
5. Conclusions A small non-resonant of 12.0 mm x 3.0 mm x 2.4 mm with a small ground clearance area of just 14 mm x 5 mm operating at the 2.4 GHz-2.5 GHz band has been proposed for being integrated in a smartwatch platform. Its performance as a function of the distance between the ground plane and the phantom hand has been analyzed. The human hand mainly introduces detuning effects and efficiency decrements. The closer the hand, the lower the antenna efficiency. A technique to mitigate the hand impact has been proposed consisting on adding an extra ground plane layer underneath the antenna board. When both layers are connected, measured total efficiency is increased from 37.0% (averaged from 2.4 GHz to 2.5 GHz) to 49.1% which is a substantial increment of 1.2dB.
The non-resonant element allows making a small antenna capable of being integrated in this kind of applications featured by significant space constraints. Its versatile design allows retuning the antenna easily through its matching network components rather than through its geometry, thus simplifying its integration process and reducing the workload that antenna customization requires. This makes the non-resonant element attractive for being integrated in multiple devices and applications since the same geometry can be used for each device, only the matching network should be customized. In terms of antenna efficiency, when the grounding is touching the phantom hand, the average efficiency across the 2.4 GHz-2.5 GHz is 15.1% and it increases up to 25.0% and 37.0% just increasing the distance up to 3 mm and 6 mm, respectively. Acknowledgements This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement Nº 674491.
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