Gain enhancement of transmitting antenna incorporated with double-cross-shaped electromagnetic metamaterial for wireless power transmission

Optik 127 (2016) 6754–6762

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Gain enhancement of transmitting antenna incorporated with double-cross-shaped electromagnetic metamaterial for wireless power transmission Bo Ma a,b , Xiao-ming Yang a,∗ , Tian-qian Li a , Xiao-feng Du a , Ming-yang Yong a , Hong-yuan Chen c , Hang He d , Yuan-wen Chen d , Ao Lin e , Jiao Chen f , Lin Zhou g a

School of Electrical and Electronic Information, Xihua University, Sichuan, Chengdu 610039, China No. 95645 Unit Troops of the People’s Liberation Army, Chongqing, Shapingba District, 400037, China c Alps Electric Co., Ltd. 1-7 Yukigaya-Otsuka-machi, Ota-ku, Tokyo 145-0067, Japan d Logistical Engineering University of the People’s Liberation Army, Chongqing, Shapingba District, 401331, China e No.94627 Unit Troops of the People’s Liberation Army, Jiangsu, Wuxi 214142, China f College of Materials Science and Engineering, Chongqing University, Chongqing, Shapingba District, 400044, China g College of Electronics and Information Engineering, Sichuan University, Sichuan, Chengdu 610065, China b

a r t i c l e

i n f o

Article history: Received 1 March 2016 Accepted 20 April 2016 Keywords: Double-cross-shaped metamaterial Gain Microstrip patch antenna Superstrate Wireless power transmission

a b s t r a c t This paper proposes a novel approach to enhance the aperture efficiency of the transmitting microstrip patch antenna (MPA) by designing a new inclusion of double-cross-shaped metamaterial (DCSM) superstrate applied in microwave wireless power transmission(WPT). Overall WPT model and transmitting antenna selection are briefly introduced and numerical simulation is conducted in HFSS. Homogenization retrieval method based on transmission and reflection coefficients is utilized to calculate the effective parameters of the DCSM unit cell, electric field and surface current distribution are displayed to qualitatively explain the occurrence of negative permittivity and permeability. The Matlab2008a script is also presented to calculate the geometry dimensions of the corresponding conventional patch antenna operating at desired resonant frequency. Simulation results indicate that the gain of the transmitting antenna increases from 7.46 dB to 12.35 dB after loading with the 6 × 3 × 1 array DCSM MTM as the superstrate, the measured gain increases from 7.1 dB to 12.21 dB thus an enhancement of 184% for the transmitting antenna’s aperture efficiency is obtained which is of significant value for the whole WPT system. For the last section of our paper, electric field of the transmitting MPA before and after loading with DCSM superstrate are illustrated to gain insight physical mechanism of the gain enhancement. © 2016 Elsevier GmbH. All rights reserved.

1. Introduction The idea of transmitting electrical energy without any medium from the supply terminal to stationary or relatively movable load, namely wireless power transmission or wireless power transfer, was initially proposed by Nikola Tesla as early as in 1904 [1]. He carried out the first experiment involving wireless power transmission via the electrical field by constructing the huge Tesla Coil on his Wardenclyffe Tower Facility, but finally failed indeed because the energy transmitted was diffused

∗ Corresponding author. E-mail address: [email protected] (X.-m. Yang). http://dx.doi.org/10.1016/j.ijleo.2016.04.107 0030-4026/© 2016 Elsevier GmbH. All rights reserved.

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in all directions and no data were collected on whether any significant amount of power would be available at any distant point in Colorado Springs. Several decades later, the attempt to transmit power via microwave beams was suggested by Peter Glaser in 1964 in the well-known SPS (Solar Power Satellite) project, in which huge amount of renewable and clean solar energy are envisaged to transmit as a microwave beam to the ground station with stringent safety continuously [2]. He also demonstrated in another subsequent research program that a microwave powered helicopter flown for ten hours at an altitude of 50 feet. The helicopter was equipped with a device called a rectenna, which was sustained solely by a 2.45 GHz microwave beam [3]. During the 1980s and early 1990s, a program called SHARP(Stationary High Altitude Relay Platform) was developed to build the first unmanned airplane,flying around a stationary point at the altitude of 22Km,whose energy refueling was planted to be implemented by means of a 2.45 GHz microwave beam transmitted from ground and received on board by a rectenna [4]. In 2007, the resonant coupling experiment conducted by Marin Soljacic and his colleagues in MIT garnered much attention around the world. A 60w light bulb attached to the received coil was lit successfully 2 m away from another transmitting coupling coil [5]. This work has since inspired many other researchers towards the more detailed understanding and wider application of the resonant coupling WPT. One year later, the Hawaii WPT experiment by John Mankins of Managed Energy Technologies set a new distance record in the long-distance WPT research [6]. The distance between the transmitting point to the receiving one is 148 km, but the received power was less than 1% of the transmitted power. The applications of wireless power transmission span over a broad range from portable consumer electronics to powering electric vehicles, as well as pervasively charging sensor or implantable biomedical devices. Generally speaking, wireless power transmission can be classified in the following four categories: inductive coupling [7], resonant coupling [8], microwave [9] and laser [10]. It is well-known that the transfer mechanism of inductive coupling can only take effect within the distance of few centimeters and its power transfer efficiency decrease with distance [7]. The disadvantage of the resonant coupling method is that the two resonant objects aimed to exchange power energy have to be tuned to resonate at the same frequency and it needs very big transfer device if we want the whole system to work in low frequency bands [8]. For short range applications where the transfer distance varies from a few centimeters to a few meters, the near-field inductive coupling or resonant coupling is feasible, but isn’t a viable solution to provide high transfer efficiency for energy transfer over a mid-range or long distance. Therefore, it is worthwhile to gain more attention in mid-range or long-distance wireless power transmission system. Laser owns the advantage of having small beam divergence, but the efficiency in generating the laser beam and converting it back into electrical energy are low compared with the microwave method [10]. The microwave method is the most suitable WPT for applications from mid-range to long-distance transmission with high efficiency. Metamaterials are artificial engineered materials composed of periodic subwavelength particles and exhibit exotic electromagnetic properties which are extensively studied in recent decades [11–16]. The metamaterial is also referred as left-handed material as the vectors E, H and k form a left-handed-like set of systems. In the field of WPT, the MTMs are also reported to increase the efficiency in literatures. However, to the best of the authors’ knowledge, the majority of contemporary WPT schemes applied the MTM into the short distance near-field inducting coupling or resonant coupling system to achieve high transfer efficiency over short distance [17–19]. The metamaterial slab was usually positioned between the transmitting and receiving coils by focusing the propagating waves and enhancing the nearfield evanescent waves. In many cases, however, the space between the coils may not be accessible to incorporate a metamaterial slab. Therefore, further investigation of the MTM based WPT method using a different approach is necessary. In this paper, distinguished from the aforementioned methods, we present the inclusion of double-cross-shaped metamaterial to enhance the aperture efficiency of the transmitting antenna as its superstrate in the mid-range or long-distance WPT system. Compared with the conventional microstrip patch antenna(MPA) which is selected as the transmitting antenna, the DCSM loaded MPA had higher gain and became more directive. The DCSM supertrate can converge the radiation width in the space and reduce half-power width through the suppression of surface wave energy radiation. In the condition that the transmitted power is fixed, the receiving antenna can collect more received power for the transmitting MPA with DCSM loading as its supertrate compared to the case without the DCSM loading. Moreover, this inclusion design can also help reduce the area of the corresponding receiving rectenna array and thus decrease the profile of the whole WPT system. 2. Overall WPT model and transmitting antenna The overall wireless power transmission model mainly consists of the transmitting-receiving-rectifying system as is illustrated in Fig. 1 [20]. The transmitting antenna in the transmitting system is a device which converts the electrical power into plane electromagnetic waves in free space. The receiving system is built mainly for receiving the transmitted microwave power and removing the harmonics presented in received waves. The rectifying system is the main apparatus of transforming the microwave to direct current. As the most significantly important element of the transmitting system, a high-efficiency transmitting antenna is desirable for the whole WPT transmitting system due to its capability of acting as a radio wave transmitter. A number of works have dealt with the efficiency enhancement for the receiving system and rectifying system, the research for the transmitting antenna is still quite limited for the mid-range or long-distance WPT system.

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Fig. 1. Constituent parts of the WPT.

Fig. 2. Geometry of the DCSM structure.

The power in watts received at the receiving antenna is calculated based on the below mentioned Friis equation for the distance of far field [21]. Pr = Pt Gr

  2 0 4d

Gt

(1)

where pt is the source power transmitted, 0 is freespace wavelength of the operating frequency, and d is the distance 2 between the transmitting and receiving antenna. The quantity (0 /4d) is declarative of the loss between the two antennas, independent of atmospheric conditions. Gr and Gt denote the gain of the receiving and transmitting antennas respectively. We can easily conclude that high gain transmitting antenna is the basic need for far-field WPT system and the transmitting antenna has to focus the energy in a narrow angular section in order to avoid undesired power diffusion as much as possible. Higher gain transmitting antenna design resulted in more received power in the receiving antenna based on Eq. (1). To fulfill the purpose of application-driven requirements, high gain and more directive transmitting antenna witnessed more and more attention within the scientific community. The researchers in [22] conducted a preliminary study of the helical antenna design which is proposed to apply in the WPT system. However, the gain of the presented helical antenna is 6.2 dB, which is evidently not satisfying in practical application. A circularly polarized microstrip patch antenna for WPT was studied in [23], but the inherent disadvantage of the low gain of the conventional MPA restricted its wider popularization. Microstrip patch antennas (MPAs) are still widely used in the microwave engineering due to their inherent advantages such as low profile, low weight and can be shaped to fit a particular requirement. Therefore, high gain microstrip transmitting antennas are a good candidate for the transmitting antenna design in WPT system. 3. DCSM unit cell design and numerical simulation Fig. 2 shows the topological structure of the DCSM unit cell. The presented copper unit cell is etched on both sides of a 1 mm thick FR4 rectangular-shaped dielectric substrate with dielectric constant of 4.6 and dielectric loss tangent of 0.02 by standard printed circuit board lithography technology. The design parameters are Ws = 8 mm,Wn = 1 mm,Ls = 10 mm,Ln = 1 mm. In the simulation model setup in HFSS, the DCSM unit cell is filled in a waveguide box structure and the perfect electric conductor(PEC) and perfect magnetic conductor(PMC) boundary conditions are assigned along the Y-axis and X-axis direction respectively. The input/output ports, namely Port1 and Port2 are defined to the up-down faces of the waveguide box along the Z-axis direction. By way of this condition setting, the DCSM unit cell is assumed to extend to infinity in the lateral directions and the incident, reflected and transmitted waves are in the form of a plane waves. The scattering parameters are simulated over the sweep frequency range of 5–15 GHz, the linear step size is set as 0.01 GHz and the solution center frequency is set as 10 GHz. The simulated magnitudes and phase spectra of the DCSM unit cell are displayed in Fig. 3. It shows that the simulated −10 dB bandwidth is 1.6 GHz, covering from 9.71 GHz to 11.31 GHz and the transmission peak is around 10 GHz which is validated in the dotted S21curve line in Fig. 3(a). It can also be observed that a change in sign occurs abruptly(from negative

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180 S11

150

0 S11 S21

-5

90

-10 S-parameters phase(degrees)

S-parameters magnitude(dB)

S21

120

-15 -20 -25 -30 -35 -40 -45

60 30 0 -30 -60 -90 -120 -150 -180

-50 5

6

7

8

9

10

11

12

13

14

5

15

6

7

Frequency(GHz)

8

9

10

11

12

13

14

15

Frequency(GHz)

(a)

(b)

Fig. 3. The S parameters. (a) for magnitude. (b) for phase. 5

real(permittivity) real(permeability)

4 Effective Parameters

3 2 1 0 -1 -2 -3 -4 -5 5

6

7

8

9

10

11

12

13

14

15

Frequency(GHz)

Fig. 4. The real part of the effective parameters.

to positive) for the transmission phase(S21) around 10 GHz and the dip in transmission phase may be used to predict of the left-handed frequency bands[24]. 4. Effective parameters retrieval and theoretical explanation In order to verify our prediction of the existence of the left-handed frequency bands based on the simulated scattering andpermeability) of the proposed parameters in Fig. 3, we calculated the constitutive parameters(the effective permittivity  2 + S2 DCSM structure according to the following four equations [25], in which X = 1 − S11 / (2S21 ), k0 denotes the wave 21 number of the incident wave in free space,and d represents the thickness of the slab sample.



z=±

[(1 + S11 )2 − S21 2 ]/[(1 − S11 )2 − S21 2 ]

exp (ink0 d) = X ± i



(1 − X)2

(2) (3)

ε = n/z

(4)

=n×z

(5)

Fig. 4 shows the results of the extracted effective parameters, in which the real part of the permittivity and permeability became negative simultaneously roughly from 9.89 GHz to 10.24 GHz and it is also referred as the so-called double negative property of the MTM. At the same time, the left-handed frequency bands around 10 GHz also confirm the validity of the conclusion drawn in [24] and conversely prove the correctness of our calculated results. Taking a further step, to interpret and understand the insight physical mechanism of the above-mentioned double negative property of the DCSM unit cell, we illustrate the electric field and surface current distribution of the proposed LHM structure. The long continuous metal strip acts as a low frequency plasmon system that is larger than the MTM operating frequency [26]. In response to the electric field, the metal strip along the Y-axis behaves like the cut-wire. The electric field shown in Fig. 5 in the surface side of the DCSM unit cell is very strong, which indicates a strong electric resonance that accounts for the expectation of a negative effective permittivity in the resonance frequency range. On the other hand, the observed surface current distribution on the upper side of the DCSM are antiparallel to that on the bottom side everywhere. Thus, the upper and bottom side’s antiparallel surface currents shown in Fig. 6(a) and (b) respectively form an equivalent virtual current loop with the aid of the displacement current that flows in the FR4 substrate. The resulted current loop

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Fig. 5. The electric field distribution on the surface of the DCSM unit cell.

Fig. 6. The surface current distribution. (a)for upper side. (b)for bottom side.

generates an induced magnetic field opposite to the external magnetic field, it is responsible for the artificial magnetism of the structure and can be regarded as a magnetic dipole [27]. It is well-known that the magnetic dipole moments can exhibit magnetic resonance under incident electromagnetic waves, thus negative effective permeability is expected to occur in the resonance frequency. By carefully adjusting the structure parameters, the electric and magnetic resonance can simultaneously occur in the same frequency and the effective negative permittivity and permeability can simultaneously exist in the same frequency bands, therefore the DCSM is expected to be a candidate of metamaterial. 5. High gain MPA configuration and results discussion The design parameters of the corresponding conventional MPA configuration can be calculated based on the following Matlab2008a script. The script can be utilized to calculate the overall geometry dimensions of the corresponding MPA operating at the desired resonant frequency. The patch size dimensions are optimized specifically to select as 13.2 mm width and 8.3 mm length to ensure that the center frequency locates at around 10 GHz. The patch is printed on a F4B substrate with thickness of 1 mm, permittivity of 2.65 and loss tangent of 0.02. A 50  coaxial probe which is used to feed the MPA was situated at the center of the rectangular patch along the X-axis, and 2.68 mm away from the Y-axis in the Cartesian coordinate. It deserved to note that the left-handed frequency band located around the operating resonant frequency of the MPA to ensure the good transmission match between the MPA and the DCSM array superstrate. clc c=3*10ˆ8;% light speed f=10*10ˆ9;% resonant frequency eps = 2.65;% the permittivity of the substrate sub = 1;% the thickness of the substrate W=0.5*c*[(2/(eps+1))ˆ(1/2)]/f*1000 % the width of patch A=1/sqrt(1+12000*sub/W); eps eff=[(eps+1)/2]+[(eps-1)/2]*A;% the effective permittivity of the substrate D1=(eps eff+0.3)*(1000*W/sub+0.264); D2=(eps eff-0.258)*(1000*W/sub+0.8); L0=0.412*sub*D1/D2; % the length of the equavilent radiating slot b = sqrt(eps eff); c=500*c/(b*f);

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Fig. 7. The simulated proposed MPA with MTM superstrate.

Fig. 8. Polar coordinate 3D simulated gain. (a)withou MTM. (b)with MTM.

L=c-2*L0 % the length of the patch h=1/[sqrt(1+12*sub/L)]; re=(eps eff+1)/2+(eps eff-1)*sub/2; Yfeed=0.5*L/(sqrt(re)) % the position of the feed point along the Y-axis The 6 × 3 × 1 array MTM is placed above the MPA with the separation distance of 15 mm in order to meet the optimum performance of the MTM loaded MPA as is shown in Fig. 7. The metallic 6 × 3 × 1 array MTM is 60mm × 60 mm, corresponding to 2␭10 × 2␭10 , in area, where ␭10 is the free-space wavelength at 10 GHz. Fig. 8(a) and (b) shows the three dimensional polar coordinate gain of the MPA before and after using the DCSM array as its superstrate. We can see that the maximum gain of the conventional MPA is 7.46 dB and the proposed composite MPA is 12.35 dB, which adds 4.89 dB. It is apparent to observe that the effect of the DCSM array superstrate on the transmitting MPA is a complex reshaping process at first sight. The main beam width of the forward direction has been compressed sharply, indicating that the gain enhancement is satisfying. Fig. 9(a) shows the fabricated MTM superstrate MPA configuration. The superstrate is fixed above the MPA using four plastic nylon column at the corners of the substrate slab. The simulated and measured reflection coefficients of the transmitting MPA before and after loading with the MTM superstrate are given in Fig. 10. The antenna was measured using an Agilent N5230A vector network analyzer (VNA). The magnitude of the simulated S-paremeters matches quite well with the measured results in both cases for the unloaded and loaded superstrate MPA. The resonant frequency for the four cases are all located at 10 GHz and the reflection coefficients of the four types do not change much. It illustrates that the proposed MCSM structure superstrate has little effect on the impedance matching of the loading antennas. The little frequency shift is mainly induced by the alignment of the measurement setup. Fig. 9(b) shows the fixed transmitting MPA in anechoic chamber room environment surrounded by artificial absorbers. The normalized radiation pattern demonstration of the simulated and measured gain for the presented MTM superstrate MPA in comparison with the bare transmitting MPA at the resonance frequency are depicted in Fig. 11. The half-power bandwidths with the E-plane are 69◦ and 20◦ before and after loading with the superstrate for the simulated patterns and 72◦ and 22◦ for the tested patterns as shown in Fig. 11(a). As for Fig. 11(b), the half-power bandwidth within the H-plane are 80◦ and 19◦ for the simulated unloaded and loaded superstrate case and 84◦ and 22◦ for the tested results respectively. The obtained measured results are in a good agreement with the simulations for both E-plane and H-plane, but we can also observe some discrepancy in the presented results, it can be due to the manufacturing process inaccuracy and far-field measuring system losses.

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Fig. 9. The photo of fabricated MTM MPA (a) and its measurement environment (b).

Fig. 10. The simulated and measured S11 before and after loading with MTM superstrate.

Fig. 11. The simulated and tested radiation pattern. (a)E-plane. (b) H-plane.

The spacer distance between the substrate and the superstrate exerts significant influence on the gain enhancement since its input impendence is quite sensitive to the spacer distance[28]. Some more experimental measurement at different spacer distance were carried out as shown in Fig. 12 and it achieves the optimum performance at the distance of 15 mm which has been verified at the previous section. The gain enhancement is of significant value for increasing the overall transmission efficiency as the aperture efficiency is greatly related to the gain of the antenna as is revealed in the following Eq. (6) [21], in which AP is the area of the antenna’s physical aperture, Aeff is the area of the antenna’s effective aperture,  is the wavelength of the operating frequency and G denotes the linear gain of the corresponding transmitting antenna. The following Eq. (7) is used to calculate the aperture efficiency. The linear gain before and after loading with the superstrate for measured maximum logarithmic gain(7.1 dB) is 5.2, and 16.65 for 12.21 dB, which are both lower than the simulated gain. In other words, the linear gain increased from 5.2 to 16.65, approximately additional 184% enhancement is obtained. That

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13 12

Gain(dB)

11 10 9 8 7 6 10 11 12 13 14 15 16 17 18 19 20 21 Spacing distance(mm)

Fig. 12. Measured gain at different spacing distance.

Fig. 13. Electric field distribution. (a)without MTM. (b)with MTM.

means the effective aperture is extended, aperture efficiency has increased by 184% after loading with the MTM superstrate and the enhancement will benefit for the improvement of the whole WPT transmitting efficiency. G=

4 4 Aeff = 2 Ap 2 

(6)

=

G 2 Ap × 4

(7)

The pictures shown in Fig. 13(a) and (b) are intended for further exploring and explaining physically the performance improvement of the microstrip antenna with the MTM loading as its superstrate at the operating frequency. It is easy to observe the great difference of the electric field distribution before and after loading with the MTM superstrate. The electric field distribution in Fig. 13(b) is more dense and congregated than that in Fig. 13(a) and the electric field intensity of the superstrate loaded MPA is more stronger and powerful than that of the unloaded one. The simulation shows a channeling or paraxial focusing of the wave energy due to the presence of the DCSM superstrate. The MTM superstrate has collimating effect on the electromagnetic wave’s transmission direction, which is similar to the congregation effect of convex lens to the radiation of light wave. The surface wave excited by the edge of the radiating patch has been suppressed greatly to some extent and the forward energy transmitting has been restricted to a narrow space to reduce the power diffusion. As referred in papers [29–31], the MPA without the MTM loading generates sphere-like waves, which means the low directivity, while the MPA with MTM superstrate generates plane-like waves concentrating to the perpendicular direction with respect to the patch ground plane, thus high directivity is achieved. The DCSM array superstrate can be employed to acts as a converter which transformed the sphere-like wave into beamlike or plane wave. The MTM controlled the electromagnetic wave propagation direction, forward radiation along the propagating direction was enhanced while the sideward radiation reduced greatly. The main lobe of the microstrip antenna with MTM superstrate points towards the Z-axis direction more evidently which are validated by plotting Fig. 13, indicating that the congregation effect in intensified gradually to radiation direction. Hence, the MPA with enhanced gain can be obtained by loading with the MTM superstrate. 6. Conclusions In this paper, differentiated from inductive or resonant coupling methods applied in wireless power transmission, we designed a new DCSM structure to form a 6 × 3 × 1 array superstrate incorporated in the microstrip transmitting antenna. Popular homogenization retrieval method was utilized to calculate the effective parameters based on the HFSS simulated S-parameters. The left-handed frequency bands locate around 10 GHz as predicted. Electric field and surface current distribution are displayed to explain the occurrence of the negative permittivity and permeability of the DCSM structure. Simulation results indicate that the gain of the MPA with superstrate achieved the maximum of 12.35 dB, increased by 4.89 dB com-

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pared with the unloaded MPA while the measured gain increased from 7.1 dB to 12.21 dB. The gain enhancement results in 184% improvement for the transmitting antenna’s aperture efficiency. Electric field of the transmitting MPA before and after loading with DCSM superstrate are also illustrated to gain insight physical mechanism of the gain enhancement. This work may help to provide another novel method for designing transmitting antenna applied in wireless power transmission. Acknowledgments The work was supported by Ministry of Education “chunhui plan” (Z2012027), the Key Fund Project of Sichuan Provincial Department of Education (16ZA0154), the Key Fund Project of Xihua University (z1320926) and the Innovation Fund of Postgraduate, Xihua University (ycjj2015104 and ycjj2016059). The authors would also like to thank Yong-jun Huang at the School of Communication and Information Engineering in University of Electronic Science and Technology of China for useful discussions and Hong Wan at Xihua University for his kind assistance in paper format revision. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

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