I. INTRODUCTION
Nowadays wireless power transmission (WPT), which is one of the most promising research hotspots, is used in many modern applications such as microwave powered aircraft [Reference East1], radio-frequency identification [Reference Kim, Mariotti, Alimenti, Mezzanotte, Georgiadis and Collado2], radio-frequency (RF) energy harvesting [Reference Soboll, Wienstroer and Kronberger3], and also widely developed in the portable electronic devices to realize the intellectualization of domestic life [Reference Mihajlovic, Grundlehner, Vullers and Penders4]. In WPT systems, the rectenna is the key component [Reference Shinohara5]. Many different antenna structures are proposed to improve the efficiency of the whole system. Sun [Reference Sun6] takes advantage of an enhanced rectenna with differential feeding to realize a better gain. However, the coaxial feeding is a non-planar design. In [Reference Song, Huang, Zhou, Yuan and Carter7, Reference Chou, Lin, Weng and Li8], the rectennas with a kind of special slot are used to boost the efficiency over a wide band, but the slots are the three-dimensional (3-D) structures that are not suitable for the integration. Various types of rectenna arrays that provide an excellent RF–DC conversion efficiency have been reported in [Reference Matsunaga, Nishiyama and Toyoda9, Reference Ushijima, Sakamoto, Nishiyama, Aikawa and Toyoda10], while matching to the diode is difficult and the size of the array is a little large.
High-directivity laser or weak-degenerativity electromagnetic wave is usually preferred for WPT. The 2.45 GHz microwave is widely applied in medical or industrial areas because of its low-attenuation and good-transmitting performance, which makes 2.45 GHz one of the best choices as transmitting energy spectrum.
In this paper, a novel 2.45 GHz rectenna unit is proposed, which is easy to set of array. It consists of two parts: one is the receiving antenna with a compact micro strip resonant cell (CMRC), whose harmonic suppression over a wide bandwidth between 3 and 8 GHz can be achieved; the other is the rectifier loaded with an inductance to enhance the RF–DC efficiency. Design and features of the proposed rectennas are described. Finally, a method of realizing large-scale rectenna arrays is depicted.
II. RECTENNA DESIGN
The proposed rectenna printed on a Taconic RF-35 substrate with a relative permittivity of 3.5 and loss tangent of 0.0018 is presented in Fig. 1. The receiving antenna is a microstrip-fed circular ring slot antenna (CSRA) with a CMRC, which would realize wideband harmonic rejection. According to the equation as follows [Reference Wong, Huang and Chen11], the inner radius r 1 and outer radius r 2 can be calculated, respectively:
$$f_0 \approx \displaystyle{c \over {\pi (r_1 + r_2 )}} \times \left( {\displaystyle{{\varepsilon _r + 1} \over {2\varepsilon _r}}} \right)^{{\rm 1/2}}, $$
where c, ε r , π(r 1 + r 2) is the speed light in free space, relative permittivity of the substrate, and mean circumference of the annular-ring-slot antenna, respectively.
Fig. 1. The proposed rectenna with harmonic suppression (r 1 = 10, r 2 = 13.8. Unit: mm).
The core mechanism of harmonic rejection is the microstrip-fed CMRC as shown in Fig. 2, which has been reviewed in [Reference Kurgan, Filipcewicz and Kitlinski12], In order to investigate the property of CMRC, we only adjust c 1 with other parameters fixed in order to simplify the design.
Fig. 2. The designed CMRC (s 1 = 0.1, s 2 = 0.2, s 3 = 0.2, w = 1.13. Unit: mm).
As shown in Fig. 3, changing c 1 could affect the bandpass response of the CMRC. Figure 4 shows the simulated results and the corresponding measured results. The measured bandwidth of less than 3-dB insertion loss and better than 10-dB return loss is approximately 1.5 GHz when c 1 = 13.6 mm. The insertion loss and reflection coefficient are 0.68 and −18 dB, respectively. The return loss in 2nd- and 3rd-order harmonic are 0.56 and 0.69 dB, respectively.
Fig. 3. The effects of c 1 on S 11 (Unit: mm).
Fig. 4. The simulated and measured S-parameter of CMRC (c 1 = 13.6. Unit: mm).
Then the proposed receiving antenna is simulated on high-frequency structure simulator (HFSS), and the simulation result is demonstrated in Fig. 5. Here, a fundamental mode of the antenna is excited at exactly 2.45 GHz. Inset in the Fig. 5 shows the radiation pattern with or without the CMRC under the fundamental mode, which denotes that the CMRC does not have much effect on the radiation pattern. From the Fig. 5, a good reflection coefficient approximately −25 dB is achieved, while the harmonic waves, at the same time, are suppressed that the return loss at 2nd- and 3rd-order harmonic were 0.4 and 0.7 dB, respectively. Notably, the return loss is less than 1 dB over 3–8 GHz.
Fig. 5. The simulated radiation pattern and S 11 of the proposed receiving antenna.
To visualize the harmonic suppression of the proposed antenna, the current distributions in the antenna are plotted in Fig. 6. It is clearly seen that the CMRC has blocked the signal at the 2nd- and 3rd-order harmonic, while only the fundamental frequency goes through successfully.
Fig. 6. Simulated current distribution diagrams of the proposed antenna and slot antenna without CMRC at 2.45, 4.9, and 7.35 GHz.
We have already reported a series-parallel rectifier with an inductance [Reference Kang, Lin, Jiang, Chen and Jiang13], which can develop the RF–DC efficiency notably as shown in Fig. 7. The series-parallel rectifier loads a proper inductance to compensate the capacitance of the diodes so that the loop of the RF can be matched quite well, and it directly connects with the receiving antenna.
Fig. 7. The series-parallel rectifier with an inductance loaded.
III. EXPERIMENTAL RESULT
To validate the concept proposed above, a rectenna operating at 2.45 GHz is fabricated on a 0.5 mm Taconic RF-35 substrate (shown in Fig. 8). The size of the rectenna is 78 × 50 mm2 and the diodes are HSMS-8202.
Fig. 8. Photograph of the fabricated antenna.
Using Agilent E8267D as signal source and standard horn antenna (about 5 dBi gain at 2.45 GHz) as transmitting antenna, we calculated the efficiency of the rectenna by measuring the output DC voltage, as shown in Fig. 9. A power amplifier with a maximum gain of 20 dBi is used so that the available transmit power is large enough, and a standard horn antenna A1 was used as transmitting antenna. Firstly, the received RF power was measured by a spectrum analyzer (Agilent N9030A) at a distance of 15 cm (far-field condition) from the transmitting horn antenna, and the receiving antenna is the designed microstrip-fed CSRA A2. Then the prototype CRSA was replaced by the rectenna A3 at the same location. The RF–DC conversion efficiency is defined as:
$$\eta (\% ) = \displaystyle{{V_{DC}^2} \over {R_L P_R}} \times 100\%, $$
where V DC is the output DC voltage of the rectenna, R L is the load resistance and the P R is the RF power received and measured by the designed microstrip-fed CSRA A2. When the received power is about 10 dBm, the measured results are depicted in Fig. 10. It figures out that the output voltage increases as the load resistance increases while the maximum value of conversion efficiency could achieve up to 70.2% when the optimum load is 1 kΩ. Good agreement is obtained. Moreover, in Table 1, the performance of the proposed rectenna is summarized and compared with the existing reports.
Fig. 9. The measuring experiment. (a) Experimental scheme (b) Experimental photograph.
Fig. 10. Measured results. (a) Output voltage versus load resistance (b). Conversion efficiency versus load resistance (P R = 10 dBm).
Table 1. Comparisons of the fabricated rectenna with the reported ones.
Table 1 illustrates the comparisons between the proposed rectenna and the previously reported ones. Compared with the works, the proposed design has the merits of compact size and relatively high efficiency.
IV. LARGE-SCALE RECTENNA ARRAY
When the transmitting RF energy is weak, antenna arrays as the receiving antenna is preferred. The high-gain antenna arrays could induce high voltage applying to the diodes. Figure 11 shows the simulated reflection coefficient and antenna directivity, respectively. Inset in Fig. 11 demonstrates the radiation patterns of the two antennas almost identical.
Fig. 11. The simulated S 11 and radiation pattern of the two antennas.
In this paper, it is very convenient to stack the proposed rectenna as a large-scale rectenna array, as shown in Fig. 12(a). There is only one rectifier combined with the array, which may limit the performance of the diode because the input voltage of the diode must be lower than the breakdown voltage. Hence, the DC accumulation is a good method to achieve a high-voltage rectenna. Figure 12(b) shows the proposed rectenna array by connecting output DC lines with the ground planes in parallel. Furthermore, it is also possible to improve the performance by increasing the slot ring antenna units in series.
Fig. 12. The large-scale rectenna array. (a) Series-stacked rectenna array (N = 2, 3, …). (b) Parallel-DC connection rectenna array.
V. CONCLUSION
A 2.45-GHz wideband harmonic rejection rectenna for WPT is proposed. The rectenna comprises a microstrip-fed CRSA and a SPR. A CMRC is inverted into the CRSA so that the harmonic suppression over a wide bandwidth (3–8 GHz) can be achieved. The RF–DC conversion efficiency of the SPR improves greatly, especially under the low input power levels by loading a proper compensating inductance. The adopted rectenna built on a low cost Taconic RF-35 substrate is measured. By up to 3rd-order harmonic rejection, the efficiency of the rectenna could achieve 70.2% with optimum load resistance 1 kΩ. Furthermore, the proposed rectenna can easily achieve the large-scale rectenna arrays by connecting output DC lines with the ground planes in parallel or increasing the slot ring antenna units in series. The relevant work is being carried out. There would be a simulation and verification on the concept of large-scale rectenna array in the future. The DC voltage is large enough for mobile phone charging so that an experimental is under way.
ACKNOWLEDGEMENTS
This work was supported in part by NSFC (NO.61571084), in part by NCET (NO.NCET-13-0095).
Zhan Yu Kang was born in Ganzhou, Jiangxi Province, China, in 1991. She received the B.S. degree in Electronic Engineering from UESTC (University of Electronic Science and Technology of China), Chengdu, China in 2009, and is currently working toward the M.S. degree at the UESTC, Chengdu, China. Her research interests include millimeter-wave and microwave devices, circuits, and systems.
Xian Qi Lin was born in Zhejiang Province, China, on July 9, 1980. He received the B.S. degree in Electronic Engineering from UESTC (University of Electronic Science and Technology of China), Chengdu, China, in 2003, and Ph.D. degree in Electromagnetic and Microwave Technology from Southeast University, Nanjing, China, in 2008. He joined the Department of Microwave Engineering at UESTC in August 2008, and has become an Associate Professor and a doctoral supervisor since July 2009 and December 2011, respectively. From September 2011 to September 2012, he was a post-doc researcher in the Department of Electromagnetic Engineering at Royal Institute of Technology (KTH), Stockholm, Sweden. He is currently a Full Professor in UESTC. He has authored over 10 patents, over 40 scientific journal papers, and has presented over 20 conference papers. His research interests include microwave/millimeterwave circuits, metamaterials and antennas. Dr. Lin is a member of IEEE and a reviewer of many well-known journals such as IEEE- MTT/AP/MWCL/AWPL, JEMWA/PIER and EL. He was the recipient of 2011 honorable mention for the national hundred outstanding doctoral dissertations, a 2012 excellent young scholar presented by UESTC, and a 2013 new century excellent talent in University presented by Ministry of Education.
Cong Tang was born in Shangqiu, Henan Province, China, in 1987. He received the B.S. degree in Electronic Engineering from PLA Information Engineering University, Zhengzhou, China in 2010, and is currently working toward the Ph.D. degree at the UESTC (University of Electronic Science and Technology of China), Chengdu, China. His research interests include millimeter-wave and microwave devices, circuits and systems.
Peng Mei was born in Suizhou, Hubei province, China, in 1993. He received the B.S. degree in Electromagnetic Field and Radio Technology from University of Electronic Science and Technology of China (UESTC), Chengdu, China in 2015, and is currently working towards the M.S. degree at the UESTC. His research interests include antenna design, metamaterial, ultra-wideband absorber design.
Wang Mao Liu was born in Anqing, Anhui Province, China, in 1990. He received the B.S. degree in Electronic Engineering from CQUPT (Chongqing University of posts and telecommunications), Chongqing, China in 2010, and is currently working toward the M.S. degree at the UESTC (University of Electronic Science and Technology of China), Chengdu, China.
Yong Fan was born in 1963. He received the B.E. degree from Nanjing University of Science and Technology, Nanjing, Jiangsu, China, in 1985, and the M.S. degree from University of Electronic Science and Technology of China Chengdu, Sichuan China, in 1992. From 1985 to 1989, he was interested in microwave integrated circuits. Since 1989, he dedicated himself to researching and teaching on subjects of Electromagnetic Fields and Microwave Techniques for many years. His main research fields are as follows: electromagnetic theory, millimeter-wave communication, millimeter-wave, and Terahertz Circuits etc. Besides, he was interested in other subjects including Broadband Wireless Access, automobile anti-collision, intelligent transportation intelligent transportation etc. He has authored and co-authored over 130 papers.
