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Design of a rectenna system for GSM‐900 band using novel broadside 2 × 1 array antenna

2017; Institution of Engineering and Technology; Volume: 2017; Issue: 6 Linguagem: Inglês

10.1049/joe.2017.0095

2051-3305

Manish Singh, Sachin Agrawal, Manoj Singh Parihar,

Full-Duplex Wireless Communications

The Journal of EngineeringVolume 2017, Issue 6 p. 232-236 ArticleOpen Access Design of a rectenna system for GSM-900 band using novel broadside 2 × 1 array antenna Manish Singh, Manish Singh Disciple of Electronics and Communication Engineering, Indian Institute of Information Technology Design & Manufacturing Jabalpur, Jabalpur, IndiaSearch for more papers by this authorSachin Agrawal, Corresponding Author Sachin Agrawal bitssachin.agrawal@gmail.com Disciple of Electronics and Communication Engineering, Indian Institute of Information Technology Design & Manufacturing Jabalpur, Jabalpur, IndiaSearch for more papers by this authorManoj Singh Parihar, Manoj Singh Parihar Disciple of Electronics and Communication Engineering, Indian Institute of Information Technology Design & Manufacturing Jabalpur, Jabalpur, IndiaSearch for more papers by this author Manish Singh, Manish Singh Disciple of Electronics and Communication Engineering, Indian Institute of Information Technology Design & Manufacturing Jabalpur, Jabalpur, IndiaSearch for more papers by this authorSachin Agrawal, Corresponding Author Sachin Agrawal bitssachin.agrawal@gmail.com Disciple of Electronics and Communication Engineering, Indian Institute of Information Technology Design & Manufacturing Jabalpur, Jabalpur, IndiaSearch for more papers by this authorManoj Singh Parihar, Manoj Singh Parihar Disciple of Electronics and Communication Engineering, Indian Institute of Information Technology Design & Manufacturing Jabalpur, Jabalpur, IndiaSearch for more papers by this author First published: 14 June 2017 https://doi.org/10.1049/joe.2017.0095Citations: 9AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract In this study, a rectenna operating at the GSM-900 frequency band has been fabricated and tested. This rectenna composed of a 2 × 1 T-shaped monopole array antenna and an energy processing circuit. In order to reduce the gap between adjacent antenna elements in the array structure, the proposed array antenna uses a ground stub. Compared with other array antennas, the proposed array antenna with the ground stub reduces the size up to 50% without affecting the gain and bandwidth. An antenna prototype is fabricated and experimentally tested. The measured antenna's gain and bandwidth are 3.2 and 152 MHz, respectively, hence showing its suitability for radio-frequency (RF) energy harvesting application. For this to be feasible, the developed array antenna is matched with the rectifier at GSM-900 using a single stub matching network. The measured result demonstrates that the proposed rectifier circuit offers the conversion efficiency of 21.2 and 63.6% for an input power of −20 and 0 dBm, respectively. Finally, the rectifier performance is attested experimentally with the developed array antenna. The rectenna's measured RF-to-dc conversion efficiency was found to be 60% at the far-field distance from the transmitting antenna. 1 Introduction Traditionally, most of the wireless sensor nodes operate by energy supplied from a battery, which has a limited lifetime and needs to be repeatedly charged or timely replacement. Radio-frequency (RF) energy harvesting where electromagnetic energy converted to dc energy is one of the most popular techniques for powering the sensor nodes [1]. Since ambient RF energy is widely broadcasted from numerous reliable electromagnetic resources hence it can ensure relatively predictable energy supply than the other harvesting techniques. Energising the sensor nodes through RF energy harvester truly alleviates the frequent battery replacement problem or even it can reduce the battery requirement altogether. However, despite the revolutionary growth in wireless technology the power density of the RF signal is still very low. Thus, it has become a challenge to increase the rectenna output voltage by harvesting as much energy as possible in such low power density environment. Considerable research effort has been placed towards the high-efficiency rectenna design using the different types of antennas such as multiband antennas [2, 3], wideband antennas [4, 5], dipole antennas [6, 7], circularly polarised antennas [8, 9] and so forth. Among them, the idea of wide/multiband rectenna attracts a lot of attention as it promises high conversion efficiency by coherently capturing more RF energy. However, compared with single band wideband energy harvester should meet both impedance matching and voltage boosting at a time [10]. Since rectifying diode is a strong function of frequency and input power hence it tends to poor impedance match and low voltage boosting for the wide frequency band. Thus, in order to collect a large amount of power, i.e. for increasing rectenna efficiency many rectennas can be connected in a series combination, forms a rectenna array [11-15]. Array antenna can be designed to enlarge the received power to the diode by focusing the beam towards the transmitter antenna. In [13], a 2 × 2 patch antenna array achieved the efficiency of 56% for an input power of 20 dBm. In [14], a rectenna array was used and 20% efficiency reported for 13.27 dB input power. In this paper, a broadside T-shaped 2 × 1 array antenna has been designed for GSM-900. The particular band is chosen because it carries a comparatively large amount of power among the available frequency band. Moreover, a processing circuit is fabricated which consist of single stub matching network and a diode detector. The measurement results agree well with the simulation results. To verify the performance of the designed rectenna, it is practically tested where 60% conversion efficiency is achieved for a received power of 0 dBm. This paper is organised as follows. Section 2 discusses the design approach of an array antenna followed by the comparison between simulation and measurement results. Section 3 describes the simulation and measurement results of the power conversion circuit and finally validates the design practically. 2 Design of 2 × 1 array antenna Figs. 1 a and b show the layout and fabricated proposed T-shaped broadside 2 × 1 array antenna, respectively. For fabrication, the conventional method of optical photolithography was performed on a RT/Duroid substrate with the thickness of 0.787 mm and dielectric constant (ɛr) of 2.3. The designed array antenna is fed by 50 Ω microstrip line. All the simulations and optimisations are carried out in CST (Computer Simulation Tool) 2014 [16]. As seen in the proposed antenna, a stub is introduced in the ground plane which acts as a reflector that helps to avoid the electromagnetic interference or coupling between antenna's elements. It also improves the impedance performance of an array antenna or in other words it improves the S 11 parameter from −16 to −40 dB. Fig. 1Open in figure viewerPowerPoint Layout and fabricated proposed T-shaped broadside 2 × 1 array antenna a Layout of 2 × 1 array antenna b Fabricated array antenna Fig. 2 a illustrates the simulated and measured S -parameter of the array antenna with and without the ground stub. It confirms that the ground stub improves the return loss of array antenna as compared with antenna without ground stub. Furthermore, the measurement result shows the proposed antenna achieves −10 dB bandwidth of about 152 MHz from 828 to 970 MHz, i.e. the entire GSM-900 band. Previously, it has been reported that for an array structure the gap between two elements should be in the range of 0.75λ –0.85λ in order to maintain the high gain and side lobe levels reduction [15]. In a compact design, a small element separation aggravates the problem of mutual coupling between the radiating elements. In literature, a variety of techniques have been proposed to reduce both, the mutual coupling and the element separation [17-19]. In [17], the element separation is reduced to λ /10 using the passive decoupling and matching network method for the ground plane size of 0.6λ × 0.6λ. In [18], uniplanar compact electromagnetic band gap (EBG) structure was used to reduce the mutual coupling with an element separation of 0.63λ. Partially extended ground plane – a decoupling structure – is used for two-port printed monopole array in [19]. The total ground plane size of 1.01λ × 0.58λ is used for an element separation of 0.25λ. However, all these designs used an electrically large ground plane. In this work, a stub is introduced in the ground plane to reduce the element separation to 0.31λ (50% shorter than [15]) with a total ground plane size of 0.29λ × 0.41λ, which is 39% smaller than [17]. This gap reduction does not influence the antenna performance anyhow rather it helps to miniature the array antenna. Fig. 2 b shows the measured gain of a single element antenna and an array antenna. As seen the array antenna achieves a maximum gain of 3.2 dB over the entire GSM band. Initially, the length of T-shaped branch has been preferred to approximately half of the wavelength at the resonant frequency. After optimisation, the array antenna parameters are (in mm): L = 100, W = 140, a = 68, b = 24.6, c = 4.5, d = 8, e = 38.8, f = 39 and g = 24. Figs. 3 a and b demonstrates the simulated and measured H - and E -field pattern of the proposed antenna, respectively. The simulated patterns are in good agreement with measured one. It is seen that radiation pattern is omni-directional, which helps to receive the signal from any direction. The half power beam width is 86° that offer a degree of freedom in orientation of the antenna array. Fig. 2Open in figure viewerPowerPoint Simulated and measured S-parameter of the array antenna with and without the ground stub a Simulated and measured S -parameter of 2 × 1 array antenna b Measured gain of single and array antenna Fig. 3Open in figure viewerPowerPoint Simulated and measured radiation pattern of 2 × 1 array antenna a H -field b E -field 3 Antenna application in RF energy harvesting A rectenna circuit is designed and tested using the developed array antenna. The schematic diagram of the rectifier circuit is shown in Fig. 4 a. One of the crucial requirements of the energy harvesting circuit is to transfer the total received power by an antenna to the rectifier circuit. Due to non-linear behaviour of diode, harvesting circuit itself exhibits non-linearity, i.e. its input impedance varies with received RF power. In this situation, the circuit performance can be controlled by the introduction of matching network between the rectifier and the antenna. An appropriate impedance matching is possible by selection of proper matching topology and its components values. It can be designed either with lumped elements (resistor, inductor and capacitor) or distributed elements (microstrip lines). However, in this situation slight change in the matching circuit parameter may alter the impedance mismatch which results in performance degradation. Thus, in order to avoid any impedance mismatch due to the small difference in elements value, stub matching is used for impedance matching. In this case, not only fabrication and optimisation process becomes so easy but the cost will also reduce. In this design process, the traces d 3, d 4 between diode and capacitor C 1 and C 2, d 6 between diode and load, and d 5 between diode and ground are adjusted to optimise the impedance matching as well as the conversion efficiency of the rectifier. Besides the matching network, the appropriate diode plays an important role in harvesting circuit performance. Since the RF energy resources are usually of low power region, the peak voltage of the signals in this region is much smaller than rectifier diode turn-on voltage [13]. Thus, a diode with very low turn-on voltage and high switching speed is required. A Schottky diode which consists of metal semiconductor junction offers low forward voltage and high switching speed. Further, its low forward voltage drop leads to lower power loss as compared to ordinary PN junction diodes. Therefore, in this work Schottky diode HSMS-285C with V th = 150 mV, Cj = 0.18 pF, Rs = 25 Ω is used for rectification. Fig. 4Open in figure viewerPowerPoint Layout and fabricated rectifier circuit a Layout of rectifier circuit b Fabricated rectifier circuit In [20], it has been demonstrated that the number of rectifying diodes or equivalently voltage multiplier stages are very much sensitive to the RF-to-dc efficiency. In low power region (≤ −20 dBm), efficiency decreases if voltage multiplier stages are increased, while for higher power region (≥ −20 dBm), an opposite effect occurs. As the demand is to harvest energy in low power region, therefore, single-series circuit with a double diode is used to convert received RF energy into dc voltage. Fig. 4 b shows the photograph of rectifier circuit fabricated on a lossy FR-4 substrate of dielectric constant (ɛr) 4.3 and height (h) 1.5 mm. The simulation has been performed in Advance Design System (ADS) 2014 [21]. The simulated and measured return loss of the rectifier circuit is shown in Fig. 5 a. The measured result is in good agreement with the simulated one. A slight difference can be accounted due to fabrication imperfections. It can be noticed that the measured impedance bandwidth for |S 11 | ≤ −10 dB covers a frequency range from 830 to 960 MHz, whereas simulated bandwidth covering the frequency range from 830 to 930 MHz. Impedance matching is a strong function of input power, due to non-linearity of the diode. This characteristic can be observed in Fig. 5 b, where rectifier's measured |S 11 | changes by varying the input power level. From figure, it can be noticed that rectifier circuit can sustain an optimal impedance matching for a broad range of input power. The impedance bandwidth is increasing considerably by increasing the input power level. Fig. 5 c shows the simulated and measured input impedance of the rectifier circuit. It can be noticed that in GSM-900 band, the measured input impedance retain the better impedance matching than the simulated one. This difference may be due to the fact that in the measurement actual rectifier diode (Schottky) is being used and in simulation spice model of the same is used which may not be so accurate. From measurement results, it can be observed that the proposed rectifier circuit obtain the almost pure resistance throughout the band. Fig. 5 d shows the simulated and measured output voltage as well as the efficiency of the rectifier circuit. It achieves the maximum output voltage of 4.98 V for an input power of 10 dBm. The result demonstrates that the maximum measured RF-to-dc efficiency is equal to 66%, whereas the simulated efficiency is 72% for an input power of 0 dBm and the load resistance of 4.7 kΩ. The photograph of the rectenna circuit is shown in Fig. 6. To investigate the rectenna performance, a measurement setup has been arranged in an appropriate manner. Fig. 7 shows the block diagram of the arrangement, where a signal generator is used to generate the signal and an antenna is used as a transmitter. Here, Agilent's vector network analyser (VNA)-E5071C is used as a power source or signal generator. At receiving site, the received power is measured at a far-field distance of 200 mm by connecting the proposed array antenna to the Agilent's spectrum analyser. Then after, the spectrum analyser was removed and rectenna (antenna + rectifier) is placed at the same location and then dc voltage is measured across the load R of 4.7 kΩ. Though, the signal is generated by the signal generator and transmitted by an antenna than after it is received by rectenna. During this process free space path loss appears, thereby the received signal strength which is transmitted from the antenna is quite less than the transmitted one. Fig. 8 compares the rectifier and rectenna's RF-to-dc conversion efficiency. A small difference is observed between rectifier and rectenna's efficiency. The result shows that the rectenna circuit achieves the maximum efficiency of 60% for a received power of 0 dBm. Fig. 5Open in figure viewerPowerPoint Simulated and measured return loss of the rectifier circuit a Simulated and measured |S 11 | of rectifier circuit b Measured |S 11 | versus input power c Simulated and measured input impedance d Simulated and measured output voltage and efficiency of rectifier circuit Fig. 6Open in figure viewerPowerPoint Photograph of proposed rectenna circuit Fig. 7Open in figure viewerPowerPoint Complete measurement setup for rectenna circuit Fig. 8Open in figure viewerPowerPoint Measured RF-to-dc conversion efficiency of rectifier and rectenna circuits 4 Conclusion The paper describes the design of an RF-to-dc energy harvesting circuit. 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