Introduction
Energy harvesting solution is proposed as a durable power source for different wireless sensor nodes distributed in a Wireless Sensor Network. This solution can bypass the main drawback for wireless sensor nodes, which is the life cycle of the primary batteries [Reference Xie, Yang and Geyi1–Reference Muncuk2]. The ambient radio frequency (RF) energy harvesting can be obtained at all places and it could be found at the night times rather than other energy harvesting. RF energy harvesters include a receiving antenna, band pass filter, a matching network, a rectifier, and a terminal load [Reference Tung3–Reference Barcak and Partal5]. A printed antenna was used to harvest the RF ambient power from the GSM system to feed the low power temperature sensor of STLM20 [Reference Din, Chakrabarty, Bin Ismail, Devi and Chen6]. To increase the harvest power the array antennas are used [Reference Elsheakh, Elsadek, Abdallah, Elhenawy and Iskander7–Reference Woelders and Granholm9]. However, the majority of the wireless communication systems are linearly polarized [Reference Guo, Yang, Shi and Chen10], as the complex environment and multipath effect of the polarization of the signal can be changed. Moreover, depolarized leads to the decrease of the efficiency of the harvest energy system. This problem can be solved by using a dual polarized receiving antenna because it can collect the electromagnetic waves regardless of the angle of polarization of the incident electromagnetic waves. Microstrip antenna is a better solution for the design of dual linear polarized (DLP) antennas and arrays because of this antenna element or array it has to be low profile and lightweight [Reference Barcak and Partal5].
There are many publications on DLP arrays previously. However, majority of the works have multilayers, dual ports, large size and complex structure which is very difficult to be integrated with the rectifier and matching network in RF energy harvesting systems [Reference Zhang, Yang, Chen, Guo and Nie11–Reference Liang, Hong, Zhao and Wu14]. Table 1 shows the comparative designs between previous research and our designed DLP array.
Table 1. The comparison of the proposed antenna and other researches
In this paper, a low-profile dual polarized 2 × 2 planar monopole antenna array is shown in Fig. 1 with a single layer and a single feeding port, designed for RF energy harvesting. This array is simulated, fabricated, and measured. This paper is organized as follows. The section “Antenna design and results” presents the analysis of single antenna element design and its simulation and measured results. In the section “Dual linearly polarized 2 × 2 antenna array”, the proposed antenna array design, and its simulation and measured results are discussed. The section “Conclusion” summarizes the results of the proposed array.
Fig. 1. The structure geometry of the proposed 2 × 2 array antenna. (a) Top view. (b) Bottom view.
Antenna design and results
The starting point in the designing of the antenna array is the single antenna element as shown in Fig. 2. This figure shows the proposed single element design steps with the 3D configuration of the final design and its fabricated photo for upper and lower substrate layer, respectively. It consists of a modified ground plane with rectangular-shaped slot and V-shaped planar monopole radiator. The design steps start with conventional rectangular monopole as shown in Fig. 2(a). A V-shaped is etched on the monopole antenna as shown in Fig. 2(a) in order to reduce the antenna size and improve the antenna bandwidth by increasing the electrical length and creating closely staggered resonant modes. Then the modified ground plane has a rectangular-shaped slot with dimensions, L s × W s, is used as shown in Fig. 2(a) to improve the antenna matching and gain. The antenna is printed on a Roger RO4003 substrate with dielectric constant of 3.55, loss tangent of 0.002, and substrate thickness of 1.525 mm as shown in Fig. 2(b).
Fig. 2. (a) Design steps, (b) 3D-geometry, and (c) photo of the fabricated single antenna element, top, and bottom layer.
The reflection coefficient of the different design steps is shown in Fig. 3(a). Then the proposed monopole antenna dimensions are optimized by the changing angle Φ of the V-shaped and the length and width (L s and W s) of the slot on the ground plane. The parametric analysis states with angle Φ is changed from 30° to 90° with step 20°. Figure 3(b) shows that when Φ decreases, the impedance matching improves and the resonant frequency decreases. This is due to as the angle Φ increases, the electrical length and the current path decrease. In addition, the effect of the slot area in the ground plane reduces. The effect of length and width of the slot on the ground plane are shown in Figs 3(c) and 3(d), respectively. By increasing the slot length and width, the antenna impedance bandwidth and matching are improved. All simulations are done by using finite element 3D electromagnetic simulator, High-Frequency Structure Simulator Ansys ver. 19. The optimized antenna dimensions are listed in Table 2. To verify the simulation results the antenna element is fabricated and measured as shown in Fig. 2(c). Figure 4 presents the simulated and measured |S 11| of the proposed antenna element, which the measured result agrees well with the simulation result.
Fig. 3. (a)The simulated design steps and (b)–(d) the effect of |S11| of different values of Φ, L s and Ws, respectively.
Table 2. Optimized antenna dimensions (unit in mm)
Fig. 4. (a) The comparison between the simulated and measured reflection coefficient versus frequency and (b) the antenna element gain.
Figure 4 also indicates that the operating frequency of the antenna ranges from 1.35 to 6 GHz for |S 11| < −6 dB, and ranges from 1.5 to 3.6 and 4 to 6 GHz for |S 11| < −10 dB. That is suitable for RF energy harvesting at GPS 1570, GSM 1800, UMTS 2100, Wi-Fi 2.4, LTE 2600 bands, and WLAN 5.2. The antenna gain is also measured and compared with the simulated results as shown in Fig. 4(b). The proposed antenna element has a maximum gain of 4.75 dBi at 2.5 GHz and average gain around 3 dBi over the operating frequency band.
Dual linearly polarized 2 × 2 antenna array
The design of the DLP antenna array should be simple and planar in order to integrate with the RF energy harvesting system. As a result, a 2 × 2 DLP antenna array is designed based on the antenna element in Fig. 2. The antenna elements are oriented such that the antenna array supports DLP as shown in Fig. 1. The feeding network for the proposed 2 × 2 DLP antenna array is shown in Fig. 5(a). The distances between the antennas in vertical and horizontal polarization antenna are 124 and 136 mm as 0.5λ o and 0.55λ o, respectively, at 2.4 GHz. It consists of 1-to-4 Wilkinson power divider [Reference Eltresy, Elsheakh, Abdallah and Elhenawy15]. The power divider is designed to be broadband, as shown in S-parameters in Fig. 5(b), to cover the frequency bands from 1 to 3.8 GHz in order to include most of the required wireless communication standards' frequencies. Figure 5(c) shows that the phase response of the four ports and the difference between port 2 with port 3, and port 4 with port 5. This figure shows that port 2 is in phase with port 3 as well as port 4 is in phase with port 5 and these two ports are 180° phase difference and the first two ports at 2.4 GHz. Then the monopole antenna elements are added as shown in Fig. 1 where the four elements in the proposed array are arranged to produce vertical and horizontal polarized radiation patterns. There is 180° phase shift between antenna element number 1 and antenna element number 3 which makes E-field of these two elements in-phase with each other and radiate vertically. As well as, for antenna element number 2 and antenna element number 4 there is 180° phase shift between them and both of these antennas are located opposite to each other, so they are in-phase with each other and radiate horizontally. Thus, in this arrangement, a DLP antenna array is realized. The isolation between two different polarization antennas is changed over the operating band as shown in table 3. The isolation between the antenna elements is about 21 dB isolation over the interesting operating band.
Fig. 5. (a) the feeding network for the 2 × 2 dual polarized antenna array, (b) reflection coefficient variation versus frequency for feeding network.
Table 3 The isolation between the different polarization antennas
The proposed array is fabricated using a CNC milling machine. The radiation characteristics of the antenna array are measured using Agilent Technologies E8364B vector network analyzer. Figure 6(a) shows the fabricated photo of array in both the top and bottom layer. Comparison between simulated and measured reflection coefficient of the antenna array is shown in Fig. 6(b). It can be seen that the array has good impedance matching in both simulated and measured results. The horizontally polarized (H-pol) and vertically polarized (V-pol) gain of the antenna array in the broadside direction is shown in Fig. 7. The peak antenna gain in V-pol and H-pol are 7.5 and 8 dBi, respectively, while the average gain is around 5.5 dBi for both V-pol and H-pol. The component gain values of the antenna array at the frequencies correspond to different wireless communication standards (GSM 1800, digital TV, WiFi, and LTE) and are listed in table 4. It could be noticed that at different wireless communication frequencies the H-pol and V-pol gain in the broadside direction are approximately the same. The differences in values between two polarizations are due to the misalignment on the vertical and horizontal antennas involved in the radiation.
Fig. 6. (a) Photo of fabricated 2 × 2 dual polarized antenna array and (b) the array reflection coefficient variation versus frequency.
Fig. 7. Simulated and measured horizontally and vertically polarized gain in the broadside direction.
Table 4. Values of the realized gain in the broadside direction at different wireless frequency applications
Comparison of the measured and simulated 2-D radiation patterns of the proposed array at XZ and YZ planes at different frequencies of 1.8, 2.1, 2.4, 2.5, 2.6, and 2.8 GHz are shown in table 5. Table 5 shows that E x and Ez components of radiation pattern are quite similar in the XY and YZ planes, the difference is due to the dissimilar of the ground plane. This means that the array is a DLP and the array can receive vertical and horizontal RF ambient power at different operating frequencies to collect more ambient power and solve the antenna alignment and the depolarization caused by the multiple path propagations which could have happened in the linear polarized signal.
Table 5. Simulated and measured normalized radiation pattern at different wireless communication frequencies; simulated (H-pol —, V-pol –.–) and measured (H-pol —, V-pol –.–)
To examine the received power of the designed single monopole element and DLP antenna array at certain frequencies, a DRG horn antenna (SAS-571) is used as Tx antenna and Anritsu MS27260 Spectrum Analyzer is used to measure the received power on the antenna element and the array for both vertical and horizontal polarizations. The input power of the Tx horn is 3 dBm from the Agilent Technology N9918A which works as a spectrum analyzer with 2 m away from the Rx antenna. Table 6 shows the list of the received RF power in dBm for both the single element and the antenna array. The received power for V-pol and H-pol of the antenna array are almost the same in the frequency band of interest.
Table 6. The ambient received power at different frequencies for single element and array power in dBm.
Conclusion
A 2 × 2 DLP antenna array is designed in this paper to harvest the ambient RF waves at the different wireless communication bands, such as GSM1800, UMTS 2100, Wi-Fi 2.4, and LTE. The single element of the array is designed to be wideband with reasonable antenna element gain 3 dBi on average. Then a wideband suitable feeding network is designed for the antenna array. The designed array is planar and uses a single substrate so that it can be easily integrated with the RF harvest system. The antenna array covers the frequency band from 1.8 to 2.9 GHz with an average gain of 5.5 dBi in both vertical and horizontal polarizations. Both of the antenna element and array are fabricated and measured. There are good agreements between the measured and simulated results.
Author ORCIDs
Dalia N. Elsheakh, 0000-0002-6168-7681
Acknowledgement
The authors would like to express their sincere gratitude to Dr Chio and Prof. Magdy F. Iskander from Hawaii Center for Advance Communication (HHCAC), Hawaii University, Honolulu, Hawaii, USA for their help and support in fabrication and measurement.
Dalia N. Elsheakh received the B.Sc., M.Sc., and Ph.D. degrees from Ain Shams University in 1998, 2005, and 2010, respectively. She was Assistant Professor and as of 2016 is Associate Professor in Microstrip Dept., Electronics Research Institute. She was Assistant Researcher at the HCAC, College of Engineering, Hawaii University, USA in 2008 and Assistant Professor in 2014 and 2018. She has published 53 papers in peer-refereed journals and 45 papers in International Conferences in the area of Microstrip antenna design. She has participated in many research projects at both national and international levels such as the Egypt-NSF-USA joint funds program and the European Committee Programs FP7 program, STDF, and ITIDA-ITAC. She is very experienced in the microwave engineering field having recently completed eight projects with one further project still pending. One of these projects was funded by the sixth European Framework Program Scientific Support Project (NTRA). She holds the award of 4th prize for the Best Distinguished Researcher in Electronics Research Institute, 2015, the award of Egyptian Government Encouragement Prize for Young Scientists in Engineering Science, 2014, and Post doctor in Hawaii Center for Advanced communications, Electrical Engineering, Hawaii University, in 2013–2014. Her Ph.D. thesis entitled “Electromagnetic Band Gap (EBG) Structure for Microstrip Antenna Systems (Analysis and Design)” has been ranked 8th best out of all Ph.D. theses granted by Egyptian Universities. She participated in Antenna arrays for Radar Project funded by Military that started from 2016 to 2018. Her roles in this project were to design and fabricate single and array antenna, as well as design package of the array column.
