A wideband antenna array for RF energy harvesting

A WIDEBAND ANTENNA ARRAY FOR RF ENERGY HARVESTING 
 Le Minh Thuy*, Dinh Quang Minh 
 Abstract: In this paper, we introduce a wideband antenna array for RF energy 
 harvesting from 3G/4G and Wi-Fi. The array consists of four parallelly linked 
 wideband antenna elements and a metallic reflector to enhance gain and suppress 
 back lobe radiation. In measurement, the antenna array possesses a wide bandwidth 
 spanning from 1.6 GHz to 2.5 GHz, fully cover the three harvested bands, and the 
 high gains between 13.5 dBi and 14 dBi. The antenna is employed in a multiband 
 rectenna for energy harvesting and placed in the ambience. The rectenna was able 
 to collect up to 0.27 mW power. 
Keywords: Antenna arrays; Monopole antennas; Wideband; Rectennas. 
 1. INTRODUCTION 
 Harvesting energy from RF signals in the ambient environment is a potential 
solution to replace batteries as the primary power supply for low-powered 
electronics devices. Due to the low power density of RF waves in the ambient 
environment, only around 1 µW/cm2 [1], it is essential to use antenna array as the 
receiver instead of a single antenna in order to harvest enough power to supply any 
device. In addition, it is desirable for the antennas to operate at multiple 
frequencies instead of only single band because there are many communication 
bands exist in the environment, namely 3G/4G, GSM, Wi-Fi,... 
 However, due to the complication in feeding and matching circuits, few 
previous works have been able to realize a wideband antenna array. The impedance 
of the antenna array can be matched at one frequency while being strongly 
mismatched at others. Therefore, most previous works on multiband energy 
harvesting employ a single multiband or wideband antenna instead of an array [2]-
[4]. For example, in [5], the quad-band rectenna uses a single bow-tie antenna with 
slots for receiving energy, which is able to operate at 840 MHz, 1.8 GHz, 2.1 GHz 
and 2.45 GHz. The classical monopole antennas are also frequently employed to 
provide ultrawide bandwidths. Yet, they have low gains due to their intrinsic 
natures. In one of a a few works on multiband/wideband antenna array for energy 
harvesting, a 1x4 array of quasi-Yagi antennas is able to achieve a bandwidth that 
fully covers the 1.8 GHz and 2.1 GHz, and a high gain around 12 dBi [6]. 
However, the nearby 2.45 GHz, 2.6 GHz or 900 MHz are still not covered. 
 In an effort to enhance the amount of power harvested, we present a wideband 
antenna array for energy harvesting from 3G/4G and Wi-Fi sources. The 
bandwidth of the proposed antenna spans wide enough to fully cover the 1.8 GHz, 
2.1 GHz and 2.45 GHz bands, while the antenna gain stays higher than 11 dBi at 
all of those frequencies. 
 The following parts of the paper are arranged as follow: section 2 provides the 
design of the antenna, section 3 provides the antenna measurement, section 4 is the 
antenna in a rectenna for energy harvesting, and finally section 5 is the conclusion. 
 2. ANTENNA DESIGN 
 The array consists of 4 elements, each one is a circular monopole antenna, 
Journal of Military Science and Technology, Special Issue, No.72A, 5 - 2021 39 
 Electronics & Automation 
horizontally arranged on a RO4003C substrate (ε = 3.55, tanδ = 0.0027) of 1.5mm 
thick. The four antennas are linked together via a corporate feeding network. A 
metallic reflector is put at the back of the antenna array as shown in figure 1. 
 Figure 1. Schematic and image of the antenna array. 
 The monopole antenna is well known for its wideband property. Therefore it is 
the right candidate for the role of element antenna in this array. By optimizing the 
geometrical dimensions, namely patch’s radius r, feed length l and width w, a 
desirable bandwidth, covering the intended frequency bands can be achieved. 
However, the antenna’s omnidirectional pattern means its gain cannot be very high. 
By connecting the monopole antennas together via a T-junction power combiner, a 
1x4 array is formed and, in theory, will significantly enhance the antenna gain. The 
task is to maintain the bandwidth as the elements are linked together. This can be 
carried out as follow: after simulating a single element in CST software, its input 
impedance is extracted and then the 4-to-1 power combiner is designed using ADS 
software with the 4 input ports having the same input impedance of the antenna 
element. The effects of each parameter w1 to w6, d1, d2, l1, l2, wg and lg on the 
combiner’s performance can be observed and changed using CST software, and 
from that, optimized parameters is obtain to achieve desirable result. 
 The monopole array, due to the omnidirectional pattern of each element, 
possesses the equally large front lobe and back lobe. To further enhance the gain, a 
reflector is placed at a distance dr behind the array, completely quenches the back 
lobe. Parameter dr along with the reflector’s size lr and wr are optimized to achieve 
the maximum gain possible without significantly affecting the bandwidth. A 
deeper insight on the role of the optimal distance dr can be understood as follow: 
By assigining dr as λ/4 of frequency f, the reflected waves from the reflector 
cancels the radiated waves at the antenna, thus does not affect the S11. Here we 
choose dr as λ/4 of the central frequency 2.1 GHz. At other frequencies, the S11 will 
be affected at a certain level. As seen from fig. 3b, as dr increases, the bandwidth is 
40 L. M. Thuy, D. Q. Minh, “A wideband antenna array for RF energy harvesting.” 
Research 
broadened and becomes narrower, the picked dr achieves a good compromise 
between deep S11 and wide bandwidth. 
 The antenna dimensions are respectively: r = 28 mm, lf = 14 mm, wf = 4 mm, w1 
to w6 are 6, 2.1, 4.8, 3, 1.3, 6 mm, d1 = 57.6 mm, d2 = 125.8, l1 = 7.5 mm, l2 = 15 
mm, lg = 35.8 mm, wg = 253.2 mm, lr = 142.6 mm, wr = 293.2, and dr = 35 mm. 
With those dimensions, the array possesses a wide bandwidth in CST simulation, 
spanning from 1.75 GHz to 2.55 GHz, fully covers the 1.85 GHz, 2.1 GHz and 
2.45 GHz bands. 
 Figure 2. Simulated S11 at different values of dr. 
 Figure 2 presents the refflection coefficient of the proposed antenna array with 
differenct value of dr from 15 cm to 50 cm. This optimal distance of 35 cm is 
chosen because the antenna bandwidth at – 10 dB of refflection coefficient is from 
1.75 GHz to 2.55 GHz as required. At the distance of 40 cm to 50 cm, the 
reflection coefficient is smaller than but the antenna array size is increase. 
 3. ANTENNA MEASUREMENT 
 Figure 3 presents the simulated and measured refflection coefficient of the 
antenna array. The simulated and measured antenna radiation pattern at three 
frequencies of 1.85 GHz, 2.1 GHz and 2.45 GHz is shown in figure 4. 
 Figure 3. Simulated and measured S11 of the antenna array. 
Journal of Military Science and Technology, Special Issue, No.72A, 5 - 2021 41 
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 a) 
 b) 
 Figure 4. (a) 3D Simulated radiation pattern of antenna array at 1.85 GHz, 2.1 
 Ghz and 2.45 GHz; (b) Simulated and measured radiation pattern of the array. 
 Solid lines are simulation results and dash lines are measurement results. 
 Figure 5. Simulated peak gain of the antenna array. 
42 L. M. Thuy, D. Q. Minh, “A wideband antenna array for RF energy harvesting.” 
Research 
 The peak gain over the bandwidth from 1.8 GHz to 2.5 Ghz is illustrated in 
figure 5. As can be observed, the peak gain is from 12.2 dBi to 14.5 dBi over the 
wide frequency band. 
 Measurement agrees well with simulation, with the measured gains being 13.5 
dBi, 14 dBi and 13.8 dBi at 1.85 GHz, 2.1 GHz and 2.45 GHz,. Respectively, and 
the bandwidth spanning from 1.55 GHz to 2.5 GHz in measurement. Such wide 
bandwidth is enough to fully cover the 3G/4G and Wi-Fi bands while the high 
gains ensure a large amount of harvested power. The half-power beam-widths in 
H-plane are around 30o to 40o for all three frequencies. Meanwhile, in E-plane are 
45o, 40o, and 60o, respectively. The measured radiation patterns have some 
differences from the simulated results due to unideal testing condition. The 
difference is more significant at larger angle, and the measured beamwidths are 
somewhat larger, especially in E-plane. However, at normal direction, we can still 
observe a good agreement between simulation and measurement. 
 The antenna array performances are comparable to earlier works [2-4] in table 
1. As shown, this proposed antenna has wider bandwidth and higher gain at three 
center frequencies 1.85 GHz, 2.1 GHz and 2.45 GHz. 
 Table 1. A comparison between the proposed antenna and earlier works. 
Ref. Frequencies Bandwidth Gain 
 [2] 2.45 GHz, 5.5 GHz ≈ 50 MHz at 2.45 GHz, 7.52 dBi at 2.45 GHz 
 ≈ 100 MHz at 5.5 GHz 7.26 dBi at 5.5 GHz 
 [4] 0.9 GHz, 1.8 GHz, 100 MHz at 0.9 GHz, 8.15 dBi at 0.9 GHz 7.15 
 2.1 GHz 400 MHz from 1.8 GHz dBi at 1.8 GHz, 
 to 2.2 GHz 8.15 dBi at 2.1 GHz 
 [6] 1.8, 2.1 ≈450 MHz from 1.8 10.9 dBi at 1.8 GHz, 
 GHz to 2.25 GHz 13.3 dBi at 2.1 GHz 
This 1.85 GHz, 2.1 ≈880 MHz from 1.62 13.5 dBi at 1.85 GHz 
work GHz and 2.45 GHz GHz to 2.5 GHz 14 dBi at 2.1 GHz 
 13.8 dBi at 2.45 GHz 
 4. ANTENNA ARRAY IN ENERGY HARVESTING APPLICATION 
 The antenna array is connected to a triple band rectifier and put under the real 
ambience testing. The consuming device is replaced with an equivalent resistive 
load of 2.7 kΩ. The rectenna was placed at a certain position inside the campus, 30 
metre away from a base station, which transmit the 1.85 GHz and 2.1 GHz bands, 
while the 2.45 GHz bands was collected from ambient Wi-Fi. The maximum 
voltage recorded was 482 mV, correspond with around 0.27 mW power. In most of 
the time, the output voltage fluctuated between 250 mV and 300 mV. One may 
expect the amount of harvested power to be much higher when the rectenna is 
placed closer to the base station. Due to the three frequencies were harvested 
simultaneously, it was difficult to tell how much power belongs to each bands. 
These amounts of power is still insufficient to fully supply an electronic devices, 
however, it is enough for some low-powered wireless sensor in sleep mode or 
some limited function. 
Journal of Military Science and Technology, Special Issue, No.72A, 5 - 2021 43 
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 Figure 6. The antenna array inside a rectenna in the ambience. 
 In ambient energy harvesting, it is believed that an antenna with wide 
beamwidth or even omnidirectional radiation pattern is more suitable compared to 
high gain antennas, because such antenna may operate under arbitrary direction of 
incidence. However omnidirectional antennas have low gains. In our testing 
scenario, the power source was known and static. In urban area, base station towers 
are everywhere, and dedicated positions near the towers are the ideal places for 
rectennas. The direction of arrival is no longer uninformed but in fact already 
known. Therefore, high gain antennas can also work well for energy harvesting 
application. 
 5. CONCLUSIONS 
 In this paper, we propose a wideband antenna array for ambient energy harvesting 
from the 3G/4G and Wi-Fi sources, corresponding with 1.8 GHz, 2.1 GHz and 2.45 
GHz bands. The antenna array is formed by four monopole antennas parallelly 
linked via a corporate feed, and backed by a metallic reflector. Measurement shows 
a wide bandwidth spanning from 1.55 GHz to 2.5 GHz, fully cover the three 
harvested bands. The array also yields high gains, from 13.5 dBi to 14 dBi. Such 
advantages make it suitable for ambient energy harvesting. The array was connected 
to a triple-band rectifier and placed in the ambience, where it was able to harvest up 
to 0.27 mW. That amount of power is enough for some low-powered electronic 
devices such as wireless sensors in sleep mode and limited functions. 
 Acknowledgement: This research is funded by the Ministry of Education and Training 
(MOET) under grant number B2020-BKA-11. 
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 TÓM TẮT 
 MẢNG ĂNG TEN BĂNG THÔNG RỘNG CHO ỨNG DỤNG THU HOẠCH 
 NĂNG LƯỢNG TỪ SÓNG ĐIỆN TỪ 
 Bài báo này đề xuất một mảng ăng ten băng thông rộng cho ứng dụng thu 
 hoạch năng lượng từ sóng điện từ ở dải tần 3G/4G và Wi-Fi. Mảng ăng ten 
 được tạo nên từ bốn ăng ten đơn cực băng thông rộng, kết nối với nhau qua 
 mạch cấp nguồn song song, với một tấm phản xạ bằng kim loại ở phía sau để 
 ngăn chặn bức xạ ở mặt sau. Kết quả chế tạo và đo đạc cho thấy mảng ăng 
 ten có băng thông rộng trải dài từ 1.55 GHz đến 2.5 GHz, hoàn toàn bao 
 trùm ba băng tần mong muốn, và độ lợi cao từ 13.5 dBi đến 14 dBi. Mảng 
 ăng ten được kết nối với một chỉnh lưu ba tần số, và thử nghiệm bên ngoài 
 môi trường. Kết quả là hệ thống thu hoạch năng lượng có thể thu được tới 
 0.27 mW công suất. 
Từ khóa: Mảng ăng ten; Ăng ten đơn cực; Ăng ten băng thông rộng; Ăng ten chỉnh lưu. 
 Received Apr 14th 2021 
 Revised Apr 23th 2021 
 Published May 10th 2021 
Author affiliations: 
 School of Electrical Engineering, Hanoi University of Science and Technology. 
 *Corresponding author: thuy.leminh@hust.edu.vn. 
Journal of Military Science and Technology, Special Issue, No.72A, 5 - 2021 45