Quad-Band 3D Rectenna Array for Ambient RF Energy Harvesting

Home , Whip antenna

Hindawi International Journal of Antennas and Propagation Volume 2020, Article ID 7169846, 23 pages https://doi.org/10.1155/2020/7169846

Research Article Quad-Band 3D Rectenna Array for Ambient RF Energy Harvesting

Fatima Khalid , Warda Saeed , Nosherwan Shoaib , Muhammad U. Khan , and Hammad M. Cheema

Research Institute for Microwave and Millimeter-Wave Studies (RIMMS), National University of Sciences and Technology (NUST), Islamabad, Pakistan

Correspondence should be addressed to Nosherwan Shoaib; [email protected]

Received 19 January 2020; Revised 23 March 2020; Accepted 8 April 2020; Published 15 May 2020

Academic Editor: Herve´ Aubert

Copyright © 2020 Fatima Khalid et al. -is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

-is paper presents a quad-band, 3D mountable rectenna module for ambient energy harvesting. With the aim of powering up Internet of -ings (IoT) nodes in practical ambient environments, a hybrid approach of combining power, both at RF and DC, is adopted using 98 MHz FM band, GSM900 (Global System for Mobile Communications), GSM1800, and Wi-Fi 2.4 GHz band. A dual polarized cross-dipole antenna featuring asymmetric slots as well as central ring structure enables multiband response and improved matching at the higher three frequency bands, whereas a loaded monopole wire antenna is used at the lower FM band. Four identical multiband antennas are placed in a 3D cubic arrangement that houses a 4-to-1 power combiner and matching circuits on the inside and the FM antenna on the top. In order to maintain stable rectenna output at varying input power levels and load resistances, a novel transmission line based matching network using closed form equations is proposed. Integrated in form of a 10 ×10 ×10 cm3 cube using standard FR4 substrate, the rectenna generates a peak output voltage of 2.38 V at −10 dBm input power. -e RF to DC conversion eﬃciency is 70.28%, 41.7%, 33.37%, and 27.69% at 98 MHz, 0.9 GHz, 1.8 GHz, and 2.4 GHz, respectively, at −6 dBm. -e rectenna also exhibits a measured conversion eﬃciency of 31.3% at −15 dBm for multitone inputs in ambient environment. -e promising results in both indoor and outdoor settings are suitable to power low power IoT devices.

1. Introduction natural environment. Technologies like solar, thermal, pi- ezoelectric, and Radio Frequency (RF) energy harvesting Recent advancements in the field of Internet of -ings (IoT) have become potential alternatives to enable self-sustain- have triggered the “smart” wave, be it homes, offices, fac- ability in sensor nodes. RF energy harvesting offers a number tories, automobiles, manufacturing, transportation, logistics, of advantages over others due to its constant availability, healthcare, agriculture, and environment. Billions of In- smaller size, and independence from environmental and ternet enabled IoT nodes deployed across the globe, gather, mechanical constraints. Energy gathered from the radio monitor, and exchange data thereby providing actionable frequency band (3000 Hz–300 GHz) can be utilized to information to the end-users. Efficient powering of these operate low power electronic devices; however, large scale nodes is a major challenge as, many a time, they cannot be deployment of RF energy harvesting systems has remained connected to the grid due to their operating environments elusive to date. and spatial distribution. Batteries that are currently used to A large body of published works is available on RF power sensor nodes need constant maintenance, are dis- energy harvesting. Applications such as prototype aerial posal, and are difficult to deploy and replace in remote and vehicles [1], remote sensing nodes [2], and medical implants inaccessible areas. Hence, charging of batteries wirelessly or [3] have been demonstrated using RF energy harvesting, designing completely battery free systems has become key albeit using large dedicated and controlled power sources. research challenges. -is not only increases the expense of the overall system but Energy harvesting offers an interesting solution for also limits the applications of energy harvesting. On the powering IoT devices by utilizing energy available in the contrary harvesting energy from ambient sources can truly 2 International Journal of Antennas and Propagation realize the concept of “energy on the go” and is expected to lumped element matching, which is lossy and cannot be reduce system costs as well as improve portability; however, implemented at higher frequencies. Few transmission line it poses serious design challenges. (TL) based matching designs have also been reported in -e basic driver for harvesting energy is a rectenna that literature. For instance [18] proposes dual band rectifier comprises an antenna and a rectifier. An impedance using transmission lines operating at 14.2 dBm input power. matching network is added before the rectifier to ensure References [19, 20] propose quad-band rectifier designs maximum power transfer between the antenna and load. -e operating at an input power range of −12 dBm to 12 dBm overall RF-DC conversion efficiency is the key parameter to and −5 dBm, respectively. All these designs are operating on evaluate the performance of a rectenna. A number of single- power levels that are higher than the ambient power levels, band, multiband, and broadband rectennas have been whereas for ambient energy harvesting, power levels are published in literature. Some single-band rectennas have expected to vary depending upon time and location. achieved high conversion efficiencies in access of 60%; Moreover, the matching network in [18] is designed for however, they have a narrowband response and operate at 14.2 dBm power level only. -erefore, a matching network is high input power levels (e.g., 0 dBm [4], 5 dBm [5], 10 dBm required whose initial parameter values can be calculated [6]), 15 dBm [7], 18 dBm [8], and 20 dBm [9]). -e average using much simpler closed form equations and then these signal strength of EM waves in the environment is around values can be optimized to cover the entire range of ambient −15 to −30 dBm, making these designs unsuitable for am- power levels. Moreover, the proposed triband matching bient energy harvesting. -erefore, multiband and broad- network is an extension of the dual band matching network band rectennas are preferred so that more energy can be reported in [21], which could only be used for two frequency harvested. For instance, [10] proposes a cross-dipole an- bands. tenna that can harvest energy from six bands including DTV In this paper a three-dimensional (3D) multiband rec- and most of the communication bands; however, it does not tenna with a novel impedance matching network is pre- utilize the FM band. Similarly, [11] uses a differentially fed sented. -e rectenna uses four multiband antennas covering slot antenna with a reflector to enhance gain and harvest GSM900, GSM1800, and 2.4 GHz with combined RF output energy from three bands. A codesigned rectenna, which fed to the rectifier. -e matching network has been designed eliminates the need of a matching network by modifying the using closed form equations for input power levels as low as receiving dipole antenna, is presented in [12], whereas [13] −30 dBm. A separate FM rectenna has been designed at proposes a stacked quad-band rectenna with an efficiency of 98 MHz and integrated with the multiband rectenna using up to 84%. A rectenna with embedded harmonic filter to DC combination of the rectifier outputs. -e simulated reject higher order harmonics generated by the nonlinear conversion efficiency of the triband and FM rectifier is up to rectifier is presented in [14]. Design of multiband and 77% and 80%, respectively, at −6 dBm input power. To the broadband rectennas is challenging due to the dependence best of the author’s knowledge, the proposed design is the of input and load impedance on multiple frequencies and a first to harvest energy from FM (98 MHz), GSM900, stable design for all frequencies and input power levels is GSM1800, and 2.4 GHz (Wi-Fi) bands simultaneously. extremely challenging to achieve. Most of the above reported -e paper is organized as follows: Section 2 introduces designs exhibit high efficiencies for a narrow range of input the proposed concept of energy harvesting at a system level. power and gather energy from a single direction only. Section 3 discusses the survey of ambient RF energy in test FM frequency band is also a potential harvesting source environment. -e antenna design for multiband and FM due to its wide availability, lower path loss, and simpler energy harvesting is discussed in Section 4. Sections 5 and 6 circuit design. -ere are very few published designs that discuss the design of the multiband rectifier and matching harvest energy from this band. References [15, 16] have network, respectively. Experimental results of rectenna in- proposed rectennas operating in the FM band using a loop cluding indoor and outdoor measurements are presented in antenna with diameter (0.81λ) and a whip antenna, re- Section 7 followed by the conclusions in Section 8. spectively. Reference [17] demonstrated a wideband FM rectenna harvesting from nearly 23% of the available FM 2. System Level Concept band. However, the total size of the antenna is nearly 1 × 0.59 m2, which is quite large to be of any use in small IoT Multiple rectenna array architectures have been studied in applications. Use of FM band in energy harvesting is limited literature that aim to enhance harvested energy. Array due to the large antenna size, miniaturizing, which degrades rectennas use either RF combination of the antenna outputs its performance in terms of gathering usable amount of or DC combination of multiple rectifier outputs. -e former, energy from environment. -us harvesting from this band shown in Figure 1(a), increases the power by combining the has not become mainstream. received RF power before rectification. In this case, rectifiers A major challenge in the design of a multiband rectenna require larger breakdown diodes to handle more power [22]. is the impedance matching network. As the rectifier is in- -e second configuration shown in Figure 1(b) combines the herently a nonlinear device, its input impedance is a function rectified DC output voltage of all antennas but reduces the of frequency, input power level, and load impedance. Hence efficiency of the system [23]. Figure 1(c) shows a hybrid designing an impedance matching network having good configuration of an array rectenna in which both the RF performance at varying frequencies and input power is very combination of incoming signals and the DC combination important. Most of the existing multiband rectennas use of rectified signals are shown for a greater voltage output. International Journal of Antennas and Propagation 3

Receiving antenna

Multiband RF impedance combination matching network

C storage Load

(a) Receiving antenna Impedance matching network DC combination

C storage Load

(b) Multiband antenna

Multiband RF impedance combination matching network DC Single combination band antenna Single band impedance Load Cstorage matching network

(c)

Figure 1: Rectenna array topologies. (a) Multiple antennas with RF combination and single multiband rectiﬁer. (b) Multiple antennas with separate rectiﬁers and DC combination. (c) Proposed hybrid topology.

-e proposed rectenna is modelled as a cube with FM band monopole multiband cross-dipole antennas on four sides and FM antenna RF monopole antenna acting as the hanging structure at the top. combination -e cubic shape is chosen to enable housing of multiple unit printed circuit boards (PCBs) in a compact form factor as well as to scavenge available energy from all directions. Based on the cubic rectenna design, a hybrid array topology that utilizes both RF and DC combinations depicted in Multiband Figure 1(c) is used. -e energy received by the multiband Rectifier + dipole matching antennas antennas is combined and transferred to the matched rec- network tifier housed within the cube. Separate rectifiers are used for multiband and FM antennas and the DC output of both is recombined. Figure 2 shows the internal components of the Figure 2: 3D cube rectenna structure. cubic rectenna. -e hybrid array topology from Figure 1(c) was selected in order to demonstrate the proof of concept In order to achieve higher DC power at the output, a design of a rectenna system that utilizes multiple signal novel concept of an RF energy harvesting (EH) tree is combinations and covers a wide frequency spectrum. proposed. -e proposed cubic rectenna module will act as 4 International Journal of Antennas and Propagation the integral unit of the EH tree. -e larger part of the rectenna, i.e., the FM antenna, can be easily integrated with the supposed branches of the tree while the multiband antennas will act as leaves (see Figure 3). -e harvested voltage can be transferred to the centre of the tree through the FM antenna. -e design also introduces two levels of DC combination: (1) the combination of DC power from FM FM and multiband rectennas (as shown in Figure 1(c)) and (2) antenna the combination of DC power from multiple rectenna modules on the tree structure. -is concept of energy harvesting can be extended to include any frequency including the AM band at the lower end and higher frequency bands. -e FM, AM, and broadcasting bands are potential Single energy sources, but their use is limited due to the large rectenna antenna size at these frequencies. -e tree structure provides module an ideal way to make use of such large antennas into an energy harvesting system. -e design approach is similar to DC existing prototypes of EH trees, which utilize solar and wind combination energy as a way towards green and sustainable power sources Figure 3: Conceptual diagram of energy harvesting system. [24, 25]. -e design emulates the process of photosynthesis, which can be seen as nature’s energy harvesting mechanism whereby fuel is created and transferred from the leaves to the rest of the tree. -e goal of the system is to harvest maximum amount of energy from the environment while the structure itself should be blended in the topography appearing practically invisible. Measurement Portable antenna spectrum 3. Ambient RF Energy Survey analyzer Analysis of ambient energy density in an area is a complex task as it depends on distance from RF sources, their transmit powers, quantity, and distribution of these sources as well as terrain. Accurate estimation of power density requires either exact path loss models acquired from experimental measurements or analytical modelling. -is is why no general solution of ambient energy density calculation exists. -erefore, power measurements in multiple locations of the desired area and subsequent averaging of the results are the most viable solution and have been adopted in Figure 4: Measurement setup. this work. -e ambient RF field measurements conducted in this work are a continuation of the tests performed in [26]. from the measurement area as mapped out in Figure 5. Multiple Wi-Fi sources are also present inside the building. 3.1. Methodology and Test Setup. -e ambient RF mea- -e equipment used for ambient energy measurement in- surements were carried out at multiple locations at the cluded a portable spectrum analyzer and a set of broadband university campus, which lies in a semiurban environment. omnidirectional whip antennas with a frequency range of Figure 4 shows the general testing setup whereas the floor few MHz to 2.7 GHz were used. With measured data for each plans of the indoor and outdoor measurement sites, mea- location, the average power density for each frequency band surement locations and distribution of RF sources like was calculated. -e antenna power factor and cable losses nearest communication towers, and Wi-Fi routers are were deembedded from the measurements. Multiple mea- shown in Figure 5. -e indoor measurement points were surement trials were conducted spreading over several days chosen to be in different parts of the building like labs and to ensure data repeatability. Since ambient RF power is not lounge. Outdoor measurement was carried out at different constant and changes with time of day and presence of active points in the grounds surrounding the campus as well as on devices in vicinity, the standard deviation of the collected the campus terrace. Together, all the measurement points data from the mean is also calculated. were located within a radius of nearly 1 km. Although a -e results of the RF field survey are shown in Figure 6, number of cellular and broadcasting sources were present in whereas Table 1 summarizes the indoor vs. outdoor available the broader vicinity, the closest cellular tower lies about power levels from different sources along with standard 0.3 km away and the closest FM source is nearly 6.2 km away frequency ranges. -e uplink and downlink bands are International Journal of Antennas and Propagation 5

Cellular tower (nearest) Outdoor grounds 0.3 km Outdoor terrace

6.2 km FM source Lab Lab (nearest)

Lounge Corridor Hall Office Office Office Lab

Measurement point Wi-Fi source Figure 5: Floor plans of testing site with nearest energy harvesting sources (distances are not realistic and have been scaled down to ﬁt image).

0 outdoor measurements show higher levels of GSM900 and GSM900 −10 broadcasting signals due to close proximity to a base station GSM1800 and a near line-of-sight from a TV transmitter. Mobile −20 communication sources show much less power indoors FM 3G −30 whereas Wi-Fi sources show relatively constant power in Wi-Fi both indoor and outdoor measurements as there are nu- −40 Broad casting merous sources present around campus. -e energy avail- −50 able in cellular communication bands and the Wi-Fi band

Ambient power (dBm) −60 shows the greatest amount of deviation depending upon time. -is can be attributed to dynamic user traﬃc. FM and −70 0 0.5 1 1.5 2 2.5 3 broadcasting energy sources show comparatively lower Frequency (GHz) deviation. Wi-Fi sources have low transmit power and are usually Outdoors found indoors. -is makes them more suited for energy Indoors harvesting for devices used inside homes and buildings like Figure 6: Peak ambient power vs. frequency in a semiurban wearable electronics and domestic sensing devices. On the environment. other hand, mobile communication and broadcasting sources can be used to power sensors in outdoor environment due to higher transmit power of base stations and their denoted by UTx and DTx, respectively, and the power omnipresence across the globe. -e results also conﬁrm the available in the environment is found by need of multiband antennas to meet practical requirements.

Wi � AePT, (1) On the basis of the above analysis, the FM, GSM900, GSM1800, and Wi-Fi bands have been exploited for the where Wi and PT are the power density of incident wave and proposed rectenna design. 3G band shows promising am- the power delivered to the spectrum analyzer, respectively, bient energy availability, which will be incorporated in our and Ae is the effective antenna aperture. -e peak available rectenna as a future work. power values are recorded between −60 dBm and −10 dBm with average ambient values between −15 and −30 dBm. 4. Antenna Design 4.1. Multiband Dual LP Antenna. To harvest energy from the 3.2. Results and Discussion. From the RF survey, FM GSM900, GSM1800, and 2.4 GHz Wi-Fi band, a multiband broadcasting, GSM900, GSM1800, 3G, and Wi-Fi were antenna consisting of a modified cross-dipole antenna is identified as potentially useful ambient RF energy sources. designed. -e cross-dipole antenna is selected based on its GSM900 shows maximum peak power whereas FM and wide bandwidth, bidirectional radiation pattern, and dual broadcasting sources appear to be heavily dependent on linearly polarized (LP) characteristics. -e antenna is made line-of-sight and Wi-Fi is very dependent on user traffic. on an FR4 substrate with relative permittivity of 4.3 and a GSM900 shows higher power level in the uplink while thickness of 1.6 mm. -e size of the PCB is selected as 2 GSM1800 and 3G show greater power in the downlink. -e 100 ×100 mm , which is 0.3λ0 × 0.3λ0 at the lowest 6 International Journal of Antennas and Propagation

Table 1: Peak ambient power level survey. Peak available power/power density Indoor Outdoor Frequency band/range (MHz) Standard deviation (σ) PT Wi PT Wi (dBm)(μW/m2)(dBm)(μW/m2) FM −45 0.0095 −29.56 0.332 2.098 (88–108) Broadcasting −54 0.00497 −27.58 2.18 5.659 (150–600) GSM900 UTx −28.41 34.5 −8.991 300 18.48 (860–915) DTx −37.5 5.91 −19.87 290 11.16 (925–960) GSM1800 UTx −60.1 0.091 −43.3 4.37 7.385 (1710–1785) DTx −45.4 3.15 −40.89 8.9 15.23 (1805–1880) 3G UTx −51.547 0.82 −31.07 91.9 10.26 (1920–1980) DTx −49 1.7 −21.45 100 8.592 (2110–2140) Wi-Fi −42.4 10.7 −47.75 3.15 10.43 (2390–2490) operating frequency of 900 MHz. -e radius and angle of the bowtie shape are selected as 50 mm and 80°, respectively. Two pairs of cross dipoles are orthogonally printed on each L side of the PCB. -e antenna is probe fed by a 50 Ω coaxial 1 cable. -e inner conductor of the coaxial cable is fed to the W top side metal and the outer conductor is connected to the 1 L bottom side. -e dipole antenna has a balanced topology and 2 W R R 2 2 1 typically requires a balun to connect to a coaxial cable. Ln However, in this case, the bandwidth is limited and the antenna is being matched to a 50 Ω impedance so omitting a Rn balun structure does not affect the overall antenna perfor- Wn mance. -is has also been done to maintain the simplicity θn and ease of integration of the cubic rectenna structure. -e same feeding structure has been employed in a number of other antenna designs without the use of a balun in cross- Figure 7: Slotted dipole arm. dipole to coaxial transition ([10, 27–29]). A simplified cross-dipole is designed for an arbitrary where θ is the angle of the cross dipole and λ is the frequency lying between 900 MHz and 2.4 GHz. -e pro- t n wavelength of the n frequency obtained using suspended posed cross-dipole resonates at 1.6 GHz with a wide th strip-line model [30]. -e parameter L is the arc length and bandwidth of nearly 800 MHz. In order to cover more bands n can be calculated by L � R θ . -e overall bandwidth can at higher frequencies (up to 2.4 GHz), the cross-dipole is n n n be increased by increasing the width (centre angle) of the modified by adding a number of slots of different dimen- cross-dipole, whereas the bandwidth of each individual band sions. Figure 7 shows the modified dipole arm with arbitrary can be fine-tuned by adjusting the slot width. number of slots. -is converts each arm of the cross-dipole -e design iterations of the antenna are shown in Fig- into a number of bent dipoles defined by a circular part ure 8. Table 2 shows the optimized dimensions of slots added (L + W sec(θ )) and a radial edge (R ) where θ is the n n n n n to dipole arm to make the antenna resonate at specific angle of the dipole arm. -e total length of each bent dipole frequencies. -e simulated S of the multiple stages of the arm can be estimated as 11 antenna design is shown in Figure 9. It can be seen that λ L + W secθ � + R � n, (2) increasing the number of slots introduces a multiband re- n n t n 4 sponse; however, matching of the antenna is degraded. To International Journal of Antennas and Propagation 7

Bow-tie cross Step 1 Step 2 dipole

Final design Step 4 Step 3

Top metal Botttom metal Figure 8: Antenna design iterations, introduction of slots for multiband response and vacant quarter ring to improve matching.

Table 2: Parameters of the proposed slotted cross-dipole antenna.

Resonant operating frequency Rn Wn θn Ln � Rnθn Ln + Wnsec(θ) + Rn λn/4 (MHz) (mm) (mm) (deg) (mm) (mm) (mm) 875 43 4 65 48.78 85.75 85.71 1730 33 2 50 28.79 43.68 41.67 2100 30 2 45 23.56 35.44 35.71 2400 15 2 40 10.47 28.08 31.25 Antenna dimensions

Wa Ra θa Rr Wr (mm) (mm) (deg) (mm) (mm) 100 50 80 4 1.6

0 achieve a good matching at all frequencies of interest the width and radius of the ring are optimized using parametric −2 analysis to 4 mm and 1.6 mm, respectively. Figure 10 shows −4 the ﬁnal antenna design along with its dimensions. Figure 11 shows the simulated and measured S11 of the proposed −6 multiband antenna. Addition of the vacant ring improves (dB)

11 −8 the matching performance of the antenna, which covers the S following bands: GSM900 (720 MHz to 940 MHz), GSM- −10 1800 (1650 MHz to 1760 MHz), and Wi-Fi (2.334 GHz to −12 2.43 GHz). -e individual multiband antenna has a peak gain of 1.6, 2.4, and 5.3 dBi at 0.9, 1.73, and 2.4 GHz, −14 0.5 1 1.5 2 2.5 3 respectively. Frequency (GHz) -e antenna exhibits dual linear polarization due to the orthogonal arrangement of the dipoles. -is allows the Cross dipole Step 3 antenna to receive energy from diversely polarized sources. Step 1 Step 4 Figure 12 shows the surface current distribution at the Step 2 frequencies of interest. It can be seen that the current flows Figure 9: -e simulated S11 of the proposed antenna versus fre- through the longest bent dipole at lowest frequency and vice quency for different design steps. versa. Figure 13 shows the 2D radiation patterns of the individual antenna while Figure 14 shows the simulated 3D radiation patterns. -e radiation patterns are of the overcome this a printed vacant quarter ring is added to the standalone antenna. -e patterns are similar in the lower structure. frequency band. -e difference arises at the higher frequency -e vacant ring is significantly able to improve the and is mainly attributed to the measurement equipment. antenna performance. It helps in the matching of the an- Also, higher order modes are excited at the higher frequency tenna in its operating bands. -e matching performance is resulting in more lobes in the radiation patterns of the greatly dependent on the dimensions of this feeding ring. To antenna. 8 International Journal of Antennas and Propagation

Top metal Substrate

Rr Coaxial cable Ra Botttom metal Conducting pin Wr Wa

θa

Top metal Botttom metal (a) (b)

Figure 10: Proposed antenna design. (a) Front view. (b) Side view.

−10

(dB) −20 11 S −30

−40

−50 0.5 1 1.5 2 2.5 3 Frequency (GHz) Simulated Measured Figure 11: Simulated and measured S11 of the proposed antenna versus frequency.

(a) (b) (c)

Figure 12: Current distribution of the antenna. (a) 875 MHz. (b) 1730 MHz. (c) 2400 MHz.

4.2. FM Band Antenna Design. An inductively loaded Loading nearer to the feed (at 33% or 66%) provides better monopole antenna at 98 MHz is designed for energy radiation performance. However, loading near the end harvesting in FM band. In order to minimize the size of results in antenna size reduction [31]. -e antenna has otherwise large FM antenna an inductively loaded coil is a 10 ×10 cm2 reﬂector at the bottom to enhance gain. added to the monopole reducing the antenna size and Figure 15 shows the design of the proposed antenna along improving performance. -e loading coil placement on the with dimensions. monopole rather than the feed point also provides greater -e loading point for the monopole can be denoted as radiation eﬃciency. -e exact location of loading point is the ratio of lower monopole length and the total length of the typically based on bandwidth and size requirement. antenna. International Journal of Antennas and Propagation 9

0 0 0 0 30 330 30 330 −4 −4

−8 −8 60 300 60 300 −12 −12

−16 −16

90 270 90 270

120 240 120 240

150 210 150 210 180 180

Simulated Measured (a) 0 0 0 0 30 330 30 330 −4 −4

−8 −8 60 300 60 300 −12 −12

−16 −16

90 270 90 270

120 240 120 240

150 210 150 210 180 180

Simulated Measured (b) 0 0 0 0 30 30 30 330 −4 −4

−8 −8 60 60 60 300 −12 −12

−16 −16

90 90 90 270

120 120 120 240

150 150 150 210 180 180

Simulated Measured (c)

Figure 13: Comparison between the simulated and measured 2D radiation patterns for XZ plane (left) and XY plane (right) at (a) 900, (b) 1730, and (c) 2400 MHz. 10 International Journal of Antennas and Propagation

875MHz 1730MHz 2400MHz X Z

Figure 14: 3D radiation patterns of antenna.

0 L MT −5

−10

L S LC (dB) −15 11 S −20

LT −25 Rc −30 90 95 100 105 110 Frequency (GHz) L ML Simulated Measured Figure 16: Simulated and measured S11 of loaded monopole antenna. LG

Figure 15: Side view of proposed loaded monopole antenna Table 3: Power combiner parameters. (LT � 55, LML � 37, LMT � 11, LC � 7, LS � 1, LG � 5, RC � 2). Parameter Z1 Z2 Z3 R1 R2 R3 Value (Ω) 93.33 78.16 64.18 120 220 470

LML m � . (3) is the output port with combined RF power and ports 2 to 5 LT are input ports. -e ports of the combiner are designed in -e required resonance is achieved at m � 0.67 with the an orthogonal arrangement so that they can be easily size reduced to 55 cm (0.18λ0) in case of loading as opposed inserted in the energy harvesting cube with connections to to nearly 160 cm when no loading coil is used. Figure 16 four antennas on the side (Figures 18(a) and 18(b)). -e shows the simulated and measured S11 while gain of 1.76 dBi drawback of the proposed power combiner is the degra- is obtained from the antenna. dation in output voltage of the rectenna due to phase RF combination of the multiband antennas has been differences in incoming signals. In an ambient environ- done using a 1 × 4 Wilkinson power combiner covering a ment, the power level and phase of available signals are frequency range of 0.5 to 3 GHz. -e 4-way combiner has completely unpredictable and random in nature, thus been designed using three 2-way combiners. Each com- making the design of an efficient power combiner con- biner consists of three semi-circular transmission line siderably difficult. -e rectenna performance in the worst sections of different widths to give a wideband response at case scenario (i.e., completely out of phase incoming sig- the frequency range of interest. -e transmission line nals) was simulated and the output voltage was noted to be impedances and resistor values for each section are shown lower than that in other cases. However, in real scenarios in Table 3. It is fabricated on FR4 substrate with thickness the ambient power is rarely completely out of phase and of 1.6 mm and relative permittivity of 4.3. -e insertion shows slight phase differences. In this work, the proposed loss, isolation, return loss, and phase of the combiner at combiner is utilized due to its superior performance over input and output ports are shown in Figure 17, where port 1 other combiner designs, simplicity of design, and ease of International Journal of Antennas and Propagation 11

0 0

−10 −20 −20

−30 −40

−40 Magnitude (dB) Magnitude Magnitude (dB) Magnitude −60 −50

−60 −80 0.5 1 1.5 2 2.5 0.5 1 1.5 2 2.5 3 Frequency (GHz) Frequency (GHz) S S S S 11 simulated 22 measured 23 measured 24 simulated S S 11 measured 12 simulated S measured S measured S S 23 24 22 simulated 12 measured (a) (b)

200

100

0 Phase (deg) −100 S S Arg ( 51, 41, S S 31, 21) −200

0.5 1 1.5 2 2.5 3 Frequency (GHz) (c)

Figure 17: Simulated and measured S parameters of the power combiner. (a) Insertion loss and return loss at input and output ports. (b) Isolation between adjacent and opposite input ports. (c) Transmission phase.

P1 P5 FM R 1 monopole R 2 antenna P2 P4 R3

Z1 Z2 Z P3 3 Multiband antenna cube

(a) (b) (c)

Figure 18: Integration of antennas to form one harvesting node. (a) Power combiner. (b) Combiner with multiband antennas. (c) Proposed 3D rectenna module. integration with the antennas and rectiﬁer. -e multiband side of the cube acts as a reﬂector of the FM antenna. -is and FM antennas are integrated to form a 3D harvesting cubic arrangement of multiband antennas helps increase node of the proposed harvesting system. -e end product is individual antenna gains to 2.34, 2.5, and 6.3 dBi at 0.9, 1.8, a 10 ×10 ×10 cm3 cube with a 55 cm hanging FM antenna and 2.4 GHz, respectively, and makes the rectenna virtually on top. It can also be seen from Figure 18(c) that the top omnidirectional. 12 International Journal of Antennas and Propagation

A number of structures (adjacent antennas, opposing 350 antennas, triangular, and cubic arrangement) were analysed 300 for the final form of the energy harvesting node. -e cubic structure was selected due to better gain and radiation 250 characteristics and ease of integration of inner components. 200 -e analysis of radiation characteristics of the antennas in the vicinity of each other has also been done. At 0.9 and 150 Real (Zin) 1.73 GHz the energy harvesting cube shows an overall gain 100 increase from 1.6 to 3 dBi at 0.9 GHz and from 2.4 to 4.72 dBi at 1.73 GHz, respectively. Similarly, at 2.4 GHz the gain 50 marginally decreases from 5.3 to 4.9 dBi. Due to the size and 0 placement of the antennas the overall gain and efficiency do 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 not alter much after the cubic structure is employed. -e Frequency (GHz) obtained radiation efficiency of the antenna system is around –30dBm –10dBm 90%, 80%, and 70% in GSM900, GSM1800, and Wi-Fi bands, –20dBm 0dBm respectively. -e choice of antenna or type of antenna ar- (a) rangement does not have a significant effect on the con- −200 version efficiency as it is measured using the RF power input to the rectifier. -e antenna arrangement only determines the level of this input RF power. −300

5. Rectifier Design −400

−500 -e three commonly used rectifier topologies for rectenna Img (Zin) include single series diode, voltage doubler, and the Grei- nacher rectifiers. A single diode is preferred when input −600 power levels are very low, whereas the Greinacher rectifier is used for applications involving higher power handling. -e −700 ambient power levels range from low to medium intensity. 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 -erefore, the voltage doubler topology is chosen due its Frequency (GHz) high power handling capability, low input voltage require- –30dBm –10dBm ment (depending on diode), and doubled output DC voltage. –20dBm 0dBm -e voltage doubler has been constructed using Skyworks (b) SMS7630 diodes and 100 nF Murata GRM188 series capacitors. -e SMS7630 has a low threshold voltage and Figure 19: Input impedance of the voltage doubler rectifier versus exhibits high sensitivity at low input power levels, thus frequency and input power levels for 10 kΩ load. (a) Real part. (b) making it ideal for power harvesting applications. A load Imaginary part. resistance of 10 kΩ is chosen as it represents the typical resistance value of IoT sensors. -e rectifier circuit is sim- impedance at 0.9, 1.8, and 2.4 GHz to 50 Ω antenna resis- ulated in an electronic design automation software, namely, tance. It is a combination of a Dual Band Impedance Advanced Design System (ADS), to accurately model the Transformer (DBIT) and a Dual-to-Triband Impedance parasitics arising due to the capacitors. Figure 19 shows the Transformer (DTBT) as shown in Figure 20 [32]. -e former simulated complex input impedance of the voltage doubler transforms the Frequency Dependent Complex Load for multiple power levels (−30 dBm to 0 dBm) at 10 kΩ. It (FDCL) impedance at the first two frequency bands of can be observed that the real and imaginary parts of the 0.9 GHz and 1.8 GHz to 50 Ω while the DTBT transforms the input impedance decrease with an increase in operating complex DBIT input impedance at the third frequency band frequency and increase with an increase in the input power of 2.4 GHz. levels. 6.1.1. Dual Band Impedance Matching Network. With an 6. Matching Network Design aim to derive a closed form solution, the schematic of DBIT is shown in Figure 21 where Z � R + jX and -e multiband impedance matching network is required L1 a a Z � R + jX . Stage I transforms the complex load im- between antennas and rectifier circuit for four frequency L2 b b pedance at f and f to pair of complex conjugate im- bands (0.098, 0.9, 1.8, and 2.4 GHz) and is designed through 1 2 pedances “Z ” such that Z | f � (Z | f )∗. a combination of a Triband matching network for 0.9, 1.8, L2 L2 1 L2 2 -e two series transmission lines of stage II transform and 2.4 GHz and a separate matching circuit for 98 MHz. ZL2 impedance to Z0 resistance at the first two frequency bands. -e characteristic impedance and length of stage I 6.1. Triband Matching Network. -e triband matching transmission line are obtained using the following set of network is required to match the complex rectifier equations [21]: International Journal of Antennas and Propagation 13

Z Z Z Dual to triband Dual band 1 2 3 l l l impedance impedance 1 2 3 transformer matching network Rectifier Z 0 Antenna + load Z f Triband matching network L1 @ 1 50 Ω Complex Z L2 @ f antenna impedance Z Z Z 2 impedance in L1 L2 Figure 20: Block diagram of the Triband Impedance Matching Network (TIMN). Stage II Stage I Figure 21: Block diagram of the Dual Band Impedance Trans- �� former (DBIT). X + X Z � R R + X X + a b R X − R X �, s a b a b R − R a b b a b a Z (4) characteristic impedance 0, which is the same as the system’s characteristic impedance. n � + Z R − R �� R X − R X �� arctan 3 a b / a b b a -e DTBTdoes not aﬀect the impedance matching that l3 � . (m + 1)βa is already achieved at the ﬁrst two frequency bands through the DBIT. Initially, only the series TL and OCSC-1 -e input impedance of stage II is found using the stubs are considered. -e input impedance of the DBIT at following transmission line equations: the third frequency band is found using the following ZL1 + jZ1 tanβal1 � equations [33]: Z0 � Zin � Z1 , (5) Z1 + jZL1 tanβal1 � ZL1 + jZ1 tanυβal1 � Zin1@f3 � Z1 � G1 + jB1, (10) Z1 + jZL1 tanυβal1 � Z + jZ tanβ l � Z � Z L2 2 b 2 , ( ) L1 2 6 where Z2 + jZL2@f3 tanβbl2 � f υ � 3, whereas the TL parameters for stage II are obtained using the f following equations: 1 � Z @f + jZ tanβ l � l � l � l � , (7) Z � Z L2 3 2 b 2 , ( ) 1 2 + L1 2 11 βa βb Z2 + jZL2@f3 tanβbl2 �

4 3 2 Z + jZ tanυβ l � Z1 + bZ1 + cZ1 + dZ1 + e � 0, (8) L3 3 a 3 ZL2@f3 � Z3 . Z3 + jZL3 tanυβal3 � where -e ZL represents the rectiﬁer’s input impedance and βc 2aR X 3 b � 0 L2 , is the propagation constant at the third frequency. Since only R − RL 0 2 OCSC-1 is used, therefore, B1 � B2 and the value of B3 can be computed using (10) and (12), thus obtaining the values of 2 2 2 2 2 2 R RL �X −R − RL � � − X R 1 + a � characteristic impedances of OCSC-1 stubs. c � 0 2 L2 0 2 L2 0 , a2R R − R � L2 L2 0 − Z B � Z B + � − Z2G2 (9) 1 0 2 tan υθ 0 2 tan υθ 0 1 tan υθ B3 � 2 2 . 2aR3X Z � 1 − Z B tan υθ� +Z G tan υθ � � d � 0 L2 , 0 0 2 0 1 RL2 − R0 (12)

3 2 2 R0 RL2 + XL2 − R0RL2� -e electrical length “θ” of the series transmission line e � . can be found using (13), which can be used to ﬁnd the R − RL 0 2 characteristic impedances of the OCSC stubs using (14). Z �� From (8), the value of 1 is obtained, which can be used 2 2 2 2 2 Z0B2 ± Z0B2 � +1 − Z0G1 � Z0B2 + Z0B2� − Z0G1 to ﬁnd the value of Z2 using (6). -e calculated values of the � , tan υθ 2 2 2 2 TL parameters are shown in Figure 22. Z0B2 + Z0B2 − Z0G1� (13)

6.1.2. Dual-To-Triband Impedance Matching Network. In B Y � 3 , ( ) order to match the third frequency band, the circuit, shown a 2 14 tan υθ1 − tan θ1 cot υθ1 in Figure 23, is used as dual-to-triband impedance transformer (DTBT). It consists of Open-Circuit-Short-Circuit From the calculated value of Za, Zb can be found using (OCSC) stubs along with a series transmission line having (15) and (16). 14 International Journal of Antennas and Propagation

4.21mm 0.43mm DTBT DBIT 34.62mm 37.67mm Rectiﬁer 1.618mm 2.16mm 0.92mm 1.03mm 8.64mm 21.58mm 3.79mm C RF source 23.95mm 1 D2 10kΩ

Z0 D1 C2 3.21mm 1.14mm 20.69mm 35.38mm

(a)

(b)

Figure 22: Schematic and prototype of the proposed Triband rectiﬁer (PCB size: 7 × 7.5 cm2).

Z f OCSC-1 OCSC-2 Yin4 = 1/ 0 @ 1 Y Z f Z f in3 = 1/ 0 @ 1 Z 1/ 0 @ 2 Za Z f c Z f 1/ 0 @ 2 1/ 0 @ 3 θ G + jB @ f θ 1 2 3 3 1

Z 0 Zb θ Zd θ θ 1 1 Z Y Z f 0 Y = 1/Z @ f in1 = 1/ 0 @ 1 in2 0 1 Z f 1/Z @ f 1/ 0 @ 2 0 2 G jB f G jB f 1 + 1 @ 3 1 + 2 @ 3 Figure 23: Block diagram of the Dual-to-Triband Impedance Transformer (DTBT).

(1 + s) � Z � Z 2 . (17) θ � , (15) c dtan θ1 1 1 + r -e overall design procedure can be summarized as 2 follows: Za � Zbtan θ1. (16) (1) -e transmission line parameters of DBIT are de- If the calculated characteristic impedances are outside termined by ﬁnding the complex load impedances at the physically realizable range of 30 Ω to 130 Ω, the design the ﬁrst two frequency bands. procedure has to be repeated by incorporating OCSC-2 stubs (2) -e input impedance (Zin1@f3) of DBIT at the third with an assumed of Zc. -e corresponding value of Zd can be frequency band is determined using equations or found using (17) as follows: simulations. International Journal of Antennas and Propagation 15

(3) -is value (i.e., Zin1@f3) is used to calculate the 0 electrical length of the series TL followed by the –5 calculation of electrical length and characteristic impedance values of the OCSC-1 and OCSC-2 stubs. –10 –15

-e calculations are initially carried out for complex (dB) –20 11 load impedances corresponding to 0 dBm input power level S obtained from Figure 19 and subsequently the design is –25 optimized in ADS to achieve triband matching at multiple –30 power levels, i.e., −30 to 0 dBm. -e schematic and pro- –35 totype of the matching network along with the parameters –40 are shown in Figure 22. -e proposed network uses closed 0.8 1 1.2 1.2 1.6 1.8 2 2.2 2.4 Frequency (GHz) form equations unlike reported works that rely solely on optimization tools. After cosimulation of the network and Meas. 0dBm Simu. 0dBm rectifier, the design is fabricated on a 0.762 mm thick Meas. −10dBm Simu. −10dBm Rogers RO4350B substrate with ε � 3.48. -e S is Meas. −20dBm Simu. −20dBm r 11 Meas. −30dBm Simu. −30dBm measured using Agilent Technologies E8362B PNA Series Figure Network Analyzer and shown in Figure 24. -e proposed 24: Simulated and measured S11 of triband rectifier for triband matching network covers the GSM-900 10 kΩ load. (870–900 MHz), GSM-1800 (1700–1730 MHz), and Wi-Fi (2360–2390 MHz) band for multiple power levels. -e 6.3.TribandandFMRectifierIntegration. In order to harvest slight shift in the measured results can be attributed to energy simultaneously from four frequency bands, a DC unknown parasitics of the SMD capacitors and diodes. -e combination of FM rectenna and triband rectenna was RF-DC conversion efficiency of the rectifier is calculated designed. Both rectennas use a single rectifier each, making using the following equation: the complete design employ two rectifiers. -e RF power of P the four triband antennas is combined and fed to one η � DC, (18) RF−DC P rectifier while the FM antenna is connected to a separate in rectifier. -e DC combination of both rectifiers is shown in where PDC is the output electrical power while Pin is the Figure 29(a). Figure 29(b) shows the fabricated prototype input RF power to the rectifier. Figure 25 shows the of the integrated quad-band rectifier design after DC triband rectifier efficiency. -e higher efficiency is combination. -e substrate used is the same as that for achieved at 0.9 GHz as compared to 1.8 GHz and 2.4 GHz. triband rectifier designed earlier. -e S11 of the integrated -is is expected due to increased PCB losses at higher multiband rectifier is shown in Figure 30. -ere is a shift in frequencies. It can be seen that the rectifier has the highest the FM frequency response as compared to standalone FM efficiency around the input power level of −6 dBm. A rectifier response shown in Figure 27 because of the ad- maximum efficiency of 77% at 0.9 GHz, 74% at 1.8 GHz, ditional capacitance, which is introduced due to the sol- and 54% at 2.4 GHz is obtained at this power level. Al- dering of matching network lumped components. -e big though multitone input could not be provided due to shift in the measured triband response compared to the equipment limitation, the simulated result for multitone simulated results can be accounted for by the soldering input is shown. It can be observed that the efficiency for losses and high frequency losses in the capacitors that were multitone input is higher than the individual single tone a part of the voltage doubler circuit. Besides this, the inputs. parasitic capacitance between the PCB traces has also caused the measured results to shift left. Figure 31 shows the simulated and measured efficiency of the integrated 6.2. FM Rectifier Design. -e matching network for the FM quad-band rectifier at frequencies of interest and multitone rectifier is a lumped LC circuit optimized to achieve a input. -e DC connection has a negative effect, which can −10 dB bandwidth over the FM band with a centre fre- be attributed to the DC combination of two dissimilar quency of 98 MHz. -e selected values of the inductor and rectenna units having different output powers and effi- capacitor for the LC circuit are 500 nH and 3.9 pF, ciencies. Due to this, the triband rectenna cannot operate at respectively. its optimum efficiency. For this reason there is a reduction Figure 26 shows the schematic and prototype of the FM of around 10% in the FM efficiency and nearly 40% deg- rectifier fabricated on Rogers RO4350B while Figure 27 radation in the simulated multitone efficiency. A possible presents the S11 for multiple power levels. Better match- solution to increase the total efficiency of the integrated ing is achieved for higher power with a maximum band- rectifier could be to improve the individual efficiency of the width of 10 MHz at −20 dBm. triband rectenna at all the frequency bands so that both the -e simulated and measured efficiency values at 98 MHz FM and triband rectifiers can achieve optimum operation. are shown in Figure 28 which highlights a good correlation. -is can be done through layout optimization of the rec- A maximum efficiency of 80% is achieved at 98 MHz for tifier design, which is also a future work of our research −6 dBm power level. group. 16 International Journal of Antennas and Propagation

80 70 60 50 40 30 20

RF-DC conversion efficiency (%) conversion RF-DC 10 0 –40 –35 –30 –25 –20 –15 –10 –5 0 Input power level to rectifier (dBm) Simu. 875MHz Simu. 2400MHz Meas. 875MHz Meas. 2400MHz Simu. 1730MHz Simu. 875 + 1730 + 2400MHz Meas. 1730MHz Figure 25: RF-DC conversion eﬃciency of the triband rectiﬁer for 10 kΩ load.

Matching Voltage doubler circuit network with load

500nH C D RF source 1 2 10kΩ

Z 0 3.9pF D1 C2

(a) (b)

Figure 26: Schematic and prototype of FM rectiﬁer (PCB size: 3.5 × 2.5 cm2).

0 100 −5

−10 80

−15 60 (dB)

11 −20 S 40 −25

−30 20

−35 efficiency (%) conversion RF-DC 88 90 92 94 96 98 100 102 104 106 108 0 −40 −35 −30 −25 −20 −15 −10 −5 0 Frequency (MHz) Input power level to rectifier (dBm) Simu. 0dBm Meas. 0dBm Simulated Simu. −10dBm Meas. −10dBm Measured Simu. −20dBm Meas. −20dBm Simu. −30dBm Meas. −30dBm Figure 28: RF-DC conversion efficiency of FM rectifier at 98 MHz Figure for 10 kΩ load. 27: Simulated and measured S11 of FM rectifier for 10 kΩ load. area was chosen as the indoor test environment. -e outdoor 7. Rectenna Measurements testing was done on an open terrace at a height of 9 m. -e rectenna was measured with single multiband antenna as -e integrated rectifier is connected to corresponding an- well as with integrated multiband and FM antennas in both tennas (Figure 32). A typical lab at ground level and 24 m2 indoor and outdoor settings. International Journal of Antennas and Propagation 17

4.21mm 0.43mm 34.62mm 37.67mm Triband DC rectenna combination 1.618mm 2.16mm 0.92mm 1.03mm 8.64mm 23.95mm 21.58mm 3.79mm C1 D2 + Z 0 C D1 2 3.21mm 1.14mm 20.69mm 35.38mm Vdc

500nH 10kΩ C1 D2 – FM Z 0 3.9pF C rectenna D1 2

(a)

(b)

Figure 29: Proposed multiband rectiﬁer. (a) Schematic. (b) Prototype (PCB size: 7 × 7.5 cm2).

0 5 −5 0 −10 −5 −15 −10 (dB) −20 (dB) −15 11 11 S −25 S −20 −30 −25 −35 −30 −40 −35 88 90 92 94 96 98 100 102 104 106 108 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 Frequency (MHz) Frequency (GHz)

Simu. 0dBm Meas. 0dBm Simu. 0dBm Meas. 0dBm Simu. −10dBm Meas. −10dBm Simu. −10dBm Meas. −10dBm Simu. −20dBm Meas. −20dBm Simu. −20dBm Meas. −20dBm Simu. −30dBm Meas. −30dBm Simu. −30dBm Meas. −30dBm (a) (b) Figure 30: Simulated and measured S11 of the integrated multiband rectiﬁer for 10 kΩ load. (a) FM band. (b) Triband. 18 International Journal of Antennas and Propagation

80 70 60 50 40 30 20 10 RF-DC conversion efciency (%) 0 −40 −35 −30 −25 −20 −15 −10 −5 0 Input power level to rectifer (dBm) Simu. 98MHz Meas. 1800MHz Meas. 98MHz Simu. 2400MHz Simu. 875MHz Meas. 2400MHz Meas. 875MHz Simu. Simu. 1800MHz 98 + 875 + 1800 + 2400

Figure 31: RF-DC conversion eﬃciency of the integrated multiband rectiﬁer for 10 kΩ load.

(a) (b)

(c)

Figure 32: Dedicated testing setup for the rectenna at −10 dBm input power using (a) single multiband antenna element, (b) single multiband antenna element combined with FM rectenna, and (c) two multiband antenna elements (all measurements have been conducted with multitone sources). International Journal of Antennas and Propagation 19

7.1. Dedicated Source Testing. A ∼6.5 dBi log periodic an- changing the distance between the rectenna and the tenna (LPDA) with a frequency range of 10 MHz–3 GHz is source antenna to achieve −10 dBm power at the input to used as transmit antenna. It is connected to R&S SME06 the rectifier. signal generator to transmit varying powers at the desired frequencies. -e rectenna module is placed in the far-field of 7.2. Ambient Testing. Rectenna testing is also carried out in the transmitting antenna. a semiurban university campus. -e testing site is ap- -e multiband antenna is initially connected to the R&S proximately 300 m from the nearest cellular tower and 5 m FS300 spectrum analyzer and the received RF power is from the nearest Wi-Fi source. -e measured ambient noted. -e distance between the proposed antenna and the power levels vary around −60 dBm to −15 dBm for the source antenna is varied to calibrate input power levels relevant frequency bands with an average power level of between −40 and 0 dBm. Note that this power level refers to −15 dBm using the proposed antenna. Figure 33(a) shows the power input to the rectifier. Next, the spectrum analyzer the test setup of the proposed rectenna in the presence of is replaced with direct connection of the proposed antenna ambient RF sources. -e output voltage from the rectenna to the rectifier circuit. -e distance of the rectenna module with a single antenna is recorded to be 92 mV. Ambient from the source antenna is again varied to measure har- rectenna testing for the 3D cube structure is also carried vested voltage at different input power levels. -e DC output out with and without the FM rectenna. -e measurement voltage across the load resistance at different power levels is setup for the cube with multiband antennas only is shown measured using a digital multimeter. -e measured values of in Figure 33(b). An output voltage of 123 mV is obtained, RF-DC conversion efficiency are found using (18) where the which is higher as compared to that of the single mul- output DC power P is found from the measured DC DC tiband antenna. Figures 33(c) and 33(d) show the ambient voltage using testing setup for complete cubic rectenna including the V2 FM rectenna. -e output voltage obtained after deem- P (dBm) � 10log�DC × 103 �. (19) DC R bedding the RF cable used for FM antenna is 315 mV, L which is higher than the preceding cases. It is observed -e energy harvested by the rectenna using a single that the FM input significantly increases the harvested antenna is shown in Figure 32(a). A Peak voltage of voltage even though the ambient power available in the 0.926 V is harvested at an input power level of −10 dBm. FM band is lower than the communication bands, as Multiple RF sources have been used to generate signals of discussed in Section 3. -is increase in output voltage can required frequencies (900, 1730, and 2400 MHz) at be attributed to the high efficiency of FM rectifier as −10 dBm. Figure 32(b) shows the test setup of the mul- shown in Figure 31. tiband and FM rectenna using a dedicated source. It can -e average available power in the environment mea- − be seen that the harvested voltage is considerably in- sured using the proposed antenna is nearly 15 dBm. -e creased due to the addition of the FM band at a similar DC power harvested by the antenna is calculated to be − input power. -is is because of the high efficiency of the 20.035 dBm using (19) where the DC voltage and load multiband rectifier at FM frequencies. Figure 32(c) shows resistance are taken as 315 mV and 10 kΩ, respectively. the test setup of the two multiband rectennas using a Using (18), the overall conversion efficiency of the rectenna − dedicated source. Due to the nonavailability of four signal for ambient measurements is found as 31.3% at 15 dBm. generators, testing is carried out using only two multiband -is is the multitone efficiency and is therefore higher than antennas of the proposed cube structure. One multitone individual efficiencies at each frequency band except the FM source is placed in front of each antenna to establish an band. -e results are in accordance with the simulated ef- ambient environment with signal incident from all sides. ficiency values of quad-band rectifier as shown in Figure 31. -e output rectified voltage is 2.38 V, which is more than -e degradation in overall efficiency is accounted by the double the voltage produced by rectenna using a single losses incurred due to DC combination of two dissimilar multiband antenna (Figure 32(a)). -is proves that use of rectennas. four antennas will roughly quadruple the voltage output. Similarly the FM antenna connected with the cubic rec- 7.3. Comparative Performance Analysis. A comparison of tenna will add to the output voltage. -eoretically the the proposed rectifier to the existing topologies is presented measured voltage should increase by a factor of 4 after in Figure 34. -e proposed triband rectifier is at least 10% doubling the number of antennas. However, due to the more efficient than the designs reported in [10, 11, 13, 36] inherent losses of the power combiner and efficiency while there is a 40% improvement in the rectifier’s efficiency limitation of the rectifier, a lesser value (2.38 V) is at 2.4 GHz as compared to [20]. practically achieved. It needs to be noted that all the A tabular comparison between the proposed rectenna results of harvested output voltage with dedicated sources and some recent rectenna designs is given in Table 4. It can are obtained while keeping the level of input power level at be seen that the proposed design is the only one that −10 dBm at all frequencies of interest. -is has been done harvests energy from the FM band along with GSM900, to maintain uniformity in measurements and due to GSM1800, and Wi-Fi bands. It not only miniaturizes the limitations of signal source to achieve higher signal level. FM element but also integrates it into the overall struc- -e constant power level was ensured by periodically ture. In addition, a hybrid combination of outputs and 20 International Journal of Antennas and Propagation

(a) (b) (c)

(d)

Figure 33: Ambient environment testing setup for the rectenna using (a) single multiband antenna element, (b) proposed cube rectenna without FM antenna, (c) proposed cube rectenna (open), and (d) proposed cube rectenna (closed).

60 [14] 50 [13] (0.915GHz) 40 ( 1.8, 2.4GHz) [14] (1.85GHz) 30 [13] 20 (0.9GHz) [11] [36] [20] (1.7GHz) 10 (2GHz)

RF−DC conversion eﬃciency (%) [19] [20] (2.4GHz) (0.9GHz) 0 −40 −35 −30 −25 −20 −15 −10 −5 0 Input power level to rectiﬁer (dBm)

875MHz Voltage doubler 1730MHz Single series diode 2400MHz Grenaicher topology Figure 34: Comparison of conversion eﬃciency of proposed rectiﬁer with existing topologies. two antennas results in an output voltage as high as 2.38 V rectenna module can greatly enhance the harvested at an input power level of −10 dBm in dedicated mode. -e voltage. -e demonstration of powering an IoT sensor rectenna also shows 315 mV harvested voltage in ambient using the proposed rectenna in ambient settings is en- settings with an average available power of −15 dBm in the visaged as a future extension to this work. Integration of environment. Although this voltage is not enough to multiple 3D mountable rectenna modules makes the power conventional IoT devices, the integration of proposed design a good candidate for energy harvesting boosting circuits and power management units with the for IoT applications. International Journal of Antennas and Propagation 21

Table 4: Comparison of the proposed rectenna with related designs. Maximum Random Input Maximum Maximum RF- conversion Ref. Frequency polarization Type of matching power harvested DC DC conversion efficiency at (year) (GHz) receiving network used level output voltage efficiency (%) multitone input capability (dBm) (V) (%) Six-band 0.55, [10] Lumped matching 0.663 at 0.75, 0.9, 1.85, Dual CP −30 to −5 67 at −5 dBm 80 at −5 dBm (2016) for each band −15 dBm 2.15, 2.45 [11] Triple-band 2.1, Differential LP −20 to 0 53 at −13 dBm Not available Not available (2018) 2.4–2.48, 3.3–3.8 matching Broadband [12] 0.9–1.1 and Dual LP Not used −25 to 0 60 at 0 dBm Not available Not available (2017) 1.8–2.5 [13] Four-band 0.9, Lumped matching Dual LP −20 to 6 84 at 5.8 dBm 84 at 5.8 dBm 0.9 at −15 dBm (2015) 1.8, 2.1, 2.4 for each band Optimization [34] Triband 0.9, 1.8 −35 to Linear based TL 56 at −10 dBm 55 at −10 dBm 0.75 at −10 dBm (2018) and 2.1 −10 matching Optimization [35] Triband 0.9, 1.8 81 with −30 dBm 0.6 at Linear based TL −30 to 10 40 at −10 dBm (2017) and 2.1 per tone 500 μW/m2 matching [15] FM band Lumped element Linear NA NA Single tone input 1.72 at −20 dBm (2013) 81.9–84.7 matching [16] Lumped element 0.619 at FM band 88–108 Linear −35 to 10 74 at 5 dBm Single tone input (2017) matching 14.2 dBm [17] Lumped element FM band 88–108 Linear −20 to 10 56 at 0 dBm Single tone input 1.5 at 0 dBm (2019) matching

8. Conclusions Conflicts of Interest A novel multiband 3D rectenna is proposed for RF energy -e authors declare that there are no conflicts of interest harvesting using a modified cross-dipole antenna and a regarding the publication of this paper. transmission line based triband matching network designed for multiple power levels and frequency bands Acknowledgments using closed form equations. -e rectenna achieves a maximum measured efficiency of 70% with −6 dBm input -is work was funded through Higher Education Com- power at 98 MHz, whereas the ambient rectenna mea- mission (HEC), National Research Program for Universities surements show a maximum achieved efficiency of 31.3% at (NRPU), Grant Program Project no. 9971, titled “RF Energy multitone inputs. It exhibits a comparable RF-DC con- Harvesting for Internet of -ings (IoT) Applications.” version efficiency and a higher output DC voltage compared to recent published designs. Moreover, the design of References the receiving antenna and the triband impedance matching network is based on closed form equations making it ge- [1] W. Brown, “Experiments involving a microwave beam to neric in nature and extendable to any desired frequency power and position a helicopter,” IEEE Transactions on range. -e 3D cubic structure enhances antenna gain and Aerospace and Electronic Systems, vol. AES-5, no. 5, enables the rectenna to harvest greater amount of energy pp. 692–702, 1969. from all directions. -is is the first published rectenna [2] F. Congedo, G. Monti, L. Tarricone, and V. Bella, “A 2.45- design that is capable of harvesting energy from the FM GHz vivaldi rectenna for the remote activation of an end device radio node,” IEEE Sensors Journal, vol. 13, no. 9, band along with GSM900, GSM1800, and Wi-Fi bands. -e pp. 3454–3461, Sep. 2013. integration of multiple 3D mountable rectenna modules [3] B. J. DeLong, A. Kiourti, and J. L. Volakis, “A radiating near- makes it suitable for powering IoT sensors in practical field patch rectenna for wireless power transfer to medical scenarios. implants at 2.4 GHz,” IEEE Journal of Electromagnetics, RF and Microwaves in Medicine and Biology, vol. 2, no. 1, Data Availability pp. 64–69, 2018. [4] B. Strassner and K. Kai Chang, “5.8-GHz circularly polarized -e data used to support the findings of this study are dual-rhombic-loop traveling-wave rectifying antenna for low available from the corresponding author upon request. power-density wireless power transmission applications,” 22 International Journal of Antennas and Propagation

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[35] S. Shen, C.-Y. Chiu, and R. D. Murch, “A dual-port triple- band l-probe microstrip patch rectenna for ambient rf energy harvesting,” IEEE Antennas and Wireless Propagation Letters, vol. 16, pp. 3071–3074, 2017. [36] M. Pinuela, P. D. Mitcheson, and S. Lucyszyn, “Ambient RF energy harvesting in urban and semi-urban environments,” IEEE Transactions on Microwave