AWideband Rectenna Using High Gain Fractal Planar Monopole Antenna Array for RF Energy Scavenging

&is paper introduces a wideband rectenna that can scavenge ambient wireless power to a range of frequency band from 0.91GHz to 2.55GHz efficiently. &e proposed rectenna is based on a wideband 2× 2 fractal monopole antenna array with omnidirectional radiation patterns and high gains of 5 to 8.3 dBi at the desired bands. An improved two-branch impedance matching technique is presented which is designed to enhance the rectifier circuit performance with a relatively low input power ranging from −25 dBm to 10 dBm. Also, a full-wave wideband rectifier that can suitably improve the RF-to-DC power conversion efficiency is designed for the rectenna. A peak efficiency of 76%, 71%, 61%, and 62% is obtained at 950, 1850, 2100, and 2450MHz, respectively. Measurement results show that a conversion efficiency of 68% has been achieved over an optimal 4.7 kΩ resistor when the simultaneous four-band input power level is −10 dBm. Moreover, an output DC voltage of around 243mV with voltage varying within 160–250mV can be achieved by gathering the low ambient wireless power inside laboratory. &is study proves that the proposed rectenna can be applied to a range of many low-power electronic applications.


Introduction
With the rapid development of wireless systems, the ambient radio frequency (RF) energy is increasing from several of electromagnetic power sources around the world, such as the mobile telephones, mobile base stations, television/radio broadcast stations, and Wi-Fi routers. Despite these ambient electromagnetic sources, an RF energy scavenging system is one of the most desirable technologies for steady charging of low-power electronic devices, because that could omit the requirement for a battery overall and save maintenance cost [1]. e RF energy harvesting by technologies of rectifying antenna (rectenna) is a practicable solution to convert the ambient wireless power to DC power.
To maximize harvested RF energy in the rectenna, the antenna in this system should be able to receive high RF energy from multiple frequencies at any directions in the environment.
us, the antenna should be of wideband operational frequency and omnidirectional radiation pattern. Moreover, since the power density of RF signals and its levels in the ambience are almost weak [2], it is essential to design antenna array consisting of more elements with higher gain to take enough power to reach optimal input power. However, designing a wideband high gain rectenna is very challenging, due to the nonlinearity behaviour of the rectifier. It has been proven that the input impedance of the rectifying circuit differs in terms of input power level, frequency, and also load impedance [3]. So far, several techniques have been presented for multiband [4,5] or wideband [6][7][8][9][10][11] rectenna designing but there are various limitations. As already mentioned in [12], wideband antennas are desirable rather than multiband antennas, because they have easier design, and they are interoperable among countries, whose assignment plans of frequency can be diverse among them. So, this paper focused on the wideband rectenna.
In [6], a wideband bent triangular omnidirectional antenna with both horizontal and vertical polarized waves is proposed for scavenging from 0.85 to 1.94 GHz, but the bandwidth is not enough for energy harvesting applications and the realized gain is relatively low. In [7], a broadband dual-polarized cross-dipole antenna with a broadband rectifying circuit with a two-branch impedance matching circuit is proposed for ambient wireless signals with a lowpower density from 1.8 to 2.5 GHz. In [8], a wideband rectifier array associated with a dual-polarized and differential-feed fractal slotted ground antenna for harvesting from 1.8 to 2.6 GHz is proposed. For low incident power densities less than 200 μW/cm 2 , a wideband differential rectifier with an Archimedean spiral dipole antenna for harvesting from 1.2 to 5 GHz is presented [9]. However, the achieved bandwidths for these three rectennas are not adequate for RF harvesting applications and the radiation pattern is not uniform and omnidirectional across the total band of interest. A three-dimensional (3D) stacked folded dipole array is designed for RF energy harvesting [10]. An array is also presented in [11] by a 3D 4-element Vivaldi antenna with the frequency range from 1 GHz to 3 GHz and rectifier circuits for energy harvesting applications. However, a 3D antenna array increases the antenna overall size, which makes the system bulky and the RF-to-DC conversion efficiency is not very high. e overall performance of a rectenna, in addition to the antenna performance, basically depends on the RF-DC conversion efficiency of the rectifying circuit. Recently, the main challenge of RF energy harvesting is how to enhance the power conversion efficiency of the rectifying circuit at low-input power levels over a wideband frequency. An appropriate matching network is one of great importance for enhancing the performance of rectifier circuit with a fixed load resistance [13,14]. In order to remove the higher order harmonics generated by the nonlinear circuit and enhance the conversion efficiency, a filter between the antenna and the rectifier can be also used [15,16]. In [6-10, 13, 17-20], several types of wideband rectifier circuits are presented. In the best case of input power level, the maximum reported power conversion efficiencies vary from 30 to 70%. However, high efficiency designs are mostly for a relatively high input power (such as levels of 0 dBm [17], 10 dBm [18], 19.5 dBm [19], and 12.8 dBm [20]) which is not suitable for ambient energy scavenging.
In this work, we designed a monopole wideband antenna array fed by a microstrip-to-slot-line transition feeding network. e feeding network is based on slot-line stubs and radial microstrip for the realization of broadband microstrip-to-slot-line transitions [21,22]. In the proposed antenna, compact size and wideband functions are obtained by using Koch fractal geometry and loaded slot on radiation stub. e compact structure and easy configuration are the main advantages of fractal monopole antenna [23,24]. e loaded slot on radiation stub can also enhance the impedance bandwidth of the antenna [25]. In the following, based on [7], a full-wave Greinacher rectifying circuit with a twobranch impedance matching network is employed in the antenna to realize the broadband rectenna with low input power levels. Simulated results and experimental measurements are carried out to evaluate the performance of the proposed antenna, rectifying circuit, and rectenna, separately.

Wideband Antenna Array Design
e layout and detailed dimensions of a single monopole fractal antenna element and 2 × 2 array are shown in Figures 1(a) and 1(b), respectively. e antenna is proposed on a FR-4 substrate with ε r � 4.4, tanδ � 0.02, and thickness h � 0.8 mm. In the configuration of the single element, a modified 2 nd iteration of Koch fractal radiation patch with an embedded rectangular-shaped thin slot is fed by a 50 Ω microstrip line. e ground plane is modified to partial rectangular plane to enhance the desired impedance bandwidth and omnidirectional radiation pattern characteristics as a monopole antenna. As shown in Figure 1(b), for array structure, a microstripto-slot-line transition feeding network is used. It has attractive features compared to conventional microstrip feed network such as low feeding and radiation losses, no need for impedance matching circuits, and extremely simple circuit structure which are mostly owing to the good performances of both the microstrip-slot parallel branch circuit and the slot-microstrip series branch circuit [21]. e performances of the single and array antennas are analyzed and optimized by using Ansoft HFSS full-wave simulator [26]. e design process of the proposed antenna array can be summarized as follows: (1) Design a radiator patch with a hexagon-shaped, microstrip feed-line with 50 Ω impedance, and a partial rectangular ground plane, in which monopole antenna can operate in a wideband frequency with the central frequency of 1.8 GHz (Antenna I).
(2) Design the first and second Koch fractal iterations that are implemented to the hexagon-shaped radiator. e geometry of Koch can be produced using an ad hoc iterative function system (IFS) algorithm designated by a set of affine transformations W. In other words, the IFS works by applying a series of affine transformations W to an elementary shape over much iteration. Various iterations of the Koch fractal are listed in Figure 2.
e IFS algorithm and design rules of Koch curves in the proposed monopole antenna can be expressed symbolically as [27] where n is the nth fractal iteration. ese transformations W can be considered successively to make the monopoles K 1 , K 2 , . . ., K n . e resonant frequency of the antenna decreases with the increase in the number of iterations, although it reduces its impedance bandwidth. e perimeter length (l perim ) and the effective area (A) of the proposed modified fractal patch geometry, at indentation angle (α � 60°) and at iteration n, are found as [27] l perim (n) � 4 2 3 (2) As shown in these formulas, the perimeter or the overall electrical length of the antenna increases with increasing the number of iterations so the decrease in resonant frequency stems from that. On the other hand, the enclosed area of the fractal antenna decreases with iteration increasing. erefore, the ratio of perimeter length to enclosed area is large when more iterations are implemented in the fractal geometry, so the impedance bandwidth decreases with this extending of iteration (Antenna II).
(3) In this stage, a thin rectangular slot is applied to the fractal radiating patch. is technique increases bandwidth, especially at the end of the frequency band [25]. Indeed, this thin slot is used for the new resonance excitation function, and hence multiresonance characteristics with wider impedance bandwidth can be made. erefore, the operating band of the antenna is improved and increased, so that it completely covers the RF energy scavenging interesting bands (Antenna III). Based on stages 1 to 3, the development designs of the single antenna are shown in Figure 3, and the corresponding simulated return loss curves are illustrated in the same figure.
(4) Based on the theory of microstrip-to-slot-line transition, the proposed feed network can be designed, and its structure and simulated results of  the design are shown in Figure 4.
e feeding network contains one slot-line on the ground plane and three microstrip lines on the radiating plane. e central microstrip line is fed by a coaxial probe and slot-line is applied to transfer the input power to the two side microstrip lines. e equivalent circuits and performance mechanisms of this feeding network for maximum transmission coefficient with details are explained in [28,29]. e fan-shaped terminal is applied to increase the impedance bandwidth [21,22]. e widths of the slot-line and microstrip of the feeding network are obtained according to the input impedance of the probe and monopole antenna. rough the analysis of simulating, it can be found that the transmission coefficient is very sensitive to the slot-line width, while the effects of changing length of the side microstrip lines and space of central and beside microstrip lines on the transmission coefficient are insignificant. e reflection coefficient (S 00 ), transmission coefficient (S 10 ), and isolation between the ports (S 12 , S 13 , and S 14 ) are only shown in Figure 4(b), because the magnitudes of the signal from the four output ports are the same. It is also important to note that the signal phases of port 1 with port 4 and port 2 with port 3 are consistent due to the symmetrical structure of the network. At the desired bands, the ports have isolation below −10 dB and therefore each port can scavenge RF energy independently. (5) Finally, based on the above design stages, the 2 × 2 antenna array is designed, and its structure is shown in Figure 1(b). In the structure, the antenna array is fed by a 50 Ω coaxial probe. e spacing between elements is obtained according to both of the electromagnetic coupling and radiation performance among elements. e adopted element spacing of the array is 0.6 λ 0 along E-and H-planes, at the central frequency of 1.8 GHz.
e proposed fractal monopole antenna array is simulated, fabricated, and measured. Figure 5 shows photographs of the fabricated prototype. e simulated and measured return loss curves of the proposed antenna array are shown in Figure 6, which appear to be in relatively good agreement. e discrepancies between the simulated data and measured results are almost due to test equipment and measurement errors. Measured result shows that the proposed fractal antenna array can cover a frequency range from 0.91 to 2.55 GHz (95%, 10 dB impedance bandwidth), which is slightly lower than that of the feed network and the single monopole antenna due to the electromagnetic coupling between elements. Nevertheless, it covers the bands of most interest for RF energy harvesting application. Figure 7 shows the measured and simulated peak gains of the array antenna in the entire band of interest. e measured gains are a little less than the simulated results, which is because of dielectric loss, the slot-line and calibration errors, and fabrication tolerance. e antenna array achieved an average gain of 6 dBi over the operating frequency and the maximum value of 8.3 dBi which occurs at about 0.95 GHz. It should be noted that the antenna gain is relatively high over the working band respecting the compact size and omnidirectional behaviour of the proposed antenna. Besides, the efficiency of the antenna decreases with the growth of frequency, so the value of gain reduces for higher frequencies.
e following values of simulated efficiency are obtained: 82.1%, 76.1%, and 72.4% at 0.95 GHz, 1.85 GHz, and 2.45 GHz, respectively. e measured radiation patterns along H (x-z) and E (yz) planes for three different frequencies of 0.95, 1.85, and 2.45 GHz are presented in Figure 8. As seen in this figure, the radiation patterns are nearly stable and uniform at the different frequencies. In addition, it is also found that the antenna maintains omnidirectional pattern in the E-pane and bidirectional pattern in the H-plane over the frequency band. e increase in cross-polarization level at low frequencies is almost by reason of hybrid current distribution excitation on the antenna radiator patch. Moreover, the side-lobes appear at high frequencies, because of the spacing between elements is very large for high frequencies.

Wideband Rectifier Design
e rectifying is a nonlinear circuit that mostly contains an impedance matching circuit for delivering the maximum power at the selected frequencies, rectifying elements to make the RF-to-DC conversion, a DC low pass filter for smoothing the ripple and storing of output DC, and a resistor load. So far, different kinds of rectifier topologies are introduced to convert the harvested wireless power to DC power [30]. e schematic of the proposed RF-to-DC circuit is shown in Figure 9 that is based on [7,20]. e RF-DC circuit contains a symmetric adjusted version of a single stage Greinacher rectifier. e Greinacher is a full-wave rectifier with high sensitivity and RF-DC conversion efficiency that is equivalent to a voltage doubler circuit with two stages that formed in a bridge type [7]. ere are two branches in the rectifier with two zero-bias low-barrier Schottky diodes in each branch. e series-mounted Schottky diode HSMS-285C from Avago is used for the rectifier circuit owing to its low turn-on voltage condition and small junction capacitance for a low input signal (forward bias voltage: 150 mV at 0.1 mA and a junction capacitance C j0 of 0.17 pF) [31].
Scavenging in a wideband operation needs the use of a rectifier circuit with operating in a wideband frequency. For this purpose, design of the matching circuit is not easy since the rectifier circuit has complex input impedance that differs with input power level, frequency, built-in characteristics of the Schottky diode, and resistance load [3]. In our layout as seen in Figure 9, a wideband two-branch matching network is accomplished and designed through the insertion of an optimized coupled line with one shorted end stub before each of the divider branches as [20]. e coupled line can be employed to realize wideband impedance transformation. Actually, it can produce impedance matching at multiple frequencies that can tune it to realize wideband matching [20]. To combine two branches, an optimized T-junction power divider is applied as seen in [32]. One of the benefits      of this scheme is that size of the impedance matching circuit is compact, because the circuit is designed on two different branches. With this two-branch impedance circuit, the rectifier can work as a function of input power level and frequency [7,20]. e upper branch is designed to acquire the circuit matched around 2.1 and 2.4 GHz, while the lower branch is designed to get the circuit matched around 0.9 and 1.8 GHz. As shown in Figure 9, capacitors C 1 and C 2 are part of the basic Greinacher rectifier circuit that can be considered as a DC-block for preventing the DC power from returning back to signal source. Meanwhile, capacitors C 3 and C 4 are used as DC-pass filter to smooth the output DC waveform and store the energy before the resistance load R L . e load R L � 4.7 kΩ is attached to the output port for collecting the DC power. e matching and rectifier circuits are simulated, analyzed, and optimized using Harmonic Balance (HB), Large Signal S-parameter (LSSP), and Momentum from Keysight Agilent ADS [33]. In order to improve the accuracy, a SPICE model for the Schottky diodes attained by the Avago Technology [31] is used in the simulation of the rectifier circuit. e chip capacitors in the design are also demonstrated using S-parameter files achieved by Murata. e proposed rectifying circuit is designed on Rogers 4003 substrate with relative dielectric constant of 3.55, thickness h of 0.8 mm, and tanδ of 0.0027. e rectifying circuit is optimized for the −20 dBm power input with input impedance of 38.1 + 19.5j at 0.95 GHz, 47.1 + 33.4j at 1.85 GHz, 31.2 + 13.3j at 2.15 GHz, and 36.5-2.7j at 2.45 GHz. e prototype of the proposed rectifier is shown in Figure 10 and it is measured in laboratory. e measured and simulated reflection coefficients |S 11 | of the designed rectifier for different input power levels, especially at lowpower levels, are exhibited in Figure 11. It can be seen that the proposed rectifier can cover the desired frequency band (|S 11 | < −10 dB) from 1.7 to 2.6 GHz with an additional frequency around 0.9 GHz band under three input power levels from −25 to −15 dBm with an interval of 5 dBm. e | S 11 | of the rectifier for three power levels is less than −10 dB for the four center frequencies of interest. e simulated results have relatively good agreement with the measured ones, in which the minor discrepancies between them are almost because of the unknown parasitic behaviour of the SMD components applied in the rectifier circuit. e RF-DC conversion efficiency of the rectifying circuit can be calculated as where V DC is the output DC voltage through the resistive load R L and P in is the input power of the rectifying circuit. e measured and simulated RF-DC conversion efficiency (with the optimal load R L � 4.7 kΩ) for three input power levels versus operating frequency are depicted in Figure 12. As observed, it is found that the proposed rectifier operates well in a wide impedance bandwidth from 0.75 to 1.1 GHz, 1.7 to 2.1 GHz, and 1.3 to 2.7 in which the efficiency is still greater than 20% at the desired frequencies for input power levels over −15 dBm. A small frequency shift occurs between the measured and simulated data, which is because of model inaccuracy of the diodes and the SMD components and fabrication tolerance. e measured and simulated conversion efficiency at the four desired frequencies in terms of input power are shown in Figure 13. During the measurement, an RF signal   generator is selected as the input source to the rectifier. e maximum efficiency at 0.9, 1.8, 2.15, and 2.45 GHz is around 76, 71, 61, and 62% with 5, −5, 10, and 11 dBm input, respectively. Compared with the simulated results, a minor error is observed, which could emerge by the manual welding process of the Schottky diode and chip capacitors. Finally, the measured and simulated conversion efficiency of the simultaneous multiband input signals in terms of input power level are shown in Figure 14. Based on [4,5], it is expected to enhance the overall RF-DC conversion efficiency of the rectifier compared to separate single band inputs. As shown in Figure 14, the proposed rectifier circuit for single, two, and three-band input signals is investigated. It can be seen that the maximum conversion efficiency is equal to 68% for low levels of power inputs when the proposed rectenna can receive three band signals simultaneously. In this case, the total RF-DC conversion efficiency enhancement is over 30% in the low levels of power that are working range of the designed rectifier.

Wideband Rectenna Measurement
e designed antenna and the rectifier are connected by a SMA connector with 50 Ω characteristic impedance in order to make a rectenna. e measurement of the proposed wideband rectenna is presented in the following.
To verify the proposed wideband rectenna performance in a realistic measurement of the harvested power, first a spectrum analyzer is connected to the proposed array antenna for integrating the power spectrum over bandwidth. An indoor laboratory environment is selected with a relatively low ambient RF power density to demeanor the test. Nearest cell phone tower site is about 400 m away from the laboratory, while a Wi-Fi router is near the test location. e received power in dBm in terms of frequency is shown in Figure 15. It is clear that the power is mostly distributed at three frequency bands which are GSM-900, GSM-1800, and Wi-Fi with average power levels of −42, −30, and −23 dBm, respectively. It should be noted that the measured overall power in the band received by antenna array is variable as a function of time. Second, the proposed antenna is substituted by the proposed rectenna and the DC output voltage is measured by using a voltage meter, as demonstrated in Figure 16. e output voltage is measured to vary around 160 to 250 mV most of the time, and it could also reach over 250 mV sometimes by reason of the change of ambient wireless energy, e.g., 243 mV as shown in Figure 16.
is voltage level can be absolutely used to run a low-power electronic device.

Conclusions
In this paper, a novel rectenna with wideband fractal antenna array and a wideband rectifier has been presented for ambient RF energy scavenging from 0.91 GHz to 2.55 GHz. Antenna array with four monopole elements is proposed based on 2 nd iteration of Koch fractal geometry. A microstrip-to-slot-line transition feeding network has been used in the antenna array to guarantee the wideband operation and improve its performance, while a thin slot has been inserted in the monopole radiating patch to enhance the frequency bands and especially cover the higher band. By investigation of reflection coefficient, realized gain, and radiation characteristics, it is found that the proposed antenna can be most proper for harvesting. A wideband rectifying circuit using   International Journal of Antennas and Propagation two-branch matching circuits by applying an optimized coupled line and a full-wave rectifier circuit has been presented to match with the ambient wireless signals in the lowpower density. e measured and simulated results have shown that the rectifying circuit has maximum conversion efficiency of around 68% for −10 dBm input power at the four bands simultaneously. e proposed rectenna exhibits an output DC voltage between 160 and 250 mV when measured in the indoor ambience. Considering the performance of the rectenna in various conditions, the presented design is appropriate for many low-power electronic devices and can be used in many wireless applications without a battery.
Data Availability e data required to replicate the proposed structures and support the conclusions are illustrated in the figures and pictures. Also, for more aid, the readers can contact the author by e-mail through mmfakharian@ugsr.ir.

Conflicts of Interest
e author declares that he has no conflicts of interest.