Multi-Frequency Solar Rectenna Design for Hybrid Radio Frequency–Solar Energy Harvester

Abstract

This paper put forward a hybrid energy harvester for collecting RF and solar energy in quad-band (GSM-900/1800, ISM-2400 and WiMAX-3500). By introducing diverse parasitic structures, good impedance matching with unidirectional radiation is achieved in the multi-band. Below the solar antenna, a low-power rectifier circuit is employed to achieve broadband rectification. Under the input power of 0 dBm, and maximum RF-DC conversion efficiency of 56.94% is realized. Accordingly, the hybrid energy harvester collects RF and solar energy individually or simultaneously, and then converts it into DC for power supply. With a light intensity of 1500 lux, the solar cell obtains 1.732 mW, and the rectenna can harvest additional 0.37–0.405 mW power. The proposed RF–Solar energy harvester has the advantages of multi-frequency operation, high gain, and high energy harvesting conversion efficiency.

1. Introduction

Solar energy, a widely accessible renewable source, offers green, pollution-free and large storage capacity, enabling autonomous power for devices without batteries [1,2]. However, a key potential problem of solar energy is its intermittency, and the night cannot provide sufficient power. Accordingly, a single solar energy supply is unfavorable for the sustainable supply of power. With the accelerated advancement of wireless communication systems, the environmental wireless power density is increasing. Using RF energy to power small power sensors can save costs and replace batteries. Thus, for various signals, especially in urban areas, using a rectenna can efficiently converts RF signals into usable DC power [3,4,5,6]. Hybrid Energy Harvesting (HEH) combines various energy sources to produce a higher electrical output [6]. A hybrid energy harvester, combining the solar cell and the rectenna, is designed to collect both optical and microwave power, ensuring stable energy supply.

Recently, many hybrid energy harvesters with solar cell have been studied with excellent performance [7,8,9,10,11,12]. M. Hamza et al. [7] and B.-Y. et al. [8] proposed dual-band hybrid energy harvesters with a low energy harvesting conversion efficiency (the product of antenna efficiency and conversion efficiency [8]). To receive multi-band electromagnetic energy, a tri-band solar Vivaldi rectenna was presented with high antenna radiation efficiency, but the conversion efficiency was not given [9]. Similarly, A. Collado et al. [10] and P. Zhang et al. [11] designed hybrid energy harvesters at the tri-band, achieving only 15% RF-to-DC conversion efficiency [10] and low energy harvesting conversion efficiency (<15%) [11]. In [12], a printed wide-slot hybrid harvester was designed at quad-band, but the energy harvesting conversion efficiency is low (15.99%@0.9 GHz, 11.3%@1.45 GHz, 30.23%@1.81 GHz and 8.21%@2.25 GHz).

To harvest solar energy and the diffused radio wave, this paper designs a quad-band (GSM 900/1800, ISM-2400 and WiMAX-3500) solar rectenna. A thin film solar cell is placed above an asymmetric dipole antenna. By adding parasitic structures, good impedance matching is realized. To convert RF to DC power, a low-power rectifier circuit is placed below the antenna for broadband rectification. The proposed hybrid harvester is characterized by multi-frequency, high gain, and high energy harvesting conversion efficiency to provide a reliable power supply.

The paper is structured as follows: Section 2 indicates the proposed solar antenna, including the electromagnetic model of thin film solar cell. Section 3 indicates the multi-frequency rectifier circuit for low power. Section 4 presents the performance of the optical/microwave rectenna, and the existing hybrid energy harvesters are compared. Section 5 concludes the paper.

2. Solar Antenna Design

In this paper, a RF–Solar hybrid energy harvester is proposed, which consists of a solar antenna and a low-power rectifier circuit. The solar antenna is composed of a solar cell, an asymmetric dipole, a feeding balun, and a three-dimensional U-shaped grounding. The solar cell is located on the top of the antenna as the radiating element. The rectifier circuit is located at the back of the antenna through the SMA connector, sharing the same ground with the antenna.

The proposed solar antenna is shown in Figure 1a, which contains three F4B-M300 substrates (relative dielectric constant of 3 and loss tangent of 0.0017) with a thickness of 1 mm. Three pairs of asymmetric bowtie dipoles are implemented on the bottom of the F4B substrate, while a pair of asymmetric parasitic triangular patches is introduced on the top of the substrate for good impedance matching. By placing two shorting posts on the dipole arm, a common mode resonance can be controlled [13]. Coupling with the asymmetric dipoles, a metallic circular patch is printed above the middle substrate layer to introduce a new high-frequency resonant mode. A solar cell acting as a parasitic patch is placed above the top substrate for unidirectional radiation with high gain. To eliminate the effect of the DC generated by the solar cell on the RF antenna performance, the substrate layer is added beneath the solar cell. To feed the antenna, the balun with three subsections operates as a wide-band transition between a single-ended coaxial feed and the balanced dipole arms, which is etched on both sides of a 1.5 mm thick F4B-M300 substrate, as shown in Figure 2. To achieve unidirectional radiation, a U-shaped metal grounding plane is designed, which is formed from a metal plate welded to two vertical metal plates at both ends in Figure 1a.

2.1. Quad-Band OCFD Antenna

Firstly, an off-center-fed dipole (OCFD) is proposed. Different from the conventional center-fed dipole, two dipole arms in different length of the OCFD antenna are asymmetric for multi-frequency operation [14]. Fed from the center at λ/4, an OCFD antenna operates at the resonant frequency of f₀ and 2f₀, where f₀ is the fundamental frequency of a center-fed antenna. It is worth noting that the antenna impedance of OCFD structure is very high. To handle the issue, a balun used as balanced–unbalanced transformer, and an impedance transformer of 1:4 is essential for the conversion of high antenna impedance to 50 Ω [15]. In this design, an OCFD antenna is modeled, with the feeding point shifting off-center of λ/4@f₀ (the fundamental frequency f₀ of 1.2 GHz). For long arm of the asymmetric dipole, low-frequency resonance is excited, while the high-frequency resonance is generated by the short arm. By adjusting the length of the arms l₁ and l₂, two resonances at 1.2 and 3 GHz are achieved. Then, by tapering the end of the long arm, good impedance can be achieved at low frequency of 0.9 GHz band, but high-frequency resonance basically remains unchanged, as shown in Figure 3a. To achieve good impedance matching at a low frequency of 0.9 GHz. Two pairs of bowtie dipoles, rotating the original asymmetric dipole counterclockwise by 17° and 34°, respectively, are introduced, as well as a pair of asymmetric parasitic triangular patches on the other side of the substrate, as shown in Design A in Figure 2.

Based on the asymmetric dipole antenna, the quad-band antenna is designed by adding the parasitic structures [16,17,18], since new resonance modes are generated for multi-frequency band operation. By sequentially loading a metal shorting post (Design B) and a vertical wall plate (Design C), two new resonance modes are excited for two resonant frequencies of 1.8 GHz band and 2.45 GHz band, respectively, as shown in Figure 3b. It can be seen that the bandwidth of the antenna is broadened in the 1.82–1.94 GHz and 2.4–2.6 GHz. Additionally, a circular patch (Design D) is added on the top of the middle F4B substrate [19]. By loading the circular patch, the TM₁₁ mode [19,20] is excited, thus providing a new resonance at the high frequency of 3.5 GHz. By adjusting the size and position of the parasitic structures, good uni-directive radiation patterns are realized at the desired quad frequency bands of 0.94–0.98 GHz, 1.85–1.89 GHz, 2.46–2.52 GHz, and 3.09–3.65 GHz.

2.2. Solar Cell Model

In this design, a commercially available thin-film solar cell (Ynvi indoor light power LEH3_50 × 50_6_10_DY) is loaded as a parasitic patch. Given that the effects of the solar cell on the antenna performance is required to be fully considered, the solar cell is modeled for co-simulation. The structure of the thin-film solar cell is shown in Figure 1b, with a thickness of 0.2 mm. It consists of a transparent PET film, a photovoltaic converter module, metal grid lines, and metal pins. The solar cell is encapsulated by a transparent PET film with a thickness of 0.05 mm, and inside it is a photovoltaic conversion module with a thickness of 0.1 mm. The photovoltaic conversion module in series–parallel is divided by five longitudinal finger electrodes and two transverse busbar electrodes. Through the grid lines, DC output generated by the photovoltaic conversion is carried to the metal pins for DC power supply. In the simulation, the metal grid lines are set to PET boundary condition. The relative dielectric constant and loss angle tangent at different frequencies for the solar cell is extracted [21], as listed in

Table 1. Relative dielectric constant and loss angle tangent of solar cell.

Frequency (GHz)	PET Film ԑr	PET tanδ	Solar Cell ԑr	Solar Cell tanδ
0.9	2.921849	0.0177	5.57–6.15	0.04–0.069
1.8	2.979557	0.00796	5.4–6.0	0.067–0.08
2.4	2 893159	0.0164	5.24–5.83	0.063–0.073
3.5	2.890049	0.0281	5.2–5.8	0.078–0.089

, where the relative permittivity of the PET film (solar cell) ranges from 2.89 (5.18) to 2.97 (6) with the frequency of 0.9–3.5 GHz, and the loss tangent is less than 0.03 (0.1).

The solar cell is placed on the top of the dipole antenna in complete coverage, forming solar dipole antenna. As a part of the antenna parasitic patch, the solar cell is coupled to the antenna, resulting in an improvement of bandwidth at 3.5 GHz frequency band, as shown in Figure 3b.

3. Multi-Frequency Rectifier Circuit Design

To convert RF power to DC power at multiple frequencies, a wideband rectifier circuit is connected to the antenna behind. In this design, a frequency-selective rectifier topology is performed in the same circuit for multi-frequency operation. With the frequency selective topology, complicated matching circuits are avoided, leading to a compact size with a simplified circuit structure and high conversion efficiency [22]. The rectifier circuit is printed on F4B-M265 substrate (dielectric constant of 2.65 and loss tangent of 0.0014) with a size of 40 mm × 45 mm × 1 mm.

The rectifier circuit with a frequency-selective rectifier topology consists of a pre-capacitor, matching network, Schottky diode (Model: HSMS-2850), L-shaped low-pass filter, and DC load, as displayed in Figure 4, where the frequency-selective topology contains two matching branches, i.e., short-circuited branch 1 (Cell A) and short-circuited branch 2 (Cell B, the capacitor C₀ and the inductor L₁). The center frequencies of Cell A and Cell B are denoted as f₁ = 2.45 GHz and f₂ = 3.5 GHz, respectively. The L-shaped low-pass filter, containing the inductor L₂ and bypass capacitor C_L, is used to smooth the DC output as well as to suppress high-frequency harmonics. The input impedance of Cell A with the electrical length λ₁/8@f₁ is Z₁, where λ₁ is the guided wavelength of the transmission line at f₁. The input impedance of the shorted branches in Cell A at different frequencies is

$${Z(f)\mid }_{l={\lambda }_1/8}=\mathrm{j}{Z}_1\tan ({\displaystyle \frac{\pi }{4}}{\displaystyle \frac{f}{f_1}})=\left\{\begin{array}{l}0,f=0\\ j{Z}_1,f=f_1\\ j0.019Z_1,f=f_2\end{array}\right\}$$

(1)

Equation (1) indicates that the input impedance of Cell A is j0.019Z₁ at f₂, which suggests that impedance of incompatible in Band 2 centered on f₂. Similarly, the input impedance of cell B at f₂ is $Z(f)\left|l={\lambda }_2/8\right.={\mathit{jZ}}_2\mathit{tan}({\displaystyle \frac{\pi }{4}}{\displaystyle \frac{f}{f_2}}){=jZ}_2$, while the input impedance at f1 is j0.01Z₂, i.e., cell B is switch off (switch on) in frequency band 1 (2) centered on f₁ (f₂) due to impedance mismatch (impedance matching). Thus, the matching branches of the rectifier circuit have good frequency selectivity, which selects Cell A in Band 1 and Cell B in frequency Band 2, contributing to broadband performance at the output.

To achieve maximum RF-DC conversion efficiency, the impedance of the rectifier circuit is necessary for conjugate matching the antenna impedance at each operation frequency band [23]. By investigation, the antenna impedance is set as the source impedance of the rectifier circuit at each of the four frequencies for co-simulation. By tuning the frequency-selective stubs (Cell A and Cell B), good impedance matching is achieved in the two bands of 2.45 GHz and 3.5 GHz. To achieve a good matching with the antenna in the 0.95 GHz and 1.85 GHz bands, a capacitor C₀ in parallel with the diode of Cell B, and L₁ connected between the two Cells is introduced for branch 2. For example, at the operating frequency of 0.95 GHz, the impedance of the circuit goes from P1 to P2 by adding the inductor L₁, turning into be inductive, and then moves to P3 with the addition of capacitor C₀, changing from inductive to resistive for good impedance matching to the antenna, as illustrated in Figure 5a. At 1.85 GHz, the impedance of the circuit changes from Q1 to Q2 in the addition of capacitor C₀, and then it varies from Q2 to Q3 with the inductor L₁, as shown in Figure 5b. The complex impedance of the rectifier circuit at the four frequencies are (29.6 − j37.3) Ω@0.95 GHz, (57.7 + j0.83) Ω@1.85 GHz, (37.5 + j16.5)@2.45 and (44.4 + j7.4) Ω@3.5 GHz, respectively, as listed in

Table 2. The complex impedance of the antenna and rectifier circuit.

Frequency (GHz)	0.95	1.85	2.45	3.5
Antenna R (Ω)	25.1 − j47.7	59.2 − j5.9	38.7 − j15	35.7 − j7.2
Rectifier circuit R (Ω)	29.6 − j37.3	57.7 + j0.83	37.5 + j16.5	44.4 + j7.4

, where good conjugate matching is implemented between the circuit and the antenna. As shown in Figure 6, for branch 1 (Cell A), good impedance matching |S₁₁| < −10 dB is achieved in the frequency band of 2.4–3.32 GHz, while the frequency bands of |S₁₁| < −10 dB are realized in the 0.9–1.12 GHz, 1.8–1.92 GHz and 3.4–3.54 GHz bands for branch 2 (cell B with L₁ and C₀). Through deploying the frequency selective structure with the desired L₁ and C₀, the frequency bands of the proposed rectifier circuit for |S₁₁| < −10 dB are 0.92–2.05 GHz and 2.44–3.68 GHz. Based on the operation frequency of the antenna (0.91–0.97 GHz, 1.8–1.89 GHz, 2.4–2.51 GHz, 3.05–3.67 GHz), the final rectenna can operate in four frequency bands: 0.92–0.97 GHz, 1.8–1.89 GHz, 2.44–2.51 GHz and 3.05–3.67 GHz.

4. Performance and Discussion

The designed multi-frequency solar rectenna was fabricated. As shown in Figure 7a, where the solar cell is stuck on the top of the three stacked F4B-M300 substrates. The balun is welded on the bottom of the F4B substrate, erecting between the F4B substrate and ground plate. The coaxial connector is inserted into the groove dug out of the U-shaped ground, and its inner conductor soldered to the balun. The rectifier circuit is printed on the F4B-M265 substrate, which is connected to the antenna behind through a pair of SMA coaxial connectors. To investigate the performance of the solar rectenna, designed solar antenna and rectifier circuits were simulated and measured individually, and the DC output power of the solar rectenna was tested under microwave or/and light irradiation.

4.1. Performance of Solar Dipole Antenna

The solar antenna is simulated and measured, and the simulation is basic agreement with the measurement. The measured (simulated) reflection coefficient is depicted in Figure 8, where good impedance, i.e., |S₁₁| < −10 dB is achieved for the frequency band of 0.9–1.08 GHz, 1.62–1.93 GHz, 2.37–2.49 GHz, and 3.22–3.65 GHz (0.91–0.97 GHz, 1.8–1.89 GHz, 2.4–2.51 GHz, 3.05–3.67 GHz). The results demonstrate that the proposed solar antenna has good multi-frequency characteristics.

Figure 9 indicates the measured (simulated) radiation patterns at frequencies of 0.95 GHz, 1.85 GHz, 2.45 GHz, and 3.5 GHz. The results indicate that unidirectional radiation patterns are achieved at the operation frequency bands. At 0.95 GHz, a simulated (measured) maximum gain of 2.34 dBi at the azimuth angle of (−4°, 0°) (2.34 dBi at the azimuth angle of (−4°, 0°)) is realized, and a maximum gain of 5.5 dBi is obtained at the azimuth angle of (−2°, 0°) (5.5 dBi is obtained at the azimuth angle of (−1°, 0°)) at 1.85 GHz. At 2.45 GHz and 3.5 GHz, the maximum gains of 7.28 dBi and 8.46 dBi at the azimuth angle of (30°, 0°) and (28.6°, 0°) (6.4 dBi and 8 dBi at the azimuth angle of (40°, 0°) and (−16, 0°)) are achieved, respectively. The simulated (measured) 3 dB beamwidths at the four frequencies are 88°, 71°, 122°, and 108.8° (67°, 65.4°, 139°, and 124.6°), and the simulated (measured) antenna efficiencies are 82.77%, 87.21%, 87.74%, and 89.4% (82.05%, 86%, 86.5% and 88.93%), respectively, as listed in

Table 3. Measured efficiencies of the solar antenna at four frequencies.

Frequency (GHz)	Realized Gain (dBi)	Directivity (dBi)	Efficiency (%)
0.9	2.34	3.2	82.05
1.8	5.5	6.16	86
2.4	6.4	7.03	86.5
3.5	8	8.51	88.93

. The reason for the frequency shift and low measured gain may be due to the actual relative permittivity and loss tangent of the solar cell, which is different from the simulation. Anyway, the simulated and measured curves have the same trend, and the measured results validate the performance of the proposed solar antenna.

4.2. Performance of Rectifier Circuit

The RF-DC conversion efficiency of the rectifier circuit is expressed by

$${\eta }_{RF-DC}={\displaystyle \frac{P_{DC}}{P_{IN}}}\times 100\%={\displaystyle \frac{V_0^2}{R_L}}\times {\displaystyle \frac{1}{P_{IN}}}\times 100\%$$

(2)

where P_DC is the DC power consumed by the load, P_IN is the input power of the rectifier circuit, R_L is the load resistor, and V₀ is the DC voltage over the load.

The conversion efficiency of the proposed rectifier circuit is simulated, as shown in Figure 10a, where high conversion efficiency η > 50% is achieved in the 0.9–3.6 GHz band under 0 dBm input power. By deploying branch 1 (branch 2), the conversion efficiency for η > 50% is obtained in the frequency band of 2.4–3.15 GHz (0.9–1.07 GHz, 1.86–1.94 GHz, and 3.36–3.6 GHz). The results indicate that high conversion efficiency across the 0.9–3.6 GHz band is contributed by the frequency selective structure.

Figure 10b indicate the relationship between the conversion efficiency of the rectifier circuit and the load at different frequencies under the input power of 0 dBm. At four frequencies (i.e., 0.95 GHz, 1.85 GHz, 2.45 GHz, and 3.5 GHz), the conversion efficiency increases with the load increasing, and reaches the peak value of 54.51%, 57.76%, 50.72% and 56.49% at the load of 350 Ω, 600 Ω, 500 Ω, and 850 Ω, and then decreases as the load increases further. It is seen that the conversion efficiency η > 50% is obtained over the four operation frequency bands, and thus the optimal load of 500 Ω is chosen. Figure 11a indicates the conversion efficiency versus frequency under the input power levels of −5 dBm, 0 dBm, and 5 dBm. It is observed that the conversion efficiency increases as the input power increasing. Considering low power in the ambient environment, the input power of 0 dBm is selected for the conversion efficiency η > 50%. Furthermore, the highest simulated conversion efficiencies of 59.8%@0.95 GHz, 67.6%@1.85 GHz, 58.8%@2.45 GHz and 58.9%@3.5 GHz, respectively, are found under the input power of 0 dBm over the optimum load of 500 Ω.

The highest measured conversion efficiency is lower than the simulation, and the discrepancy may be the 50 Ω coaxial connector used in the experiment of the rectifier circuit, which is different from the antenna impedance in the co-simulation. By investigation, the conversion efficiency with the input impedance of 50 Ω is simulated, as shown in Figure 11b. It is observed that the conversion efficiency with 50 Ω is lower than that with antenna impedance, but the conversion efficiency with 50 Ω stays above 50% in the frequency band from 0.9 to 3.6 GHz, which is in accordance with the measurement.

4.3. Performance of Solar Cell Rectenna

A.

Under the RF signal radiation.

Measurement of the proposed solar rectenna was conducted in the anechoic chamber, as illustrated in Figure 12. The signal transmitter provides the RF signal, which is amplified by a power amplifier and then transmitted to the horn antennas for the four frequency bands (Standard Gain Horn: HD-9SGAH15N/HD-22SGAH10N/HD-32SGAH10N) with a gain of Gt = 15 dBi@0.95 GHz, Gt = 10.5 dBi@1.85 GHz, Gt = 13 dBi@2.45 GHz and Gt = 16.7 dBi@3.5 GHz through a directional coupler. The horn antenna and the solar rectenna are polarization-aligned, and the distance between the designed solar rectenna and the horn is R = 0.9 m. Using the Friis formula [24], the received power of the rectenna can be calculated by

$$P_{\mathrm{r}}={\displaystyle \frac{P_{\mathrm{t}}{G}_{\mathrm{t}}{G}_{\mathrm{r}}{\lambda }^2}{{\left(4\pi R\right)}^2}}$$

(3)

where P_t is the transmit power, G_t is the horn antenna gain, G_r is the receive antenna gain, and R is the distance between the horn antenna and the proposed rectenna.

For the input power of 0 dBm, at the operating frequencies of 0.95 GHz, 1.85 GHz, 2.45 GHz and 3.5 GHz, the power transmitted by the horn is 13.7 dBm@0.95 GHz, 20.9 dBm@1.85 GHz, 20 dBm@2.45 GHz, 17.7 dBm@3.5 GHz. The conversion efficiency of the rectenna with different loads is shown in Figure 11b, where the measured results are consistent with the simulation. Over the optimum load of 500 Ω, the conversion efficiency of 50.93%@0.95 GHz, 53.8%@1.85 GHz, 54.58%@2.45 GHz and 57.37%@3.5 GHz is achieved under the input power of 0 dBm. Furthermore, by changing the transmitted power of the horn, the conversion efficiency of the rectenna with different input power is measured in Figure 11a. The measurement is in basic agreement with the simulation. It is found that the conversion efficiency increases as the input power increasing, and the conversion efficiency of 54%@0.95 GHz, 51.61%@1.85 GHz, 50.7%@2.45 GHz and 52.65%@3.5 GHz, respectively, are found under the input power of 0 dBm over the optimum load of 500 Ω. The low measured conversion efficiency may be due to the actual components, i.e., capacitor, rectifier diode and inductor, which is different from the SPICE model.

B.

Under the light source.

As shown in Figure 13, the current-voltage (I–V) curves on the solar cell were measured with a 1500 lux light illumination. In the case of the solar cell only (solar rectenna), the measured open-circuit voltage and short-circuit current were 4 V (4 V) and 0.6 mA (0.58 mA), respectively. In both cases, the maximum DC output power was 18.8 mW (voltage = 3.44 V, current = 0.546 mA) and 1.84 mW (voltage = 3.51 V, current = 0.524 mA), respectively. The maximum DC output power of the solar cell rectenna is reduced by 2% compared to the solar cell alone.

C.

Under microwave and light source.

The DC combined efficiency under microwave and solar radiation was investigated, which is defined as

$${\eta }_{combining}={\displaystyle \frac{P_{DC\_Hybrid}}{P_{DC\_Solar}+P_{DC\_RF}}}\times 100\%$$

(4)

where P_{DC_Solar} is the DC power obtained by the solar cell with an optimal load of 6900 Ω, and P_{DC_RF} is the DC power obtained by the rectenna and the best load is 500 Ω@0.95 GHz, 700 Ω@1.85 GHz, 900 Ω@2.45 GHz, 700 Ω@3.5 GHz. P_{DC_Hybrid} represents the DC power obtained by the hybrid RF–Solar rectenna with an optimal load of 7900 Ω@0.95 GHz, 7900 Ω@1.85 GHz, 8100 Ω@2.45 GHz, 8100 Ω@3.5 GHz. The measured output power of the solar cell and the rectenna at different loads is shown in Figure 14, where the measured power P_{DC_Solar} is 1.732 mW, the power P_{DC_RF} is 0.37 mW @0.95 GHz, 0.405 mW @1.85 GHz, 0.374 mW@2.45 GHz, 0.382 mW@3.5 GHz as the antenna input power of 0 dBm, and the power P_{DC_Hybrid} is 2.066 mW@0.95 GHz, 2.0152 mW@1.85 GHz, 2.062 mW@2.45 GHz, 2.0451 mW@3.5 GHz. Finally, the measured DC combined efficiency is 98.29%@0.95 GHz, 94.3%@1.85 GHz, 98%@2.45 GHz, 96.74%@3.5 GHz.

The performance of the proposed solar rectenna is compared with the existing hybrid energy harvesters, as listed in

Table 4. Hybrid energy collection devices.

Ref.	Freq. (GHz)	Ant. Eff.	Rec. Eff.	Electric Dim.	Gain (dBi)	Tot. Eff.	Hybrid Energy Harvesting
[7]	1.85 2.45	N/A	52.1% 42.1%	N/A	3.84@1.8 GHz 5.45@2.4 GHz	N/A	0.1929 mW @100m W/cm2 (−10 dBm)
[8]	3.5 5	33.4% 57.3%	54.67% 6%	0.304 × 0.202 × 0.0012	2.24@3.5 GHz 4.03@5 GHz	18.26% 3.44%	1000 uW 250 uW @210 lux (14 dBm)
[9]	0.95 1.87 2.45	84% 78% 70%	N/A	N/A	0.04@0.95 GHz 3.91@1.87 GHz 5.28@2.45 GHz	N/A	3.06 uW 2.28 uW 0.42 uW @17.4 mW/cm2 (−10 dBm)
[10]	0.85 1.85 2.45	N/A	15%	0.0369 × 0.0315 × 0.0001	−3.27@0.85 GHz 2.18@1.85 GHz 2.23@2.45 GHz	N/A	N/A
[11]	1.7 1.9 2.1	21.2% 22.3% 13.5%	57.5% 65% 61.5%	0.183 × 0.183 × 0.18	1.3@1.9 GHz	12.2% 14.5% 8.3%	N/A
[12]	0.9 1.45 1.81 2.25	65% 65.3% 65% 66.2%	24.6% 17.3% 46.5% 12.4%	0.48 × 0.48 × 0.0048	4.1@0.9 GHz 4.2@1.45 GHz 5@1.81 GHz 4.8@2.25 GHz	15.99% 11.3% 30.23% 8.21%	0.688 V @100 μW/cm2 (−20 dBm)
This Work	0.95 1.85 2.45 3.5	82.05 86 86.5 88.93	56.94% 54.5% 54.11% 56.56%	0.39 × 0.27 × 0.081	2.34@0.95 GHz 5.5@1.84 GHz 6.34@2.45 GHz 8@3.5 GHz	48.49% 56.59% 50.08% 51.76%	2.066 mW 2.0152 mW 2.062 mW 2.0451 mW @1500 lux (0 dBm)

. Our design has the merits of quad-band operation, stable unidirectional pattern with high gain and high efficiency. Although [9] has high antenna efficiency at 0.95 GHz, a low antenna gain of 0.04 dBi is obtained. The designs in [7,11] have a high conversion efficiency, but low antenna efficiency and gain. Furthermore, the quad-frequency solar rectenna is designed in [12], but the energy harvesting conversion is low.

In our design, the solar cell used as a radiating patch is placed on the top of the antenna for high gain, and the rectifier circuit is placed at the back of the rectenna to convert RF power to DC power. The proposed quad-frequency hybrid RF–Solar rectenna has multi-frequency characteristics and high gain, as well as high energy harvesting conversion efficiency.

5. Conclusions

In our design, a solar rectenna with a cascaded frequency-selective rectifier circuit is proposed at four frequency bands, i.e., GSM-900, GSM-1800, ISM-2400, and WiMAX-3500 for hybrid RF–Solar energy harvesting. The energy conversion efficiency of existing hybrid energy harvesters is very low, mostly around 10%, and few designs operate in quad-band. Compared with existing hybrid energy harvesters, the design in this paper can work in four frequency bands while achieving high energy conversion efficiency, as well as high gain and directional radiation performance. High energy harvesting conversion efficiency of 48.49%@0.95 GHz, 56.59%@1.85 GHz, 50.08%@2.45 GHz and 51.76%@3.5 GHz is acquired. The proposed hybrid RF–Solar energy harvester has multi-frequency characteristics, high gain and high energy harvesting conversion efficiency, which can be widely used in power supply for low-power electronic devices and sensors. In real-world environments, such as urban settings, the RF power is extremely low. Therefore, in future designs, ultra-low rectification diodes can be used to ensure efficient RF output.