The rapid advancement of the Internet of Things (IoT) has positioned wireless sensor networks as a cornerstone of modern technology. These networks, comprising thousands of interconnected sensor nodes, are deployed across diverse environments to monitor parameters such as temperature, humidity, noise, electromagnetic fields, and seismic activity. However, the reliance of most sensor nodes on conventional batteries presents a significant bottleneck. Once depleted, these batteries render nodes non-functional, and manual replacement becomes logistically challenging and economically prohibitive, especially in remote or hazardous environments. Consequently, achieving energy self-sufficiency for these nodes has emerged as a critical research imperative. Given that the average power consumption of typical low-power sensor nodes is less than 10 mW, environmental energy harvesting presents a viable and sustainable solution. Single-source energy harvesting, however, often suffers from intermittency and instability—solar energy is abundant but weather-dependent, while RF energy is persistent but offers lower density. To address these limitations, hybrid energy harvesting systems that combine multiple sources are essential. This thesis presents a comprehensive investigation into a photovoltaic-RF hybrid energy harvesting system, with a focused study on solar panel integrated antennas and the design of high-efficiency rectifier circuits for low-power inputs, aiming to achieve reliable energy autonomy for IoT sensor nodes.
The core objective is to integrate solar panels seamlessly with RF harvesting antennas without compromising the performance of either subsystem. The design philosophy emphasizes large, unobstructed solar panel areas for maximum solar harvesting, while leveraging the metallic backplane of the solar panels to enhance antenna performance. A critical challenge is the orthogonal requirement for the antenna’s radiation direction and its aperture direction. The aperture (solar panel) must face the sky to capture sunlight, whereas the main beam of the antenna must be directed horizontally towards RF sources such as cellular base stations. Traditional antenna designs with broadside radiation are unsuitable for this purpose. Therefore, this research focuses on developing antenna configurations that provide end-fire or side-fire radiation patterns when integrated with solar panels.
This thesis tackles the energy supply problem for IoT sensor nodes by employing a photovoltaic-RF hybrid energy collection method. By carefully considering the material, shape, illuminated area, and antenna integration method of the solar panels, as well as radiation direction and aperture direction, we designed a solar panel integrated reconfigurable antenna, a solar panel integrated dual-beam antenna, a solar panel integrated Yagi antenna, and a low-power broadband rectifier circuit. Comprehensive testing of the integrated system validates its feasibility and reliability.
Fundamental Antenna Parameters
Antennas are fundamental components in wireless communication systems, responsible for converting guided waves into radiated electromagnetic waves or vice versa. The performance of an antenna is characterized by several key electrical parameters, which are critical for design and optimization in energy harvesting applications.
Input Impedance, Reflection Coefficient, and VSWR
The input impedance \( Z_{in} \) of an antenna is defined as the ratio of the voltage \( U_{in} \) to the current \( I_{in} \) at its input terminals:
$$ Z_{in} = \frac{U_{in}}{I_{in}} = R_{in} + jX_{in} $$
where \( R_{in} \) represents the resistance (comprising radiation resistance and loss resistance), and \( X_{in} \) is the reactance. The reflection coefficient \( \Gamma \) quantifies the mismatch between the antenna impedance \( Z_{in} \) and the transmission line characteristic impedance \( Z_0 \):
$$ \Gamma = \frac{Z_{in} – Z_0}{Z_{in} + Z_0} $$
The voltage standing wave ratio (VSWR) is related to the reflection coefficient by:
$$ VSWR = \frac{1 + |\Gamma|}{1 – |\Gamma|} $$
For efficient operation, a VSWR less than 2 (or \( |S_{11}| < -10 \) dB) is typically desired.
Impedance Bandwidth
The impedance bandwidth is the frequency range over which the antenna’s VSWR is below 2 or the return loss is greater than 10 dB. It can be expressed as absolute bandwidth \( \Delta f = f_H – f_L \) or relative bandwidth:
$$ BW = \frac{f_H – f_L}{f_0} \times 100\% $$
where \( f_H \) and \( f_L \) are the upper and lower frequencies of the operating band, and \( f_0 \) is the center frequency. For solar panel integrated antennas, achieving a wide bandwidth is crucial to capture RF energy from multiple sources operating at different frequencies.
Radiation Pattern and Directivity
The radiation pattern describes the spatial distribution of radiated energy. Directivity \( D \) is defined as the ratio of the radiation intensity in a given direction to the average radiation intensity over all directions:
$$ D = \frac{4\pi U_{max}}{P_{rad}} $$
The gain \( G \) incorporates the antenna efficiency \( \eta \):
$$ G = \eta D $$
High gain is beneficial for RF energy harvesting as it allows the antenna to capture more energy from distant, low-power RF sources.
Polarization
Polarization describes the orientation of the electric field vector. For energy harvesting, circular polarization or dual-polarization can be advantageous as they allow for the reception of signals with arbitrary linear polarization, maximizing energy capture in dynamic environments. The axial ratio (AR) is the key parameter for circular polarization:
$$ AR = 20 \log_{10} \left( \frac{E_{max}}{E_{min}} \right) $$
An AR of less than 3 dB is generally considered good circular polarization.
Feeding Techniques for Patch Antennas
Direct feeding methods such as microstrip line feeding and coaxial probe feeding are simple but offer narrow bandwidth. For wideband applications, indirect methods are preferred. Aperture-coupled feeding, where the feed line is isolated from the radiating patch by a ground plane with a coupling slot, can achieve significantly wider bandwidths and is particularly suited for solar panel integrated antennas as it allows for the RF circuitry to be independent from the solar panel circuit. The following table compares various feeding techniques relevant to solar panel antenna design:
| Feeding Method | Bandwidth | Advantages | Disadvantages | Application in Solar Panel Antennas |
|---|---|---|---|---|
| Microstrip Line | 2-5% | Simple, easy fabrication | Narrow bandwidth | Limited use |
| Coaxial Probe | 2-5% | Flexible impedance matching | Narrow bandwidth | Limited use |
| Aperture Coupling | >10% | Wide bandwidth, isolation of circuits | Multi-layer structure | Ideal for hybrid systems |
| Proximity Coupling | >10% | Wide bandwidth | Complex design | Suitable for multi-layer integration |
| Coplanar Waveguide (CPW) | Wide | Single-layer, easy integration | Potential for surface waves | Good for solar panel integration |
Solar Panel Integrated Side-Fire Radiation Energy Harvesting Antennas
To meet the requirements of hybrid energy harvesting, which demands that the solar panel faces the sun while the antenna beam aligns with RF sources, two types of solar panel integrated side-fire antennas have been developed. The metallic backplane of the solar panel, typically made of aluminum, provides excellent electrical conductivity, allowing it to function as part of the antenna radiating structure. By employing the solar panel as a periodic superstrate, the antenna’s bandwidth and gain can be enhanced. Placing these solar panel periodic superstrates on the top layer ensures they are not obstructed by the antenna structure. The chosen feeding mechanisms are designed to ensure that the solar energy collection circuit and the RF energy collection circuit are independent, eliminating the need for complex isolation circuitry.
Solar Panel Integrated Pattern-Reconfigurable Antenna
This first design is a mechanically reconfigurable antenna integrated with solar panels for broadband and high-gain operation. The fundamental principle is based on the fact that a traditional microstrip slot antenna radiates bidirectionally with low gain. By introducing a periodic superstrate, the radiation pattern can be transformed from bidirectional to directional, significantly enhancing the forward gain. Figure 18 demonstrates this effect, showing that the gain improves dramatically with the addition of either a metallic or a solar panel periodic superstrate, as compared to a simple slot antenna.
The key innovation is to achieve a non-broadside radiation pattern. By moving the solar panel superstrate relative to the feeding structure, the beam can be steered. The simulation results, as shown in Figure 19, reveal that as the periodic superstrate is displaced horizontally from the center, the main beam tilts to approximately ±30°. This mechanical reconfiguration allows the antenna to adapt to the direction of the strongest RF source, while the solar panel on top remains optimally oriented towards the sun.
Antenna Structure and Design
The configuration of this antenna is presented in Figure 20. It has a total size of 233mm × 200mm × 3mm and is designed for a center frequency of 2.5 GHz. The structure comprises three layers: the top layer, which is a 1×4 array of solar panel cells acting as the periodic superstrate on substrate 1; the middle layer, which is the metal ground plane with a rectangular slot etched in its center; and the bottom layer, which is the microstrip feed line on substrate 2. The solar panel cells measure \( W1 \times W2 \) with a spacing of \( W3 \) between them. The coupling slot has dimensions \( W4 \times L4 \), and the feed line is \( W5 \times L5 \). The substrate materials have a relative permittivity of 2.65 and thicknesses of \( H1 \) and \( H2 \), respectively. The detailed parameters are listed in Table 2 of the original text, which is replicated below:
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| L1 | 200 | W1 | 140 |
| L2 | 170 | W2 | 31 |
| L3 | 233 | L4 | 60 |
| W4 | 3.0 | L5 | 135.5 |
| W5 | 3.1 | H1 | 1 |
| H2 | 2 |
Figure 21 illustrates the current distribution on the solar panel superstrate for Mode 1 (beam at -30°). It is evident that the current is primarily concentrated on the solar panel cells directly above the coupling slot, with a non-uniform phase distribution across the elements. This phase difference, where currents on some elements are opposite in direction to others, is responsible for steering the main beam. Similarly, Figure 22 shows the current distribution for Mode 2 (beam at +30°), where the current concentration shifts to the solar panel cells on the opposite side of the slot.
Parametric Analysis
A parametric study is conducted to optimize the antenna performance. The length \( W1 \) of the solar panel superstrate has a significant effect on the resonant frequency and impedance matching. As shown in Figure 23, increasing \( W1 \) shifts the resonant frequency lower. A value of 140 mm provides the best combination of broad bandwidth and good matching. The width \( W2 \) of the solar panel also influences the matching, with 31 mm yielding the optimal performance (Figure 24). The width \( W5 \) and length \( L5 \) of the microstrip feed line primarily affect the impedance matching. The optimal values are found to be 3.1 mm and 135.5 mm, respectively, as depicted in Figures 25 and 26. The dimensions of the coupling slot (\( W4 \) and \( L4 \)) also play a role in the matching, and their optimal values are 3.0 mm and 60 mm, respectively (Figures 27 and 28).
Fabrication and Measurement
A prototype was fabricated as shown in Figure 29. Figure 29(a) shows the top view with the 1×4 solar panel array mounted on the dielectric substrate using conductive adhesive. Inductors (33nH) are used to isolate the RF signal from the DC solar panel output, and via holes are drilled for assembly. Figure 29(b) displays the bottom view with the microstrip feed line. Figure 29(c) shows the drying oven used for curing the conductive adhesive. The simulated and measured \( |S_{11}| \) curves for both modes are presented in Figures 30 and 31. The measured \( |S_{11}| < -10 \) dB bandwidths are 2.28-3.05 GHz for Mode 1 and 2.30-3.02 GHz for Mode 2, which align well with the simulated bandwidths of 2.26-2.92 GHz and 2.26-2.91 GHz, respectively. The slight shift to higher frequencies is attributed to fabrication and assembly tolerances. The measured gain within the operating band varies between 5-10.8 dBi for both modes. The normalized radiation patterns at different frequencies are shown in Figures 32 and 33, confirming that the main beam is directed at -30° in Mode 1 and +30° in Mode 2, with good agreement between simulation and measurement. Outdoor testing in sunny conditions showed the solar panel producing 614.3 mV and 227.7 mA (Figure 34), confirming its proper functionality. The result of these measurements are summarized in the table below:
| Parameter | Mode 1 (Beam at -30°) | Mode 2 (Beam at +30°) |
|---|---|---|
| Simulated Bandwidth (|S11|<-10 dB) | 2.26 – 2.92 GHz | 2.26 – 2.91 GHz |
| Measured Bandwidth (|S11|<-10 dB) | 2.28 – 3.05 GHz | 2.30 – 3.02 GHz |
| Gain Range in Band | 5.2 – 11 dBi | 5 – 10.8 dBi |
| Max Gain at 2.5 GHz (Sim/Meas) | 10.9 / 8.7 dBi | 11.0 / 10.6 dBi |
| Solar Panel Output | 614.3 mV, 227.7 mA | |
Solar Panel Integrated Dual-Beam Antenna
To increase the angular coverage for RF energy harvesting from multiple sources, a solar panel integrated dual-beam antenna is proposed. This design builds on the concept of the reconfigurable antenna but employs a 1×6 solar panel superstrate to produce two simultaneous beams. As illustrated in Figure 35, the two modes of the previous reconfigurable antenna are conceptually combined into a single structure. The antenna achieves a main beam at both +30° and -30°, expanding the spatial range from which RF energy can be harvested. A metal reflector is placed at the bottom to suppress backward radiation and enhance the forward gain.
Antenna Configuration
The structure is shown in Figure 37. The overall dimension is 228mm × 190mm × 2mm. The antenna consists of four layers from top to bottom: a 1×6 array of solar panels, a dielectric substrate with a thickness of 2mm, a coplanar waveguide (CPW) feed structure, and a metal reflector plate placed 20mm below the substrate. The top view (Figure 37(c)) shows the solar panel superstrate on the substrate, with a total width \( W1 \) of 190mm. The bottom view (Figure 37(d)) shows the CPW feed with two inverted “L” shaped slots etched from the ground plane. The horizontal slot has dimensions \( W3 \times L3 \), and the vertical slot has dimensions \( W4 \times L4 \). The distance between the two slots is \( W5 \). The dimensions are provided in Table 3:
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| L1 | 228 | W1 | 190 |
| L2 | 24 | W2 | 144 |
| L3 | 4.8 | W3 | 44 |
| L4 | 116.4 | W4 | 0.3 |
| G1 | 3.5 | H1 | 20 |
| H | 2 | W5 | 3.8 |
The use of a CPW feed simplifies the design to a single-layer structure, avoiding the multi-layer fabrication requirements. It also allows for easy integration of the DC bias lines for the solar panels without complex isolation structures, as the RF ground and DC ground are inherently separate to some extent.
Parametric Analysis
The width \( W2 \) of the solar panel superstrate affects the matching at higher frequencies, as shown in Figure 38. A value of 144mm is chosen as it offers a good trade-off between bandwidth and matching. The CPW slot width \( W3 \) is critical for determining the operating frequency and matching. The analysis in Figure 39 shows that a width of 44mm provides the best overall matching. The ground plane width \( W1 \) mainly impacts the low-frequency matching (Figure 40), and 190mm is the optimal value considering both matching and bandwidth. The distance \( H1 \) between the antenna and the reflector plate influences the matching across the band. As per Figure 41, a distance of 20mm is selected as it provides balanced matching at both low and high frequencies.
Experimental Verification
The fabricated prototype is shown in Figure 42. Figure 42(a) portrays the top view with the solar panel array. The positive and negative terminals of the solar panels are connected in series via metallic traces and wires. Inductors (27nH) are used for RF-DC isolation, as shown in the magnified view in Figure 42(d). Figure 42(b) shows the CPW structure on the bottom, and Figure 42(c) is the side view showing the reflector. The simulated and measured \( |S_{11}| \) curves are displayed in Figure 43. The measured operating band (\( |S_{11}| < -10 \) dB) is from 2.11 to 2.81 GHz, close to the simulated band of 2.13 to 2.74 GHz. The gain within the band ranges from 6.6 to 9.7 dBi. The radiation patterns at 2.4 GHz, 2.5 GHz, and 2.6 GHz are shown in Figure 45. The measurements confirm the dual-beam radiation pattern, with the maximum radiation direction pointing at both +30° and -30°. The measured gain at 2.5 GHz is 9.5 dBi. The test set-up in the anechoic chamber is shown in Figure 44. The six solar panels, connected in series, produced an open-circuit voltage of 3.833V and a short-circuit current of 468.6mA under strong sunlight (Figure 46), validating the solar energy harvesting functionality. The measured key performance indicators are summarized below:
| Parameter | Simulated | Measured |
|---|---|---|
| Bandwidth (|S11|<-10 dB) | 2.13 – 2.74 GHz | 2.11 – 2.81 GHz |
| Gain Range in Band | – | 6.6 – 9.7 dBi |
| Max Gain at 2.5 GHz | – | 9.5 dBi |
| Main Beam Directions | +30° and -30° | |
| Solar Panel Output | 3.833 V, 468.6 mA (6 panels in series) | |
Solar Panel Integrated Yagi Antenna
For harvesting RF energy from distant base stations while simultaneously collecting solar energy, the end-fire radiation characteristic of the Yagi-Uda antenna is highly advantageous. The Yagi antenna is a classic example of an end-fire array, where the main beam is directed along the axis of the array, which is parallel to the plane of the antenna. This makes it ideal for scenarios where the solar panel aperture must face the sky and the antenna beam must point towards the horizon. By integrating solar panels as the parasitic director elements of a microstrip Yagi antenna, a compact and efficient hybrid energy harvesting solution is achieved.
Operating Principle of Yagi Antennas
A Yagi antenna consists of a driven element, a reflector, and one or more directors. The reflector, being slightly longer than the driven element, is inductive and helps to suppress backward radiation. The directors are slightly shorter than the driven element and are capacitive. As stated in the theory, the current on the reflector lags the voltage by 90° due to its inductance, and after traveling a quarter-wavelength path, it lags by an additional 90°, resulting in a net 180° phase difference that cancels the backward signal. Conversely, for the directors, the current leads the voltage by 90° due to their capacitance, which after a quarter-wavelength path cancels out, resulting in a phase advancement that enhances the forward signal. This mechanism creates a directional end-fire radiation pattern.
Antenna Structure
The proposed solar panel integrated Yagi antenna is shown in Figure 49. The overall size is 142mm × 66mm × 1.5mm, with a center frequency of 2.5 GHz. The top surface of the substrate hosts the driven element and the reflector, which are made of metal and connected via a co-planar strip line. The feed is a microstrip line on the bottom side, connected to the driven element through a shorting via. The four parasitic director elements, which are the solar panels, are placed on the top surface. To maximize the solar panel area within the limited space, four additional parasitic patches are added on both sides of the directors. These elements act as directors as well and significantly increase the total area of the solar panels, thus improving the solar energy harvesting capability. The design parameters are listed in Table 4:
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| L1 | 24.5 | W1 | 8 |
| L2 | 28 | W2 | 13 |
| Rw | 18 | d0 | 15 |
| d1 | 8 | d2 | 7 |
| d3 | 18 | dr | 25 |
| X1 | 142 | Y1 | 66 |
| L3 | 39 | W3 | 2 |
| L4 | 5 | W4 | 1 |
| L5 | 9 | H | 1.5 |
The addition of the side parasitic patches does not significantly degrade the antenna’s impedance matching. Figure 49(d) compares the \( |S_{11}| \) with and without these patches. The relative bandwidth remains at approximately 34%, and the gain variation is less than 1 dB, confirming that the modification is effective in increasing the solar panel area without compromising antenna performance.
Parametric Analysis
A parametric study is performed to optimize the Yagi antenna design. The length of the driven element \( L1 \) has a strong influence on the operating frequency. As shown in Figure 50, a length of 24.5mm places the center frequency correctly at 2.5 GHz. The spacing between the directors \( d1 \) affects the bandwidth and matching. Figure 51 shows that a spacing of 8mm offers a good balance. The reflector width \( Rw \) primarily affects the low-frequency matching; a value of 18mm is optimal (Figure 52). The distance \( d0 \) between the driven element and the first director is crucial for impedance matching. From Figure 53, a distance of 15mm is selected. The parameters \( d3 \) and \( d2 \) related to the side parasitic patches have a minor impact on the matching, and their optimal values are determined to be 18mm and 7mm, respectively (Figures 54 and 55).
Fabrication and Measurement
The prototype is fabricated as shown in Figure 56(a) (top view) and Figure 56(b) (bottom view). The solar panels are mounted on the top substrate, and the SMA connector is soldered to the microstrip feed line on the bottom. The measured and simulated \( |S_{11}| \) curves are compared in Figure 57. The measured bandwidth for \( |S_{11}| < -10 \) dB spans from 2.03 GHz to 3.28 GHz, which is broader than the simulated bandwidth of 2.16-3.02 GHz. The gain across the operating band ranges from 6.9 to 9 dBi. The radiation patterns measured in an anechoic chamber (Figure 58) are displayed in Figure 59. The normalized patterns at 2.2 GHz, 2.5 GHz, and 2.8 GHz show good agreement with simulations, confirming the end-fire radiation with the main beam pointing in the direction of the directors (90°). The maximum gain reaches 9.5 dBi. Under sunlight, the eight solar panels connected in series produced a voltage of 4.73V and a current of 111.9mA (Figure 60), demonstrating effective solar energy collection. The performance is summarized in the table below:
| Parameter | Simulated | Measured |
|---|---|---|
| Bandwidth (|S11|<-10 dB) | 2.16 – 3.02 GHz | 2.03 – 3.28 GHz |
| Relative Bandwidth | 33% | – |
| Gain Range in Band | – | 6.9 – 9 dBi |
| Max Gain | – | 9.5 dBi |
| Radiation Pattern | End-fire | |
| Solar Panel Output | 4.73 V, 111.9 mA (8 panels in series) | |
Low-Power Broadband Rectifier Circuit
The rectifier circuit is the second critical component of the RF energy harvesting system. It converts the AC RF signal received by the antenna into usable DC power. Since the ambient RF energy density is typically very low, the rectifier circuit must be specifically designed for low-power inputs (e.g., -10 dBm). The efficiency of the rectifier circuit is primarily dependent on the choice of diode and the impedance matching between the antenna and the rectifying circuit. The design process is depicted in the system block diagram in Figure 61, which includes input filtering, impedance matching, the rectifying stage, and output filtering.
Diode Selection
The choice of the Schottky diode is paramount. The diode’s parameters, such as series resistance \( R_S \), zero-bias junction capacitance \( C_{j0} \), and breakdown voltage \( V_b \), significantly influence the rectification efficiency. To maximize efficiency, a diode with low \( R_S \) and \( C_{j0} \) and a high \( V_b \) is required. Three common Schottky diodes, SMS-7630, HSMS-285C, and HSMS-286C, are evaluated using a simple half-wave rectifier circuit at 2.5 GHz. The test circuit is shown in Figure 62. The simulated efficiency versus input power is plotted in Figure 63. For input powers below 5 dBm, the HSMS-285C exhibits the highest efficiency. Since our application focuses on harvesting low ambient RF energy, the HSMS-285C diode is selected.
Rectifier Topology
Two rectifier topologies are considered: half-wave and full-wave (voltage doubler). The half-wave rectifier uses a single diode and conducts only during one half of the RF cycle. The full-wave rectifier, as shown in Figure 65, employs two diodes and a capacitor to rectify both halves of the cycle, effectively doubling the output voltage and increasing the efficiency. The full-wave topology is chosen for its superior performance, particularly at low input power levels. The design, based on the analysis of the diode selection, utilizes the HSMS-285C diode, with the load resistance \( Z_{out} \) optimized at 3.2 kΩ to achieve peak efficiency.
Circuit Design and Matching
The initial rectifier design, as per the schematic in Figure 70, does not achieve optimal performance due to impedance mismatch. The initial simulation results in Figure 71 show a poor return loss (\( |S_{11}| > -10 \) dB). To improve this, a single-stub matching network is employed, as shown in the schematic in Figure 75. The stub parameters are optimized to match the impedance of the rectifier circuit to the 50-ohm source impedance. This is a crucial step to ensure maximum power transfer from the antenna to the rectifier. After matching, the simulated \( |S_{11}| \) is improved significantly. The efficiency of the rectifier is strongly dependent on the load impedance. As shown in Figure 76, the maximum efficiency is achieved when the load resistance \( Z_{out} \) is 3.2 kΩ.
Simulated and Measured Performances
The simulation results of the matched rectifier are presented in Figures 77 and 78. Figure 77 shows the rectifier efficiency as a function of input power at 2.5 GHz. It is observed that the efficiency peaks at an input power of 0 dBm, reaching 67%. At -10 dBm, the efficiency is 42%, which demonstrates its capability for low-power operation. The broadband behavior is illustrated in Figure 78, which shows the frequency response of the efficiency for different input power levels. For an input of -10 dBm, the efficiency remains above 20% over the frequency range of 2.3-2.7 GHz, indicating a wide operating bandwidth. For higher input power (0 dBm), the efficiency exceeds 40% from 2.35 to 2.80 GHz. The fabricated rectifier circuit is shown in Figure 79. The measured results are depicted in Figure 80. At input powers of 0 dBm, -5 dBm, -10 dBm, and -15 dBm, the measured maximum efficiencies are 63%, 53%, 40%, and 25%, respectively. These values are in good agreement with the simulations, with minor discrepancies attributed to component tolerances and fabrication variations. The key performance characteristics of the rectifier are listed in the table:
| Input Power | Simulated Max Efficiency | Measured Max Efficiency | Bandwidth (Eff > 20%) |
|---|---|---|---|
| 0 dBm | 67% | 63% | 2.35 – 2.80 GHz |
| -5 dBm | 55% | 53% | 2.32 – 2.75 GHz |
| -10 dBm | 42% | 40% | 2.30 – 2.70 GHz |
| -15 dBm | 27% | 25% | 2.30 – 2.67 GHz |
System Integration and Testing
To validate the overall system performance, the designed rectifier circuit is integrated with the three solar panel integrated antennas. The experimental setup, shown in Figure 81, uses a horn antenna as the RF transmitter. The solar panel integrated antennas act as receivers, and their output is connected to the rectifier circuit. The system’s output voltage is then measured under different conditions, including varying distances between the transmitter and receiver and different input power levels. The results, presented in Table 5, demonstrate that all three antennas are capable of harvesting RF energy and converting it to a DC voltage. For instance, at a distance of 0.5 meters with an input power of 20 dBm, the reconfigurable antenna, dual-beam antenna, and Yagi antenna produce output voltages of 760 mV, 550 mV, and 560 mV, respectively. As expected, the output voltage decreases with increasing distance due to path loss. This system-level test confirms the end-to-end functionality of the proposed hybrid energy harvesting scheme.
| RF Power | Distance | Reconfigurable Antenna | Dual-Beam Antenna | Yagi Antenna |
|---|---|---|---|---|
| 20 dBm | 0.5 m | 760 mV | 550 mV | 560 mV |
| 1.0 m | 660 mV | 310 mV | 380 mV | |
| 15 dBm | 0.5 m | 406 mV | 280 mV | 320 mV |
| 1.0 m | 350 mV | 145 mV | 206 mV | |
| 10 dBm | 0.5 m | 205 mV | 122 mV | 144 mV |
| 1.0 m | 158 mV | 57 mV | 84 mV |
Conclusion and Future Work
This thesis has presented a comprehensive study on hybrid energy harvesting systems, focusing on the integration of solar panels with high-performance antennas for RF energy harvesting. The primary challenge of differing aperture and radiation directions was addressed through three innovative antenna designs. The solar panel integrated pattern-reconfigurable antenna achieves a broad bandwidth of over 25% and a maximum gain of 10.5 dBi, with mechanical beam steering between ±30°. The solar panel integrated dual-beam antenna expands the coverage by simultaneously radiating in two directions, achieving a 25% bandwidth and a gain exceeding 9 dBi. The solar panel integrated Yagi antenna leverages end-fire radiation for collecting energy from distant sources, demonstrating a wide 33% bandwidth and a peak gain of 9.5 dBi. All three designs successfully integrate solar panels on the top layer without obstruction, and their feeding mechanisms ensure effective isolation between the RF and DC paths. In addition, a low-power broadband rectifier circuit operating at 2.5 GHz was designed. The rectifier, utilizing the HSMS-285C diode and a single-stub matching network, achieved a measured efficiency of 40% at an input power of -10 dBm with an operating bandwidth exceeding 400 MHz. System-level tests integrating the antennas with the rectifier confirmed the feasibility of the entire hybrid energy harvesting system, demonstrating that it can reliably convert ambient RF and solar energy into usable DC power for low-power IoT sensor nodes.
Future research directions will focus on several key areas. Extending the integration concept to full antenna arrays is a natural progression to significantly increase the harvested power for practical applications. While this study focused on using solar panels as the radiating superstrate, other integration methods, such as utilizing them as the antenna ground plane or developing optically transparent antennas, should be further investigated. Finally, the solar panel integrated dual-beam antenna currently has a relatively high profile. Future work will aim to develop low-profile designs. Additionally, the design of a specialized feed network for solar panel integrated antenna arrays could enable electronic beam scanning, allowing the system to dynamically track high-power RF sources, thereby further enhancing the efficiency and adaptability of the hybrid energy harvesting system. Overall, the research conducted in this thesis provides a solid foundation for the realization of self-sustaining, energy-autonomous wireless sensor networks powered by a combination of solar and RF energy.