Multi-Frequency Solar Rectenna Design for Hybrid Radio Frequency–Solar Energy Harvester
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsPaper is interesting and well described. I recommend the following: a) regarding the rectifier the performance over a broader input power range could be presented especially towards lower input powers ( at list simulation results). b) regarding the prototype, the layout regarding the dc output circuit of the solar cell and how it is connected to the cell could be described in more detail…how does this dc circuitry affect the antenna performace? c) it would be interesting to comment on whether the presence of RF power affect the dc output of the solar cell and if yes how does it vary with RF power ? And inversely does solar power affect rectifier performace?
Author Response
Response to Reviewer 1:
Thank you very much for reviewing our manuscript. We have revised our manuscript as follows:
- regarding the rectifier the performance over a broader input power range could be presented especially towards lower input powers ( at list simulation results).
Answer：The rectification efficiency of this rectifier circuit at different input powers (0 dBm to -15 dBm) was simulated and measured, and the results are shown below.
To evaluate the performance of the rectifier circuit across a broader range of input power levels, particularly at lower input powers, we conducted simulations and measurements of the power conversion efficiency (PCE) of the rectifier circuit at input power levels ranging from 0 dBm to -15 dBm. The simulation and measurement results of the rectifier circuit's conversion efficiency under different input power levels (0 dBm to -15 dBm) are shown in Figure 1 below. As illustrated, it can be observed that the simulated conversion efficiency aligns well with the measured trend. Moreover, as the input power increases, the conversion efficiency of the rectifier circuit also improves. At -5 dBm, the maximum PCE measured at 1.79 GHz was 49.43%. At -10 dBm, the maximum PCE measured at 1.77 GHz was 37.04%, and at -15 dBm, the maximum PCE measured at 1.75 GHz was 23.31%. As shown in Table 1 below, at -5 dBm, the PCE values at the four frequency points were 44% @ 0.95 GHz, 47% @ 1.85 GHz, 38.2% @ 2.45 GHz, and 37.76% @ 3.5 GHz. At -10 dBm, the PCE values at the four frequency points were 32.24% @ 0.95 GHz, 33.22% @ 1.85 GHz, 25.74% @ 2.45 GHz, and 23.68% @ 3.5 GHz. At -15 dBm, the PCE values at the four frequency points were 17.66% @ 0.95 GHz, 19.52% @ 1.85 GHz, 12.8% @ 2.45 GHz, and 10.71% @ 3.5 GHz.
Table1 PCE of rectifier circuit at four frequencies under different input power
|
|
0.95 GHz |
1.85 GHz |
2.45 GHz |
3.5 GHz |
|
-5dBm |
44% |
47% |
38.2% |
37.76% |
|
-10dBm |
32.24% |
33.22% |
25.74% |
23.68% |
|
-15 分贝 |
17.66% |
19.52% |
12.8% |
10.71% |
2. regarding the prototype, the layout regarding the dc output circuit of the solar cell and how it is
connected to the cell could be described in more detail…how does this dc circuitry affect the
antenna performace?
Answer：
The structure of the solar cell is shown in Figure 2. The surface of the solar cell is encapsulated
with a transparent film, while the interior consists of an energy conversion module. First, the light-
collecting film gathers optical energy, which is then converted into electricity by the internal energy
conversion module. The solar cell contains metal grid lines (five vertical and two horizontal grid
lines) that perform series-parallel connections of the DC output from the photoelectric conversion.
The converted DC power is then delivered to the metal pins at both ends of the solar cell, which
ultimately output the generated electricity.
When integrating the antenna with the solar cell, careful consideration must be given to the
solar cell's structure and metal pins, as they can influence the antenna's electromagnetic
characteristics. Therefore, co-simulation is necessary. Since the solar cell consists of both metallic
and non-metallic parts, the metallic portions can couple with the antenna body, affecting its
radiation performance. In the simulation, the metallic parts are added according to the actual
antenna's dimensions and position. As shown in Figure 3, the solar cell affects the surface current
distribution of the antenna, while coupling currents are also induced on the solar cell itself. Due to
this coupling effect between the solar cell and the antenna, the co-simulation results demonstrate an
improvement in antenna gain. Specifically, the gain increases from -0.2 dBi @ 0.95 GHz, 5.38 dBi
@ 1.85 GHz, 7.7 dBi @ 2.45 GHz, and 8.2 dBi @ 3.5 GHz to 3.22 dBi @ 0.95 GHz, 6.54 dBi @
1.85 GHz, 8.28 dBi @ 2.45 GHz, and 9.5 dBi @ 3.5 GHz, as illustrated in Figure 4.
3. it would be interesting to comment on whether the presence of RF power affect the dc output of
the solar cell and if yes how does it vary with RF power ? And inversely does solar power affect
rectifier performance?
Answer：
The presence of RF power has no effect on the DC output of the solar cell. Under the
same illumination conditions, we measured the DC output of the solar cell at different power levels.
The actual test results are shown in Figure 5. From the experimental results, it can be observed that,
for example, at an operating frequency of 2.45 GHz, the solar cell consistently produced a DC
output voltage of 0.777 V when the RF power was varied at 5 dBm, 0 dBm, and -10 dBm.
Therefore, we can conclude that the presence of RF power does not affect the DC output of the
solar cell, as illustrated in Figure 6
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article "Multi-frequency Solar Rectenna Design for Hybrid RF-Solar Energy Harvester" is submitted for review. The topic is relevant. The work has a certain novelty, in my opinion, not fully formulated, and also has scientific and practical significance. The authors propose the design of a hybrid energy harvester for collecting radio frequency and solar energy in four bands. Among the results obtained, it is worth noting the efficiency of converting a radio frequency signal to a constant one at the level of 56.94%. At the same time, it is not clear how much better it is than its analogues. There are a few comments about the work: 1. The introduction is written very concisely and does not fully give an idea of the relevance and degree of elaboration of the issue.; 2. The structure does not comply with the principles of IMRAD. Sections 2, 3, and 4 do not allow us to evaluate the research methodology, as well as obtaining specific results.; 3. Formula 1 on page 6 is not formatted correctly. In formulas 2 and 4, the sign "X" is used for multiplication, I would recommend replacing it with "×"; 4. On page 5, the drawing is positioned incorrectly, with a check on the table; 5. The conclusion is written very concisely, in fact, duplicates the abstract. There are no specific results. Advantages over other analogues, etc. are not shown. 6. Half of the sources in the list of references are older than 5 years, and there are also references to non-scientific publications (17, 18, 25). In general, many links are designed incorrectly. I would recommend reviewing the list of references. In general, based on the topic, the article is very relevant, has scientific and practical significance. I believe that it can be published after significant improvements and elimination of comments.
Author Response
Response to Reviewer 2:
Thank you very much for reviewing our manuscript. We have revised our manuscript as follows:
1. The introduction is written very concisely and does not fully give an idea of the relevance and degree of elaboration of the issue.;
Answer：
According to the reviewers' comments, the article has been updated to include an explanation of the application requirements and design rationale for the solar cell antenna used in hybrid energy harvesting. This solar cell antenna, which combines a solar cell and a rectifying antenna, can collect both optical and microwave energy separately and simultaneously, ensuring stable energy supply while generating higher power output. The added content is located in line 24 of the introduction section and has been highlighted. The additional content reads:
With the accelerated advancement of wireless communication systems, the enviromental wireless power density is increasing. Using RF energy to power small power sensor that 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. Hybrid Energy Harvesting combines various energy sources to produce a higher electrical output. A hybrid energy harvester, combining the solar cell and the rectenna, is designed to collect both optical and microwave power, ensuring stable energy supply.
2.The structure does not comply with the principles of IMRAD. Sections 2, 3, and 4 do not allow us to evaluate the research methodology, as well as obtaining specific results.;
Answer：
As reviewer suggest, to evaluate the research methodology, the simulation and test results in Chapters 2 and 3 have been moved to Chapter 4.
3. Formula 1 on page 6 is not formatted correctly. In formulas 2 and 4, the sign "X" is used for multiplication, I would recommend replacing it with"×";
Answer：
As reviewer suggest, the sign "X" has been modified to "×".
In Page 7, Section 4.2, Row 245 and Page 10, Section 4.3, Row 324, the revised formulas 2 and 4 are as follows:
4. On page 5, the drawing is positioned incorrectly, with a check on the table;
Answer：
Figure 4 has been improved for good presentation.The picture is misaligned when the article is translated, and it has been modified. Please see the revised manuscript or attached file.
5. The conclusion is written very concisely, in fact, duplicates the abstract. There are no specific results. Advantages over other analogues, etc. are not shown.
Answer：
Our design has the merits of quad-band operation, stable unidirectional pattern with high gain and high energy harvesting conversion efficiency, which can be widely used in power supply for low-power electronic devices and sensors. Add the following text at line 361 of the body and highlight it. The text reads:
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 advantages of multi-frequency operation, high gain and high energy harvesting conversion efficiency, which can be widely used in power supply for low-power electronic devices and sensors.
6. Half of the sources in the list of references are older than 5 years, and there are also references to non-scientific publications (17, 18, 25). In general, many links are designed incorrectly. I would recommend reviewing the list of references.
Answer：
We have deleted the outdated and redundant references [1][2][7][21][23][28] from the previous version, and added the newly published references [6][16][17] in the new version. We corrected the incorrect citation formats [17][18] of the previous Chinese references , updated them to references [14][15] , and deleted the duplicate reference [16] and the working manual [25].
[6] Citroni, R.; Mangini, F.; Frezza, F. Efficient Integration of Ultra-low Power Techniques and Energy Harvesting in Self-Sufficient Devices: A Comprehensive Overview of Current Progress and Future Directions. Sensors 2024, 24, 4471.
[16] Lin, X.; Weng, Z.; Hong, Y.; Zhang, Y. A Wideband Circularly Polarized Dipole Antenna with Compact Size and Low-Pass Filtering Response. Sensors 2024, 24, 3914.
[17] Li J F, Chen Z N, Wu D L, et al. Dual-beam filtering patch antennas for wireless communication application[J]. IEEE Transactions on Antennas and Propagation, 2018, 66(7): 3730-3734.
Author Response File: 
Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsPlease, refer to the attached document. Thanks!
Comments for author File: Comments.pdf
Author Response
Response to Reviewer 3:
Thank you very much for reviewing our manuscript. We have revised our manuscript as follows:
(-) I recommend adding a statement at the conclusion of the introduction that outlines the organization of the document. For example: "The paper is structured as follows: Section 1... Section 2... and so forth." This will provide readers with a clearer road map of the content that follows.
Answer：
The content has been added at the end of the introduction, on line 50 of the main text, and highlighted. The content reads: The paper is structured as follows: Section II indicate the proposed solar antenna, including the electromagnetic model of thin film solar cell. Section III indicate the multi-frequency rectifier circuit for low-power. Section IV presents the performance of the optical/microwave rectenna, and the existing hybrid energy harvesters are compared. Section V concludes the paper.
(-) To improve clarity for readers, consider adding a section at the end of the paper that lists all abbreviations along with their full forms. This addition will facilitate better understanding and accessibility of the material.
Answer：
The abbreviations have been added and highlighted in the manuscript in Page 11, Row 373.
|
Abbreviations |
Full form |
|
OCFD |
Off-center-fed Dipole |
|
PET |
Polyethylene terephthalate |
|
DC |
Direct Current |
|
IoT |
Internet of Things |
|
RF |
Radio Freqency |
|
I-V |
Current-Voltage |
|
Rectenna |
Rectifying Antenna |
(-) The plagiarism detection software indicates a 23% matching rate in the text. Therefore, I recommend that the authors undertake a comprehensive revision of the entire manuscript to reduce this matching threshold.
Answer：
Regarding the reviewers' concerns about the duplication rate, I have used the plagiarism detection software PaperYY to check the article's duplication rate and corrected any duplicate items. The results show that when tested on the PaperYY website, the duplication rate is only 9.1%.
(-) In introduction section, you might consider including the following citation in your text (it includes studies on solar an RF harvesters): Citroni, R., Mangini, F., & Frezza, F. (2024). Efficient Integration of Ultra-low Power Techniques and Energy Harvesting in Self-Sufficient Devices: A Comprehensive Overview of Current Progress and Future Directions. Sensors (Basel), 24(14), 4471.
Answer：
It has been added, and highlighted in the manuscript. The article has been added as reference [6] in the text and highlighted.
Reference [6] focuses on the importance of minimizing power consumption and maximizing energy efficiency to improve the autonomy and longevity of these sensor nodes. It examines current advancements, challenges, and future direction in ULPDT, ES, PMU, wireless communication protocols, and EHT to develop and implement robust and eco-friendly technology solutions for practical and long-lasting use in real-world scenarios.
The relevant research to us in this article is that thin-film solar cells can operate more efficiently and maximize their performance and lifespan when combined with EHT technology. It is also noted that with the rapid growth of wireless technology, the wireless power density is increasing due to various electromagnetic sources such as mobile base stations, TV towers, and Wi-Fi routers. Using radio frequency (RF) energy to power small sensor nodes that operate at microwatt-level power has become popular to save costs and avoid battery replacement. Therefore, signals from various sources, especially in urban areas, can be converted into electrical energy using rectifying antennas (rectennas). This component can effectively convert RF signals into usable DC power.
(-) While this is merely a reviewer's suggestion, I recommend utilizing keywords that more effectively highlight the central focus of the paper. The authors have the opportunity to include a maximum of five keywords that capture the essence of their research.
Answer：
As reviewer suggest, to capture the essence of our design, the keywords have been modified to “Hybrid RF-Solar Energy Harvester, Multi-frequency rectenna and Solar rectenna”.
(-) In the row 10, are reported “maximum RF-dc conversion efficiency”. It is better to write “maximum RF-DC conversion efficiency”. Also row 225.
Answer：
The "maximum RF-dc conversion efficiency" has been changed to "maximum RF-DC conversion efficiency" in the article. The changes are located on line 11, line 172, line 244, and line 277 in the main text, and are highlighted.
(-) The paper mentions a maximum RF-DC conversion efficiency of 56.94% under 0 dBm input power, but the measured efficiencies are lower. What is the reason for this discrepancy between simulation and measurement?
Answer：
The maximum RF-DC conversion efficiency of 56.94% mentioned in the article under 0 dBm input power is an actual measured value. The simulated maximum RF-DC conversion efficiency is 58.12%. The simulation results are consistent with the measured values, with only a 1.18% difference between the maximum values. The possible causes of this discrepancy may include losses due to coaxial cables, line losses between component and circuit solder joints, and errors between the actual and simulated values of the substrate dielectric constant and electrical components, leading to impedance mismatch between the antenna and the circuit.
Taking the rectifier circuit using F4BM265 dielectric substrate as an example, according to the datasheet, the relative dielectric constant of F4BM265 dielectric substrate is 2.65±0.5, so its actual value ranges from 2.6 to 2.7. Through simulation, when the circuit's substrate dielectric constant is 2.6, the maximum rectification efficiency at 0 dBm is 57%, which is 1.12% lower than the efficiency at a dielectric constant of 2.65. At this time, when the circuit's substrate dielectric constant is 2.6, the maximum rectification efficiency at 0 dBm is 57.32%, which is 0.8% lower than the efficiency at a dielectric constant of 2.65.
(-) performance under different light intensities and RF power levels is not thoroughly explored. How does the device perform under varying environmental conditions, particularly in low-light scenarios?
Answer:
In the indoor environment, a light source is used to input light to the system, and the light intensity input to the system is controlled by adjusting the intensity of the light source. At a frequency of 2.45GHz, an HD-22SGAH10N horn antenna is used to input radio frequency (RF) signals to the system, and the RF power input to the system is controlled by adjusting the input power of the signal source.
The hybrid energy harvesting system was tested under light conditions of 270 lux, 1000 lux, and 1500 lux, and varying RF input power from -10 to 10 dBm. The DC output power curves of the hybrid energy harvesting system under different input RF and light conditions are shown in Figure 2. As the RF power increases, the output power of the system also increases. When the input RF power is 0 dBm, the output power of the system is 0.255 mW@270 lux, 0.962 mW@1000 lux, and 2.263 mW@1500 lux, respectively. When the input RF power is -10 dBm, the output power of the system is 0.245 mW@270 lux, 0.746 mW@1000 lux, and 1.732 mW@1500 lux, respectively. Meanwhile, as the light intensity gradually increases, the output power of the system also increases. Under low light conditions (270 lux), the output power provided by the system is 0.245 mW@-10 dBm, 0.255 mW@0 dBm, and 0.45 mW@10 dBm, respectively. The results show that this system can provide stable output power in low-light environments, enabling it to power low-power systems.
(-) The choice of materials (e.g., F4B substrates) and their associated losses at different frequencies is critical. How do these losses affect the overall efficiency and performance of the rectenna?
Answer：
The designed antenna in this paper uses F4B-M300 substrates as the dielectric substrate (relative dielectric constant of 3 and loss tangent of 0.0017). As shown in Figure 3(a), the dielectric constant of the substrate is 3±0.06. By analyzing the substrate's datasheet, it can be found that as the dielectric constant of the substrate gradually increases, the operating frequency of the antenna shifts towards lower frequencies in the medium and high frequency bands. The results are shown in Figure 3(b). When the dielectric constant is 2.94, the antenna operates at 0.915GHz-0.94GHz, 1.835GHz-1.915GHz, 2.43GHz-2.52GHz, and 3.11GHz-3.73GHz. When the dielectric constant is 3.06, the antenna operates at 0.905GHz-0.945GHz, 1.835GHz-1.91GHz, 2.43GHz-2.52GHz, and 3.05GHz-3.67GHz. Since the actual F4B substrate only has a thickness of 1mm for F4B-M300, and according to the datasheet in Figure 3(a), it has a stable loss tangent of 0.0017 at low frequencies (<10 GHz), therefore, after comprehensive consideration, the F4B-M300 substrate with a dielectric constant of 3 and loss tangent of 0.0017 is adopted.
(-) Were the measurements conducted in an anechoic chamber? How do real-world conditions (e.g., urban environments, interference) affect the performance of the proposed system?
Answer：
① Yes, we conducted the tests in an anechoic chamber.
② In real-world conditions, the RF signals present in urban environments are very low power and constantly changing. Therefore, our system needs to use low-power tubes in the future to ensure efficient RF output.
(-) What specific validation methods were used to ensure the accuracy of simulations against measured results?
Answer：
For the S-parameter results of the antenna, a vector network analyzer Agilent N5230A PAN-L was used to test the input reflection coefficient of the fabricated antenna, and the results were compared and verified with the simulation results from ANSYS HFSS. For the antenna radiation pattern results, a microwave anechoic chamber was utilized to test the two-dimensional radiation pattern and gain of the fabricated antenna. The E-plane and H-plane results were tested by adjusting the antenna's orientation, as shown in Figures 4(c) and 4(d). Meanwhile, a compact range was used to test the three-dimensional radiation pattern of the antenna, as shown in Figure 4(b), and the final test results were compared and verified with the simulation results from ANSYS HFSS. For the rectifier circuit results, a signal generator SIYI 1433 was used, with Rohde & Schwarz power meter N1914A and power sensor E4412 for power calibration, and Rohde & Schwarz multimeter 34461 for voltage measurement. The rectification efficiency of the fabricated rectifier circuit was obtained and compared with the simulation results from ADS, as shown in Figure 4(e).
(-) Can you elaborate on the techniques used to achieve impedance matching at multiple frequency bands?
Answer：
We will separately describe the impedance matching techniques for multi-band implementation from two parts: the antenna and rectifier circuit.
①For multi-band antenna design, the multi-band dipole antenna designed in this paper is based on the characteristics of asymmetric dipole antennas. By adding parasitic components, multi-frequency point design of the antenna is achieved. The parasitic components are integrated into the asymmetric dipole, introducing new resonance modes to realize a flexibly controllable wideband. The added parasitic structures can be divided into horizontal and vertical types. The horizontal parasitic structures are printed on the substrate simultaneously with the main radiation patch, while the vertical parasitic structures use non-metallic columns or metal plate-like structures based on the main radiation structure. By adjusting the size and relative position of parasitic elements, each added structure can excite circularly polarized radiation at new frequency bands.
Firstly, we adopt an OCFD antenna as the basis, which can operate in two frequency bands. By sequentially loading a metal shorting post (Design B) and a vertical wall plate (Design C), two new resonance modes are excited, corresponding to resonant frequencies of 1.8 GHz and 2.45 GHz bands, respectively. Subsequently, a stacked structure is employed by adding a single-sided circular metal microstrip patch (Design D) on top of the existing metal microstrip substrate. A circular patch with a radius R from the center M0 of the substrate is loaded, and the surface current distribution on the circular metal patch is observed, as shown in Figure 5. When the frequency is near the high frequency f=3.5 GHz, the current mainly flows in the horizontal direction, with the same current direction on the left and right edges. The simulation results match the theoretical analysis. This indicates that at the high frequency f=3.5 GHz, the TM11 mode of the circular metal patch is excited, thus adding the circular metal patch provides a new resonance point at the high frequency. By loading the circular patch, the TM11 mode 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, 3.09-3.65 GHz. By coupling a thin-film solar cell above the antenna substrate through a stacked structure, a multi-frequency solar cell antenna capable of simultaneously absorbing light and radio frequency energy is formed. Due to the irregular shape of the metal parts of the solar cell, different tuning effects may be introduced at multiple frequency points. The irregular shape may lead to different equivalent capacitance values of the patch at different frequencies, thereby allowing for independent tuning of the original four resonant points and enhancing the resonant characteristics at each frequency point. Ultimately, the solar cell antenna can operate in the frequency ranges of: 0.91-0.97 GHz, 1.8-1.89 GHz, 2.4-2.51 GHz, and 3.05-3.67 GHz.
②For rectifier circuit: The input impedance of Cell A(Cell B) with the electrical length λ1/8@f1 is Z1(λ2/8@f2 is Z2). where λ1(λ2)is the guided wavelength of the transmission line at f1(f2). The input impedance of the shorted branches in Cell A(Cell B) at different frequencies is:
So the impedance of two different bands (2.8 GHz and 3 GHz) is controlled independently by two matching branches with different lengths, so that the two branches are matched at two frequencies respectively. From the power reflection coefficient Equation 5, the maximum transmission of power can only be realized if the source impedance is made conjugate to the port impedance.
Using the antenna impedance at quad-frequency to as the signal source impedance. Connect capacitor C in parallel with the diode of Cell B and L in series with the branch to adjust the impedance of branch 2. Different bands are adapted by dynamically adjusting the component parameters (L, C and etc.). Using the optimized design of ADS simulation software, the design objective is rectification efficiency greater than 50% at all bands. Then, by taking into account the capacitance and inductance values of database, the optimal parameters of the simulated circuit (L1, C0 and etc.) are finally determined. The simple ADS simulation circuit designed is shown in Fig.6. Considering the match of different components connecting, adding MLIN with different lengths and widths of the connecting wires to reduce reflection is needed, as shown in Fig.7.
The impedance change of branch 2 with added L1 and C0 at frequencies 0.95 GHz and 1.85 GHz is analyzed by Smith circle diagrams as shown in Fig. 6. The impedance matching point at the operating frequency 0.95 GHz changes from low resistive to inductive from P1 to P2 (with the addition of inductor L1), and then from inductive to purely resistive from P3 (with the addition of capacitor C0 again), realizing a good antenna impedance matching with the rectifier circuit. The impedance matching point at 1.85 GHz goes from Q1 to Q2 (adding capacitor C0), which changes from highly resistive to capacitive, and then to Q3 (adding inductor L1 again), which changes from capacitive to purely resistive, and achieves a good impedance matching between the antenna impedance and the rectifier circuit at 1.85 GHz. Finally, the complex impedances of the rectifier circuit are good conjugate matches with the antenna. And he proposed rectifier circuit is able to provide a good input return loss (|S11| < -10 dB) over the operating range of the antenna, which indicates that the antenna and the rectifier circuit are well-matched over a wide frequency range.
(-) What are the next steps in the research and development of this technology? Are there plans to test the rectenna in real-world applications?
Answer：
①In the next step, the integrated power management system is considered to adopt, achieving high efficiency energy management, outputting a stable voltage so as to directly power low-power scenarios and devices such as IoT sensors and wearable devices.
②This experiment combines solar cells and rectifying antennas in series connection to obtain the output power of the hybrid system. However, due to the instability and unpredictability of environmental energy, the output voltage fluctuates. In practical applications, most devices require stable input voltage. Therefore, an energy management system is considered, using chip BQ25570 to convert variable electrical energy into constant output voltage, as shown in Figure 8. Finally, under the operating frequency of 2.45 GHz, we tested the output voltage of the hybrid energy harvesting system under different light intensity conditions. As shown in Figure 9, when the light intensity is 71 lux and 320 lux respectively, the hybrid energy harvesting system can output a stable voltage of 2 V, thus providing stable power supply for low-power systems.
(-) What is the expected variability in power output due to fluctuations in RF signal strength and solar irradiance?
Answer：
The system was tested under 270lux, 1000lux and 1500lux of light respectively. The DC output power curves of the hybrid energy harvesting system at different input RF are shown Fig. 6 below. The DC output power increases with increasing light intensity. And the DC output power increases with increasing input RF power. When the light intensity is 270 lux, the system's DC output power gradually increases from 0.18 mW to 0.45 mW as the RF input power varies from -12 dBm to 9 dBm. When the light intensity is 1000 lux, the system's DC output power gradually increases from 0.71 mW to 1.3 mW as the RF input power varies from -12 dBm to 9 dBm. When the light intensity is 1500 lux, the system's DC output power gradually increases from 1.69 mW to 4.23 mW as the RF input power varies from -12 dBm to 9 dBm. When the RF input power is 0 dBm, the system's DC output power is 0.26 mW, 0.96 mW, and 2.26 mW respectively at light intensities of 270 lux, 1000 lux, and 1500 lux, gradually increasing as shown in Fig.10 below.
(-) What specific low-power electronic devices and sensors are envisioned to benefit from this hybrid energy harvester considering the low output power?
Answer：
This hybrid energy harvester can power distributed sensing networks (passive RFID tags), wearable devices (heart rate/blood oxygen sensors) and smart city infrastructure (bridge strain sensors, passive RFID tags). These devices achieve “perpetual life” in low power scenarios. Passive RFID tags have extremely low power consumption, typically in the microwatt (μW) range. For example, the Impinj Monza series (UHF passive tag chip) has a peak power consumption of 5-20 μW, and the NXP UCODE series (UHF tag chip) has a read/write operation power consumption of 3-15 μW. Therefore, the output power of our system can fully meet the requirements of such low power systems.
(-) Always include aspace between the numerical value and the unit of measurement. Please review the entire paper for consistency. For example, (15.99%@0.9 GHz, 11.3%@1.45 GHz, 30.23%@1.81 GHz and 8.21%@2.25 GHz), 40 mm×45 mm×1 mm and so on. Please check whole paper.
Answer：
Spaces have been added between all numerical values and units throughout the text to standardize the structure. The added spaces are highlighted in the document.
(-) Some of the images listed below are quite small, making it challenging future readers to comprehend them fully. Please make the necessary adjustments. In Addition, figure 6, the Smith chart is covering the efficiency (%) results. Please. Check it.
Answer：
Change the structure from displaying three images per column to two images per column. This layout not only avoids overlapping other content but also enlarges the images, making them easier for readers to view clearly.
(-) The conclusion section should include also the main achievements and pioneering research as
well as mention future directions more tangibly.
Answer：
The main achievements is that integrating multi-frequency solar cell antennas and rectifying
circuits to form a multi-frequency hybrid energy harvesting device, and 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.
This content has been added to line 367 of the conclusion section in the main text and highlighted
for emphasis.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsAfter reviewing the article again, it is clear that the authors have finalized it well.
1. The introduction, conclusion and structure have been finalized.
2. Formulas have been edited.
3. The drawing has been corrected.
4. The list of references has been updated.
As I have already noted, the article is relevant, has scientific novelty, scientific and practical significance. In conclusion, the advantages of the model developed by the authors are well shown.
The article is recommended for publication.
Reviewer 3 Report
Comments and Suggestions for AuthorsPlease, refer to the attached document. Thanks!
Comments for author File: Comments.pdf
