A Study on a Compact Double Layer Sub-GHz Reflectarray Design Suitable for Wireless Power Transfer

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work is interesting.

reflection ability of varactor diode-based reconfigurable reflector array with multiple 8-type patches operate at 865.5 MHz is investigated. an infinite array and a finite array are analyzed by numerical simulations with generalized geometrical optics method and Floquet mode amplitude optimization method. A prototype consisting of 36 elements are build and tested. The measured results are consistent with the calculated ones.

 

 

I have some concerns:

 

Minor issue:

 

Motivations to choose an 8-type patch, among others: C O E types, may be given.

 

In the experiment, voltage vector is used to apply different voltage to the diodes in 6 panels. How this voltage vector is generated may be given. Equipment to generate this voltage vector may be pointed clearly in fig. 13a.

 

Experimental results are given in Fig.15-16, theoretical or numerical results may be given also in these 2 figures, so the read have a quick understanding of what happened.

Author Response

Comment 1: Motivations to choose an 8-type patch, among others: C O E types, may be given.

Response 1: Thank you very much for your kind suggestions. The motivation for the choice of the phase shifter configuration is given below:

• The choice of the RRA design is dictated by several practical considerations, as well as the findings of an extensive numerical study of the performance of various RRA phase shifter (metal patch) configurations carried out by our research team. The study was aimed at minimizing the reflection magnitude variation (closer to the uniform one) while maintaining a sufficiently wide reflection phase range. Additionally, a size constraint was imposed on the phase shifter dimension (width) along the beam scanning direction to reduce the phase profile discretization error which, in turn, may lead increased side lobes.
• We by no means pretend that the proposed RRA configuration is the most optimal in terms of the reflection magnitude uniformity and phase agility since more sophisticated configurations demonstrating superior properties are likely to exist; therefore, we will continue our efforts to develop even more advantageous designs. Nevertheless, the RRA phase shifter design proposed in the paper has several advantages, making it a promising candidate for WPT systems. Firstly, the design is simple from the fabrication standpoint—no complicated and expensive manufacturing process is needed, as the prototype employed in the experimental studies presented in the paper was fabricated using a milling machine intended for creating PCB board layouts.
• During the abovementioned study, the following phase shifter configurations were examined: two solid patches (capacitive patches), O(D)-shaped patches and C-shaped patches. It was found that the solid patches exhibited the worst behaviour, as the magnitude exhibited a large dip in the vicinity of the resonance. The O-shaped patches gave better results, owing to additional inductances due to the narrow strips obtained by introducing a rectangular window to each metal patch. Regarding the C-shaped patches, they showed slightly larger phase variation than their O-shaped counterparts.
• Finally, based on the observation that the gap between the adjacent phase shifter patches in the same column has a little effect on the RRA behavior, a decision was made to combine two adjacent pairs of O-shaped patches which resulted in the current 8-shaped configuration. Specifically, combining the O-shaped patches proved to affect the phase shifter magnitude and phase response slightly. At the same time, 8-shaped (combined) patches require fewer biasing wire pairs (half the number of wires in the O-shaped patch-based design), which further reduces the adverse effect of the biasing lines on the RRA efficiency.
• • Also, it was found the impact of the variation in the reflection magnitude on the RRA performance is more severe than that of the incomplete (< 360°) phase range. Hence, in cases where it is not possible to achieve a full phase cycle of 360° and sufficiently low magnitude variation, the phase range can be reduced in favour of a more uniform magnitude response. Other configurations have not yet been studied, but we plan to perform a more in-depth investigation in our future research.

   

  Also, to address the reviewer's comment regarding the selected RRA configuration, the following three paragraphs explaining the motivation behind the choice of the RRA phase shifter topology have been added to the modified version of the manuscript (Page 10):

  The choice of the RRA design is dictated by practical considerations and substantiated by findings of an extensive numerical study of various RRA phase shifter (metal patch) configurations carried out for an infinite RRA model described in what follows. The study aimed at minimizing the reflection magnitude variation (closer to the uniform one) while maintaining a sufficiently wide reflection phase range. Additionally, the phase shifter dimension (width) along the beam scanning direction was constrained during the study to minimize the phase profile discretization error, leading to increased side lobes.

  The following phase shifter configurations were examined: two solid patches (capacitive patches), O(D)-shaped patches and C-shaped patches. It was found that the solid patches exhibited the worst behaviour, as the magnitude exhibited a large dip in the vicinity of the resonance. The O-shaped patches gave better results, owing to additional inductances due to the narrow strips obtained by introducing a rectangular window to each metal patch. Regarding the C-shaped patches, they showed slightly larger phase variation than their O-shaped counterparts.

  Finally, based on the observation that the gap between the adjacent phase shifter patches in the same column has a little effect on the RRA behaviour, a decision was made to combine two adjacent pairs of O-shaped patches which resulted in the 8-shaped configuration. To be more specific, combining the O-shaped patches only slightly affects the magnitude and phase responses. At the same time, 8-shaped (combined) patches require fewer biasing wire pairs (half the number of wires in the O-shaped patch-based design), which further reduces the adverse effect of the biasing lines on the RRA efficiency. In this configuration, the same voltage is applied to the pair of diodes mounted on a phase shifter via two wires, each connected to a respective 8-shaped patch of the phase shifter. 

Comment 2: In the experiment, voltage vector is used to apply different voltage to the diodes in 6 panels. How this voltage vector is generated may be given? Equipment to generate this voltage.

Response 2:  We highly appreciate your valuable comment on the diode voltage generation methodology followed in our study on the RRA.  For the reader's convenience the following concise description of the procedure employed to retrieve the diode bias voltages which, in turn, serve as a good initial guess for the experimental optimization process aimed at maximizing the amount of power in the desired direction (Page 24):

 

To summarize, the diode voltage finding procedure is almost identical to that for the calculation of the reactances followed in the theoretical analysis given in Section 2. To find the required diode voltages, the following steps must be taken:

1. Calculate the phase profile period for the desired power reflection angle assuming the normal incidence
2. Find the required phase incrementwhereis the unit cell width;
3. Usingfrom the previous step, find a reflection phase value,, for each phase shifter;
4. Retrieve the phase response from the values of the time delay between the incident and reflected sine waves measured with the digital oscilloscope for uniformly configured RRA at different voltage levels ranging from 0 V to 5 V;
5. Perform smoothing of the experimental phase curve using a Centred Moving Average Filter (CMAF) with a window size of 10 samples;
6. Construct a least-squares-based polynomial approximation of the phase curve using the MATLAB built-in function polyfit(). The phase is expressed as a function of the varactor diode voltage;
7. Generate a set of reactance values with a step size of 0.1V and calculate the reflection phase value for each of them using the polynomial approximation;
8. Find the diode reactance values, which ensure the required set of discrete reflection phase values,using the following MATLAB built-in function: Voltages = inperp1(ReflectionPhase, DiodeVoltage, RequiredPhaseValues).

 

Regarding the description of the voltage controller board mentioned in the paper, the following brief outline of the DAC and microcontroller-based voltage control circuitry has been added to the amended version of the manuscript (Page 23):

For each RRA unit cell (phase shifter), the varactor diode capacitance control is ensured using several 12-bit MCP4728 DACs (Digital-to-Analog Converters). A microcontroller unit (MCU) connected to a PC adjusts the DAC's voltage levels. The MCU outputs are connected to several digital-to-analogue converters (DAC) intended to produce and apply required bias voltages to varactor diodes mounted on the metal patches of the RRA.  An RL lowpass filter circuit is inserted between the DAC output and the pair of wires to reduce the adverse effect of the parasitic TEM transmission lines formed by the wire pairs connecting the edges of the 8-shaped patches with the DAC outputs.

Comment 3: Experimental results are given in Fig.15-16, theoretical or numerical results may be given also in these 2 figures, so the read have a quick understanding of what happened.

Response 3: We appreciate your valuable comment on the need to improve the quality of the result presentation. In the updated version of the paper, the experimental results displayed in Figures 15 and 16 are accompanied by the calculated ones for better result interpretation and comparison.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In the paper, a reconfigurable reflectarray is designed, manufactured, measured and tested. Results of HFSS simulations, measurements and tests are mutually compared.

I find the paper being a practical engineering contribution worth to be published.

[Originality]

If my understanding is correct, structure of the reflectarray was adopted from [A]. The paper [A] should be added to references and briefly commented in the manuscript.

 [Validity]

Functionality of the design was verified by HFSS simulations. Modeling outputs were compared with measurements:

- Fig. 14 shows reflection phase versus bias voltage. The deviation is significant but rather well commented.

- Fig. 15 and 16 show radiation patterns of the metallic plane and the reflectarray with the main beam oriented to 30 and 45 degrees. I do not understand why the metal sheet (blue line) shows different pattern for different reflectarray settings (should be commented).

But I do not see any relation between the designed reflectarray and the wireless power transfer (WPT). I would remove WPT from the paper.

 

[References]

[A] T. Makdissy; I. Hassoun; Coupled slots varactor-tuned unit cell for single linear polarization reflectarrays at C-band. IEEE International RF and Microwave Conference. Kuala Lumpur (Malaysia): IEEE, 2020. DOI: 10.1109/RFM50841.2020.9344776

Author Response

Comment 1: If my understanding is correct, structure of the reflectarray was adopted from [A]. The paper [A] should be added to references and briefly commented in the manuscript.

 [A] T. Makdissy; I. Hassoun; Coupled slots varactor-tuned unit cell for single linear polarization reflectarrays at C-band. IEEE International RF and Microwave Conference. Kuala Lumpur (Malaysia): IEEE, 2020. DOI: 10.1109/RFM50841.2020.9344776

Response 1: The configuration proposed and described in the paper, kindly provided by the reviewer, closely resembles the one developed in our study in that it is also composed of two layers, both of which are endowed with varactor diodes to enable electronic reconfigurability. However, in the paper by Makdissy and Hassoun, the RRA is designed to operate at a much higher operating frequency (5.1 GHz). While conceptually similar, the physical and technological constraints imposed on the RRA operating at 5.1 GHz and the one proposed in our paper, whose operating frequency is as low as 865.5 MHz, differ significantly. Specifically, the 5.1 GHz main problems are conduction and dielectric losses, as well as those due to the diodes, while the varactor diode capacitance variation range is not of primary concern. Meanwhile, at 865.5 MHz, the situation becomes almost the opposite – the current technological process cannot ensure wide varactor diode capacitance value ranges, which, in turn, considerably limits the reflection phase variation range.  Nevertheless, we would like to express gratitude to the review for indicating to the paper by Makdissy and Hassoun who made a significant contribution to the study of RRA and we have cited the paper in the updated version of the manuscript. 

Comment 2: Fig. 15 and 16 show radiation patterns of the metallic plane and the reflectarray with the main beam oriented to 30 and 45 degrees. I do not understand why the metal sheet (blue line) shows different pattern for different reflectarray settings (should be commented)

Response 2: We are thankful for this comment concerning the difference between the patterns obtained for the same metal plate and those that appear differently in different figures. In fact, the metal plate's radiation patterns are completely identical, except that in the diagram comparing the pattern of the RRA configured electronically for the 30° reflection with that obtained for the metal plate, additional values are added (measured at 15° and 20° angles). This has been done for better comparison and visibility. While the same values could be shown in the other three diagrams, it would lead to a visually larger peak of the metal sheet than that of the RRA, which, in turn, would, from the point of view of the author's results, make comparisons and interpretation less convenient. Our fault was to omit any explanation of this apparent difference between the pattern shapes.

To avoid misinterpretations of the result presented in the manuscript in the updated version, the radiation pattern presented in Figures 15 and 16 is plotted for the same set of angles, including 15° and 20°.

Comment 3: But I do not see any relation between the designed reflectarray and the wireless power 

Response 3: Thank you very much for your kind suggestions. Although in the present study, two conventional antennas were used to characterize the power reflection efficiency of the proposed RRA design and validate the theoretical results, in real-life WPT systems, the receiving antenna is combined with an RF-DC converter (rectenna). The power conversion efficiency of the rectenna is critical for the overall performance of the WPT system. However, 

the inherent non-nonlinearity of the converter components (Schottky diodes) results in non-linear dependence of the PCE upon the received power. Thus, to minimize the amount of wasted power, the RF-DC converted must be operated in the most or nearly the most efficient regime, which, in turn, requires maintaining a certain received power level. The problem is that due to high path losses as well as the presence of various scatterers in an indoor environment (furniture components, chairs, ventilation elements, etc.) where the WPT systems are to be deployed, the amount of power reaching the rectenna is too low to guarantee the efficient operation of the RF-DC converter. 

The proposed RRA is capable of ensuring sufficient amounts of reflected power at a desired angle of up to 50°. This capability of redirecting power coming from a power source in an antenna-based WPT system can be exploited to increase the power received by a rectenna. Additionally, the RRA is electronically reconfigurable, enabling receiver tracking to ensure the power necessary for efficient RF-DC converter operation. Using relatively cheap RRAs such as the proposed one is a more cost-effective solution than using high-gain high output power amplifiers to compensate for the power level decrease due to multiple reflections and scattering.

Additionally, we are currently preparing a paper showing the results of an experimental study where a WPT involving a multi-hop node intended for power-carrying wave amplification was used in conjunction with the RRA described in this paper. The results show that using the RRA results in much higher received power levels in the absence of line-of-sight (LoS) propagation due to various types of obstructions present in the laboratory or any other indoor environment. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper proposes a reflect array operating at 865.5 MHz, which can control the angle of the reflected beam. The control principle of the reflected beam has been explained, and a prototype has been manufactured to verify the correctness of the principle and the simulated results. This paper is innovative in the research of reflective arrays. However, to enhance the paper, I have some comments that should be addressed:

1. Figures 4 and 5 show the reflection coefficient phase and magnitude as a function of the varactor reactance calculated. How to obtain the reactance value as shown in Figures 4 and 5? Please supplement this process.

2. The maximum simulated reflected beam angle can achieve 80°, while only 45° measured reflected beam is shown. Please explain the difference between these two results.

3. The comparison with state-of-the-art designs about the control of the reflected angle and the complexity of the design to highlight this design is lacking.

4. The professionalism of chart drawing needs to be improved, such as the size of the legend in Figures 4, 5, 7, 8, et al.

Comments on the Quality of English Language

Minor editing of English language required.

Author Response

Comment 1: Figures 4 and 5 show the reflection coefficient phase and magnitude as a function of the varactor reactance calculated. How to obtain the reactance value, as shown in Figures 4 and 5? Please supplement this process.

The reactance finding procedure adopted in the present study comprises the following steps:

1. Calculate the phase profile period for the desired power reflection angle and incidence angle
2. Find the required phase increment (difference in phase between any two adjacent phase shifters along the beam scanning direction);
3. Usingfrom the previous step find a reflection phase value,, for each phase shifter;
4. Obtain the phase response curve via the full-wave analysis using Ansys HFFS for a finite RRA model. Namely, the reflected phase of a uniformly configured RRA model with finite dimensions (all elements have the same reactances) is retrieved for different values of the diode reactance in the range from 10 Ω to 100 Ω;
5. Construct a least-squares-based polynomial approximation of the phase curve using the MATLAB built-in function polyfit(). The phase is expressed as a function of the varactor diode reactance;
6. Generate a set of reactance values with a step size of 0.1 Ω and calculate the reflection phase value for each of them using the polynomial approximation;
7. Find the diode reactance values, which ensure the required set of discrete reflection phase values,using the following MATLAB built-in function: Reactances = inperp1(ReflectionPhase, Reactance, RequiredPhaseValues).  

Comment 2: The maximum simulated reflected beam angle can achieve 80°, while only 45° measured reflected beam is shown. Please explain the difference between these two results.

Response 2: We appreciate your comment. The main approach adopted in the theoretical analysis and experimental study is based on the generalized reflection law (Generalized Geometrical Optics Approximation). Namely, the calculated results shown in Figures 9 and 10 and the measured ones exhibited in Figures 15 and 16 were found for the RRA whose reactances (in the theoretical study) and voltages (in the excremental study) were calculated based on the phase curves (calculated or experimentally obtained). Unfortunately, this method can ensure adequate RRA performance only for relatively small angles – up to about 40° – 50°. However, to ascertain the proposed RRA performance at extreme reflection angles, an alternative approach based on the Floquet theory 

was used to achieve reflection power peaks at angles as large as 70° and 80°. Since the theoretical analysis in the case of the Floquet theory is far more computationally intensive (all the computations were performed on a computer with 256 GB RAM and Intel family processors with a total number of 64 cores), the excremental investigation would be prohibitively long due to the need for real-life measurements in place of calculations. For this reason, the authors abandoned the idea of performing Floquet Theory-based RRA configuration in the experimental part of the present study.

Furthermore, the theoretical results, some part of which are presented in the paper (Figures 11) reveal that even though the proposed design is theoretically capable of forming a narrow beam at such extreme angles as 80°, the power reflected per a single supercell is so small, that a more or less adequate reflection efficiency can be ensured when the dimensions of the RRA are on the same order as those of typical indoor environment, e.g., an office room, which would make the use of such large RRAs entirely impractical.

 

For the sake of clarity regarding the lack of experimental results for large reflection angles, the following paragraph has been inserted in the updated version of the manuscript (Page 21):

 

The results obtained by means of the Floquet-based RRA synthesis approach allow one to find such RRA diode reactances that can deflect the beam by up to 80°. By contrast, the generalized reflection law (Generalized Geometrical Optics Approximation) based approach can ensure adequate RRA performance only for relatively small angles (up to about 40° – 50°). Unfortunately, the theoretical analysis based on the Floquet theory is substantially more computationally intensive—the reactance value optimization was performed on a machine with 256 GB RAM and several Intel family processors with a total of 64 cores. Performing a similar RRA design would be prohibitively long due to the need for real-life measurements instead of calculations. For this reason, the authors abandoned the idea of performing Floquet Theory-based RRA configuration in the experimental part of the present study.  

Comment 3: The comparison with state-of-the-art designs about the control of the reflected angle and the complexity of the design to highlight this design is lacking.

Response 3: We are very thankful for your valuable suggestion concerning the design performance comparison of the proposed design with the existing solutions; however, it is quite difficult to accomplish properly. The reason is that The vast majority of studies on reflectarrays reported to date are focused on developing new and improving existing solutions of either of the following two types:

• RRAs intended for use as low-profile reflectors increasing the gain of a horn antenna or dipole antenna and, therefore, are a viable and efficient substitute for bulky metallic reflectors that can ensure only a fixed configuration unless equipped with some motors to shift and rotate them;
• RRA is designed to serve as a building block of the more sophisticated and emerging concept of Intelligent Reflecting Surfaces (IRS), whose purpose is to enhance the data signal level via sophisticated beamforming.

Both classes of the RRA are used at frequencies significantly higher than the one chosen for the present study – starting with few GHzs. This makes the comparison of the existing RRAs and the proposed one improper since designing RRA operating in the sub-GHz range requires engineers addressing challenges that differ significantly from those faced by RRAs designed for higher frequencies. Specifically, the main efficiency limiting factors at higher frequencies are conduction losses, dielectric losses, and losses introduced by the varactor diodes (or PIN diodes). In contrast, at lower frequencies, the losses mentioned above become less pronounced, which allows for the use of FR-4 material as a substrate. However, the main problem is achieving a broad reflection phase variation range while ensuring adequate reflection magnitude uniformity using inexpensive varactor diodes.

To the best of the author's knowledge, the only contribution to date addressing the sub-GHz reflectarray design problem is made by Zainud‑Deen—el. 1, where the operating frequency of the RRA is 740 MHz.  However, the paper presents only simulation results, and their reflectarray was designed for use as a reflector, ensuring a gain of 26.2 dB.

Furthermore, only a few of the recently published studies are concerned with WPT, and moreover, they employ focussing reflectarrays operating at high frequency. In our study, by contrast, a working frequency of 865.5 MHz was chosen to reduce path-loss and the cost of RF-DCT converter, thereby enabling affordable WPT systems requiring no expensive high-frequency high-power amplifiers, as the main objective of our study was to ensure the power received power level that guarantees the operation of the RF-DC converter in the nearly optimal regime.

To highlight the novelty of the proposed RRA design, as well as the research area the following sentence has been added to the updated version of the manuscript:

To the best of the author's knowledge, no RRA intended to improve the performance of antenna-based WPT systems operating in the sub-GHz range has been designed and studied both theoretically and experimentally so far.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Thanks for your efforts in revising the manuscript. All of my comments have been addressed. I have no further comments.