Reconﬁgurable Antennas for RF Energy Harvesting Application: Current Trends, Challenges, and Solutions from Design Perspective

: Due to the widespread use of low-power embedded devices in both industrial and consumer applications, research into the use of alternate energy sources has been sparked by the requirement for continuous power. Due to its accessibility and ability to be implanted, RF energy is always taken into consideration among the traditional energy sources that are currently available. There is a signiﬁcant necessity for efﬁcient RF front-ends, which must provide effective circular polarization (CP) features, effectiveness, feasibility from a design standpoint, and optimal usage of ambient RF signals accessible in the environment. So, for understanding their utilization in RF energy harvesting, a metasurface reﬂector-inspired CP-printed reconﬁgurable antenna integrated with a Greinacher voltage divider (GVD) rectiﬁer circuit is reported. It offers broadband CP with fractional bandwidth > 25%, CP gain > 8.35 dBic, and directional radiation with the 3 dB angular beamwidth > 100° in the 3.5/5 GHz bands. With the integration of the rectiﬁer circuit, a theoretical DC output > 4.8 V at 12 dBm is obtained. The acceptable impedance bandwidth, axial ratio bandwidth, antenna gain, antenna efﬁciency, and directional radiation with a 3 dB angular beamwidth value are studied and subsequently matched with the trade-offs (usage of diodes, complexity of DC biasing circuits, and attainment of polarization reconﬁgurability) obtained from the state of the art. A comprehensive study of the reconﬁgurable antennas is reported to highlight the ﬁndings as a widespread solution for these limitations in RF energy harvesting application.


Introduction
The purpose of this particular review article is to provide a comparative focus on the probable utilization of reconfigurable antennas integrated with the rectifier circuit for the purposes of energy harvesting applications. In general, the user-friendliness of RF energy harvesting technology, which is accomplished through the utilization of ambient RF signals, has paved the way for a way to save costs and eliminate the requirement for periodic maintenance with regard to the device. The many different kinds of environmental energy sources that are currently accessible are disregarded as possibilities. Among them, the potential of radio frequency (RF) energy as the possible alternative source of energy for replacing batteries in sensors is determined to be superior. This is because it is typically implemented with the assistance of the internet of things (IoT) and wireless sensor networks (WSNs), for the various applications [1,2]. So, with the demand for low-power embedded devices (LPEDs), there is a need for research directed towards the improvement of power requirements for the particular scenario [3,4] when there is the involvement of multifunctional electronic components [5][6][7][8][9][10]. For this type of environment, we require a batteryless operation due to their limited lifespans. Additionally, one of the major limitations is the processing of battery waste, which ends up in a landfill. It becomes worse because a large number of embedded devices are used in our day-to-day life for different activities, where the battery is the only energy source for their operation. So, the phenomenon of RF energy harvesting (i.e., shown in Figure 1 and Table 1) has become an appealing prospect for reducing cost and the requirement for periodic maintenance at regular intervals, making the devices self-sustaining in nature. Figure 1. Constituent elements of RF energy harvesting (RF-EH) system [7]. Table 1. List of commercially available RF energy sources [7] (information about emission power levels is decided by the Federal Communication Committee). Of the great variety of available CP antenna systems, it has gained wide and tremendous popularity due to the ever-growing trend in the areas of system integration toward performance improvement in the wireless applications. Their modeling and attainment of low-profile features of the RF front-ends have been the subject of extensive research, which will always remain a great technical challenge and for which counter-related issues still need to be resolved with a new design (including a feasible solution). So, despite the progressive developments made in deploying the RF front-ends, there is still room for improvement in terms of the following characteristics (i.e., prerequisites of RF front-ends for maximizing outcomes from the application perspective) [11].

Frequency of Operation
Due to such types of characteristics, these printed monopole antennas (PMAs) are usually used for the UWB bands (3.1-to-10.6 GHz). However, its implementation in the sub-6 GHz frequency bands (1-to-6 GHz) is quite restricted in number. On the contrary, their limited presence and its failure to achieve viability due to their non-responsive behavior towards CP characteristics (3 dB axial ratio bandwidth + CP gain) are one of the main limitations of printed monopole antennas. The technology itself suggests that an effective RF front-end is vastly required for implementing the design philosophy from an application perspective (primary objective) [12][13][14][15][16][17].
Here, the comprehensive analysis carried out in this review paper will focus on the realization and understanding of its current trends and the challenges of reconfigurable antennas from a design perspective. In addition, understanding the frequency of operation, the performance of antenna metrics: -10 dB impedance bandwidth (IBW), 3 dB axial ratio bandwidth (ARBW), antenna gain, antenna efficiency, and radiation pattern attributes are quite significant. Further, issues such as the transition from LP to CP, the attainment of a directional pattern, and scrutinizing trade-offs before going for the optimal solution are needed from an application point of view. Hence, along with the comprehensive review, a feasible solution has been provided that, to an extent, minimizes the system complexity and solves the limitation of designing reconfigurable antennas. The main highlights are as follows: 1.
Reviewing the considerations of RF energy harvesting, analyzing its potential utilization of ambient RF signals, and addressing the growing presence of IoT, WSNs, and RFIDs.

2.
Proposing and investigating a metasurface reflector-based CP reconfigurable antenna, aiming to advance the practical implementation of efficient RF energy harvesting systems.

3.
Evaluating the RF energy harvesting capability through integration with the GVD rectifier circuit. Theoretical analogies are derived for parameters such as RF-to-DC power conversion efficiency (η • , %) and DC harvested voltage (V out , V) to assess system performance.

4.
Presenting a theoretical framework for demonstrating circular polarization (CP) and emphasizing the importance of finding a DC biasing mechanism for polarization reconfigurability, particularly the dynamic switching from LP to CP.
In light of the proposed solution from the design perspective, a circularly polarized printed monopole antenna (CP-PMA) inspired by the incorporation of metasurfaces (MS) in the form of a reflector is proposed for the RF-EH application [11]. Since the primary source of input are EM waves coming out in the form of ambient RF signals from the various wireless communication bands (i.e., mostly in the sub-6 GHz frequency bands), microwave antennas are considered an intrinsic part of the RF-EH system. So, its practical realization and consideration of the performance trade-offs, such as the achievement of enhanced CP characteristics, broadened bandwidth responses (IBW/ARBW), high CP antenna gain, consistent antenna efficiency, and a directional radiation pattern with the good boresight strength at both principal planes, are so significant in nature [11]. Before delving into the design aspects of an efficient RF-EH system, a unified understanding and exploration of cutting-edge RF front-ends must be reviewed, with an eye to the usage of diodes (number), complexity of DC biasing circuits, and the attainment of polarization reconfigurability. Because, the trade-offs for usage of diodes and complexity of DC biasing circuit gives an overall idea about the performances regarding the attainment of polarization reconfigurability.
Subsequently, a comparative focus with its concurrent analysis on the reconfigurable antennas in RF energy harvesting   It is designed, simulated, and investigated by using the CST microwave studio suite (EM simulation part) and ADS solver (circuit part). Furthermore, the results obtained from our investigation are in accordance with the attainment of trade-offs such as broadband circular polarization (CP), high antenna gain, consistent antenna efficiency, and lesser structure complexity, due to the elimination of extra biasing circuits. Finally, on execution, it can generate a DC harvested voltage (V out ) of around 5.8 volts and the RF-to-DC power conversion efficiency of up to 75% with the integration of the GVD rectifier circuit.
Thereby, the proposed metasurface (MS) antenna exhibits improved 3 dB axial ratio bandwidth and CP antenna gain. Most importantly, the existing antenna trade-offs, i.e., usage of diodes (number), complexity of DC biasing circuits, and the attainment of polarization reconfigurability) in the same field of interest are resolved. Adding more into it, the number of diodes used in the previous case is two or more, which literally adds congestion through the incorporation of additional DC biasing circuits. Due to this, the attainment of polarization reconfigurability could not be achieved to its maximum extent, looking from the application perspective.
A comparative focus on this ground, along with its state-of-the-art references, is highlighted in Table 2 and Figures 2-5, respectively. In the next section, the complete design perspective of a reflector-inspired CP-based reconfigurable antenna is presented, along with its corresponding outcomes. It will act as a generic solution for improving the state transition for reconfigurability [140], aimed at addressing the requirements of various wireless applications. It is also observed that, for the selection of a reconfiguration technique, there is a dependency on the property of reconfigurable antenna geometry. That is why an antenna designer selects the technique in such a way that it satisfies generic imposed constraints and at the same time and completes the antenna designing process very efficiently.  Figure 2. Overview of the frequency reconfigurability antennas for RF energy harvesting applications (a) [111], (b) [112], (c) [113], and (d) [114].  [122], (c) [123], and (d) [124].

Case Study: Solution from Design Perspective
In this section, the metasurface reflector-based CP reconfigurable antenna integrated with a rectifier is taken as a case study and investigated for the RF energy harvesting. It illustrates the achievement of design trade-offs such as broad 3 dB axial ratio bandwidth, higher antenna gain, consistent antenna efficiency, and a directional pattern with a wider 3 dB angular beamwidth [11]. Along with the realization of this reconfigurable antenna, a proper understanding of emission power levels at various frequencies is needed, as shown in the Table 1. These reconfigurable antennas presented in Table 2 have used two or more diodes for their operation. The presence of a larger number of diodes increases the circuit complexity. Additionaly, the coverage for antennas in the sub-6 GHz band is found to be limited. So, a more practical solution needs to be realized, as briefed earlier in the previous section.
Thus, a metasurface reflector-inspired reconfigurable broadband circularly polarized Y-shaped monopole antenna is proposed for the 3.5/5 GHz bands. Here, it consists of a Y-shaped radiating monopole and a 50 Ω microstrip feed line for input excitation. The traditional monopole antenna with a partial ground plane is limited in its ability to achieve polarization and pattern attributes. Therefore, the partial ground plane is shorted with a parasitic conducting strip (PCS L ) by using a BAR 50-02V RF PIN diode from Infineon Technologies. It is capable of generating the horizontal and vertical components required to witness the existence of CP fields. Such an occurrence of events is often confirmed by surface current distribution, electric field distribution, and a far-field radiation pattern [11].
To meet the trade-offs, a metasurface (MS) reflector is placed just below the Y-shaped radiator at an air gap of 20 mm. As a result, the 3 dB axial ratio bandwidth is improved from 460 MHz to 1.23 GHz (a rise of 3.08 times) with an enhancement of CP antenna gain from 3.6 dBic to 8.45 dBic (a rise of 2.35 times). Hence, for the RF-to-DC conversion, a multi-stage GVD rectifier is incorporated. The RF-to-DC conversion efficiency (η o , %) is calculated on the basis of Equation (1). Here, the DC output voltage (V out ) is greater than 4.8 volts, with an RF-to-DC conversion efficiency of greater than 55% at 12 dBm across the operating frequency bands. Their detailed behavioral insights are shown in Figure 6a-d and Table 3, respectively.
Besides implementing the CP RF front-ends (CP-PMAs), detailed theoretical insights are presented for understanding the different behavioral aspects in hope of understanding the existence and behavior of CP for PMAs, the placement of reflectors below radiating structures, performance analysis related to the enhancement of IBW and ARBW, enhanced CP antenna gain, and attainment of directional patterns with improved front-to-back ratio (FBR) for the MS. As a result, the proposed contribution includes the achievement of performance trade-offs. In the end, the usage of a PIN diode (BAR 50-02 V) from Infineon Technologies with a simple biasing circuit is reported, which minimizes the overall complexity of the system by alleviating the use of additional DC biasing circuits.
In the next section, an analogy about the theoretical aspects of CP backed by surface current distribution, the study of electric field pattern in the context of the plane wave equation, and correlation with a Cartesian plot of the far-field normalized pattern is highlighted with respect to the proposed antenna. With insights into DC biasing mechanisms, the intuitions behind the attainment of directional radiation pattern characteristics are also presented as a generic solution to the observed limitations.
Phenomenon-D: Exploration for RF energy harvesting as a prospective application.

Transition from LP-to-CP
For a better understanding of circular polarization (CP) characteristics, the axial ratio (AR ≤ 3 dB) is considered the primary indicator. Here, CP analysis is backed by (a) surface current distribution, (b) electric field distribution, along with its theoretical insights with respect to the plane waves and its modeling with relationship to the far-field observables, and (c) a normalized radiation pattern, can be seen in Figures 7a-d, respectively.
The first primitive approach to analyzing CP is through the surface current distribution. At stage-2, due to the absence of parasitic conducting strips (PCS), no connection is established with the partial ground plane. Induced surface currents on the horizontal edges of the partial ground plane are found in the opposite direction, canceling each other out and leaving only the vertical surface currents on the monopole arm. Hence, a linearly polarized (LP) wave is generated (OFF-state). In stage-3, when one of the parasitic conducting strips (PCS L ) is shorted with the partial ground plane, through a BAR50-02V PIN Diode from Infineon Technologies, the surface currents on PCS L and partial ground plane are rearranged in such way that the resultant currents on the upper edge of PCS L and the lower edge of the partial ground plane do appear in the same direction, inducing out horizontal surface currents. The presence of both horizontal (x) and vertical (y) components in terms of horizontal and vertical currents leads to the generation of CP characteristics (ON-state).
The second primitive approach to analyzing CP is through electric field distribution. When PCS L is connected to partial ground plane through the BAR50-02V PIN Diode, the CP reconfigurable antenna realizes LHCP in the +z direction (i.e., outward in nature). The rotation of electric field vectors changes in a clockwise direction, with a change in the phase from 0°to 90°at f = 4.5 GHz. Therefore, an orthogonal change in the electric field pattern confirms the generation of left-handed circularly polarized waves (LHCP).
The third primitive approach to analyzing CP is through normalized radiation pattern. It correlates with the relative power from the normalized radiation pattern (i.e., the cartesian plot) at f = 4.5 GHz. In the broadside direction, the antenna has excellent LHCP characteristics. The LHCP is stronger than the RHCP by -21 dBic in their desired bands.
The explanations of circular polarization (CP) have been explored using the following approaches: I, II, and III. It is related to the analysis of transverse components in that farfield observations are frequently treated as far-field vector components, which correspond significantly to the polarization varsities, i.e., the linear, circular, or elliptical, respectively.

Intuition Behind Implementation of Metasurface Reflector
The design and development of a metasurface reflector are investigated in this case by evenly dividing the entire rectangular patch of 1.78λ • × 1.48λ • into grid-slotted sub patches of 12 × 12 cells, with each cell of being 0.13λ • × 0.009λ • and an intermediate gap of 1 mm between them. As a consequence, TM 10 mode is found on each sub-patch that contributes towards the generation of quasi-TM 30 mode. The presence of such higher-order modes frequently results in a broadened impedance and 3 dB axial ratio bandwidth [141,142]. When the metasurface reflector comes into contact with the radiator, it redirects out one-half of the radiated waves in the opposite direction and partially shields objects on the other side. Such behavior is seen due to radiated waves from a reflector-based printed monopole antenna that consist of (a) directed waves from the Y-shaped monopole radiator and (b) reflected waves from the metasurface reflector. It exhibits a directional radiation property, as shown in the Figures 8-10. In addition to it, the average CP antenna gain is enhanced by 2.35 times, the 3 dB axial ratio bandwidth is also improved with a rise of 3.08 times, and a fractional bandwidth of >25% is attained. Thus, the changes in this type of characteristic are often attributed to the movement of incoming currents and the generation of stronger orthogonal fields over the desired CP frequency bands [143][144][145][146][147][148].

Understanding DC Biasing Mechanism
By connecting an extended partial ground plane and one of the parasitic conducting strips with the aid of a single BAR 50-02V PIN Diode (ON-state) from Infineon, polarization reconfigurability is achieved. So, the equivalent of an Infineon BAR 50-02V PIN Diode is simulated in the CST microwave studio with a 3Ω forward resistance in the ON-state and a 5 kΩ reverse parallel resistance with 0.15 pF reverse parallel capacitance in the OFF-state.
Here, the forward current of the 18 mA forward input voltage of 1 V is measured, with 0.89 V as the voltage drop across the PIN diode as the extended partial ground plane is connected to the BAR 50-02V PIN Diode in this instance. Thus, a minimal loss is introduced in this proposed antenna, which reduces complexity at the time of practical realization shown in Figures 11-14.

RF Energy Harvesting: Prospective Application
For incorporating the phenomenon of RF energy harvesting into the real-time aspect, there is a need to convert these ambient RF signals into a DC output, for which highly efficient and wideband rectifier circuits (GVDs) are required and need to be appended with the impedance matching network (IMN) to the CP RF front-ends. Henceforth, the viability of the entire system significantly depends depending on the performance of these individual units [RF front ends (antennas) + impedance matching network (IMN) + rectifier circuits (GVDs)]. Thereby, the overall working idea remains with the development of an extensive system that fulfills the increasing power requirements to carry out the robust operations of the multi-functional electronic components for both consumer and commercial usage, provided that the proposed RF front-ends are able to achieve the performance trade-offs before integrating them with the proposed rectifier circuits. To satisfy these types of needs, the design and implementation of CP-printed monopole antenna (CP-PMAs) operating in the sub-6 GHz frequency bands with enhanced performance due to incorporation of metasurfaces (MS) embedded with the multi-stage rectifier circuit systems (GVDs) being thought to be very important in terms of performance trade-offs [12]. Furthermore, the entire scenario (Y-shaped monopole antenna + metasurface reflector) is represented by Equations (2) and (3). Therefore, the overall results resulted in the powering of sensors found in low-power devices (such as wearables, and medical and healthcare plug-based kits), which require a constant DC output voltage of around 2.4-5.5 V to operate. In short, this particular technology will gradually play a crucial role in designing efficient systems. Although it faces so many issues, overcoming these challenges gives rise to an era of clean, green, and sustainable energy for the 5G/6G [149][150][151][152][153][154][155] applications of recent times.
In a practical testing scenario of the polarization reconfigurable mechanism electronically, the BAR50-02V PIN RF Diode as a switch from Infineon with package P-SC79-2-1 is used. The voltage conditions are applied for ON-state (forward voltage, V F = 0.95 V) and for OFF-state (reverse voltage, V R = 50 V) when applied to the RF PIN diode through the DC power supply. From the radiation graphs shown, LHCP and RHCP are isolated by >15.5 dBic (simulated) and >25.2 dBic (measured) indicating that the LHCP is dominant as compared to RHCP with front-to-back ratio (FBR) ranging from −15 dBic to −23.5 dBic, as shown in Figure 13.

Conclusions
This review article presents a comprehensive summary of the recent advancements in reconfigurable antenna technology for RF energy harvesting. Since electromagnetic waves constitute the main input, microwave antennas are regarded as an integral component of the RF energy harvesting system. Thus, it is important to address its practical realization and trade-offs. However, before beginning the designing process, it is necessary to have a thorough understanding of the principles of RF transmission, insights into rectification behavior, and an investigation of reconfigurable antennas in RF energy harvesting. At first, a wideband, CP, reconfigurable Y-shaped monopole antenna is reported. Conventional antennas were unable to accomplish trade-offs. Improvements in IBW, ARBW, CP antenna gain, antenna efficiency, and attainment of the directional pattern are seen in the case study with the addition of the metasurface reflector. In addition, the conditions such as usage of diodes (number), the complexity of DC biasing circuits, and the attainment of polarization reconfigurability are also taken care of. The three-stage Greinacher voltage doubler and LC matching (IMN) rectifier circuit are integrated into the metasurface reflector-inspired CP reconfigurable antenna (GVD). Here, the designed rectifier circuit is examined for input power levels (P in ) ranging from 0 to +20 dBm, V out is of >4.86 V with the η o as >55% at 12 dBm when simulating at the ADS platform with a load resistance (R load ) of 2.2 kΩ, which is quite enough to efficiently power the sensors in internet-of-things (IoT) and wireless sensor nodes (WSNs).