Regular paperA compact broadband circularly polarized printed monopole antenna using twin parasitic conducting strips and rectangular metasurface for RF energy harvesting application
Graphical abstract
Introduction
With the immediate growth of low power embedded devices for consumer and commercial usage; a significant research directed towards the improvement of power requirements of these devices are required. The implementation of RF energy harvesting can be persuaded by using electromagnetic spectrum. Thus, it becomes appealing prospect for reducing cost and the need of periodic maintenance from device point-of-view [1]. In such cases, microwave antennas are considered as the intrinsic part of RF energy harvesting. Their effectiveness is judged with the traits of circular polarization (CP) such as better signal matching, irrespective of the orientation of antenna [2]. Hence, the choice of microwave antennas is quite significant for achieving the trade-offs (broadband CP, better antenna gain and consistent antenna efficiency). Here, printed monopole antennas are chosen over other classification of antennas; due to its unique features: low-profile, lesser expensive, reasonable efficiency, satisfactory radiation pattern, along with good time domain utility [3]. Prior to that, there is a significant need of single antenna element possessing multiple features [4], [5], [6], [7]; especially its viability for the achieving circular polarization [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. CP monopole antennas are designed by adopting various methods, such as modification in feeding techniques [8], asymmetrical ground plane [9], electromagnetic metasurfaces [10], [11], [12], [13] and coupler networks [14]. Similarly, there are quite significant number of instances, where DRAs also play an important role [15], [16], [17], [18], [19]; especially for Wi-MAX and WLAN (IEEE 802.11) applications. Along with the frequency of operation; researchers are looking for the intent of RF front-ends in terms of structural complexity (mainly practical realization) and performance of the evaluation metrics over desired bandwidth. To design broadband CP monopole antenna, researchers have put their focus on modification in feeding techniques [8]; where, broadband dual CP patch antenna with wide beamwidth is reviewed out. Here, the patch is excited by four cross slots via microstrip line with multiple matching segments placed underneath the ground plane, with a reported axial bandwidth of 21%. But the implementation of asymmetric ground plane [9] for Wi-MAX and WLAN are quite limited. That's why, the designing of microwave antennas in terms of broadband impedance and axial bandwidth using the above techniques [8], [9] is quite challenging.
In the recent times, electromagnetic metasurfaces like artificial magnetic conductors (AMCs) are used for improving the performance of antenna. In [10], circularly polarized patch antenna using artificial ground structure with rectangular unit cells is used to provide axial bandwidth of 25.2%. Later, the same concept is utilized for modified-feed printed antenna, which reported axial bandwidth of 23% [11]. A corner square patch is inserted between the lattice of metallic square patches and ground to offer axial bandwidth of 23.4% [12]. In other case, regarding the inclusion of metasurfaces with antenna [13], where the broadband CP traits are achieved; along with the increase of structural complexity. Concurrently, with the usage of coupler networks; although CP is achieved, but it failed to achieve necessary trade-offs [14]. Before, advancing towards the generic solution, a comphrensive literature study is persuaded for CP-DRAs. Next, a rotated staircased DRA is proposed with a center frequency of 5 GHz. Although, it operates out in the desired frequency band, but axial bandwidth is reported to be 18.5% [15]. In [16], a wideband CP trapezoidal DR excited by an inclined by a slot, where 21.5% axial bandwidth is achieved. Similarly, stair-shaped DR excited by the combination of a microstrip feed line and a trapezoidal conformal strip resulted in 22% axial bandwidth [17]. Further, with the co-axial probe and a square slotted DR, the axial bandwidth is extended upto 24% [18]. Although, CP traits are investigated in the reported cases [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]; but during the course of study, it is observed that structural complexity of the structure also increased. Prior to that, for practical realization of DRAs, extra time and extra work is required in the design stages to process out the ceramic materials [19]. Subsequently, the trade-offs such as broadband circular polarization, broadside gain, consistent antenna efficiency with minimized system complexity are not achieved in the reported cases [8], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Thus, shifting its viability towards the negative side.
In this article, a low-profile, compact and efficient broadband CP monopole antenna employing twin parasitic conducting strips and rectangular metasurface is investigated for RF energy harvesting application. The proposed antenna is a combination of Y-shaped linearly polarized printed monopole antenna with partial ground plane. For LP-to-CP transition, partial ground plane and one of the parasitic conducting strip (PCSL) are shorted. By employing a rectangular metasurface, as a parasitic patch just below the upper edge of FR-4 substrate; broadband CP characteristics are obtained. In addition to it, a significant improvement in terms of impedance bandwidth, axial bandwidth and antenna gain are also observed. The proposed antenna exhibits a measured impedance bandwidth of 2.62 GHz (3.27–5.89 GHz) and 3 dB axial bandwidth of 1.21 GHz (4.21–5.42 GHz). Moreover, it yields out an average antenna gain of 4.92 dBi with consistent antenna efficiency of ≥72% in the desired frequency bands. A comparative focus of various antenna parameters with existing antenna designs of same field-of-interest are tabulated in Table 1. It indicates that the proposed antenna is suitable for multiple wireless applications. With consideration of broadband CP and improved antenna performances in 5 GHz ISM band, it can be utilized for RF energy harvesting. For compensating such developments, theoretical context of time domain analysis and implementation of rectifier circuit are also persuaded.
Section snippets
Antenna geometry
The geometry of proposed broadband circularly polarized Y-shaped printed monopole antenna with rectangular metasurface is shown in Fig. 1. Proposed antenna comprises of Y-shaped linearly polarized monopole antenna with partial ground plane. For the transition from LP-to-CP, partial ground plane and one of the parasitic conducting strip (PCSL) are shorted by using a metallic strip. Due to shorting, there is a generation of circularly polarized radiated waves. It confirms the presence of both
Experimental results
The proposed broadband CP Y-SMA is fabricated and characterized to validate the simulated outcomes. Fig. 6 shows the fabricated prototype. It is fabricated by using PCB prototyping mechanism and measurement of S11 parameter is persuaded by using Agilent N5247A vector network analyzer. Far-field parameters like axial ratio, antenna gain and radiation pattern are measured in anechoic chamber. Simulated and measured S11 characteristics are shown in Fig. 7(A). The measured impedance bandwidth is
Time domain analysis and implementation of multi-stage rectifier circuit
Time domain analysis is quite crucial for broadband antennas. It is performed by considering the arrangements: at a) side-by-side and b) face-to-face. These identical radiating structures are kept at a distance of 30 cm shown in Fig. 10(A), (B). A Gaussian pulse is used for analyzing the signal behaviour characteristics, shown in Fig. 10(C). Fig. 10(D) depicts group delay variation and is found to be within 2 ns in the desired frequency bands. Subsequently, variation in isolation and magnitude
Conclusion
A low-profile, compact and efficient monopole antenna with twin parasitic conducting strips and rectangular metasurface for broadband circular polarization is proposed. It demonstrates that, due to the shorting of partial ground and one of the parasitic conducting strip (PCSL), there is transition from LP-to-CP. With incorporation of rectangular metasurface as a parasitic patch just below the upper edge of substrate, a significant improvement in impedance bandwidth [rise of 3.33 times], axial
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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