Design of Wideband Circular-Slot Antenna for Harvesting RF Energy

Department of Electronics and Telecommunication Engineering, Ahmdu Bello University, Zaria 810211, Nigeria Department of Medical Equipment Technology, College of Applied Medical Sciences, Majmaah University, AlMajmaah 11952, Saudi Arabia College of Engineering, Al Ain University, Al Ain 64141, UAE Institut National de la Recherche Scientique (INRS), Montréal H5A 1K6, QC, Canada Institute of Integrated Micro and Nano Systems, School of Engineering, University of Edinburgh, Edinburgh EH9 3FF, UK Department of Signal eory and Communications, Universidad Carlos III de Madrid, Leganés 28911, Madrid, Spain


Introduction
e ability to handle high electromagnetic (EM) energy is one of the important features of antennae used for the RFEH module [1,2]. RFEH technology has recently piqued the interest of researchers as an additional source of energy that provides an alternative solution to short-life batteries [3,4]. Mobiles phones and other related wireless devices have been penetrating the market since 1980 [1,5]. Hence, the rising demand for the long operational life of a battery remains an open challenge [6,7]. In RFEH systems, a rectifying antenna (rectenna) harvests the energy via a combination of a source antenna and RF-recti er [8,9]. e source antenna picks up the incoming signals, which are then transformed by an RFrecti er into a useable low power dc supply [10]. RFEH technology is considered among the sources of green energy by utilizing and shielding humanity from potentially harmful radiation [11].
us, wireless medical implanted devices (WMID) largely facilitated the emergence of applications in healthcare systems such as wirelessly capsule endoscopes, neural implants, retinal prostheses, various neural recording microsystems, spinal cord stimulators, and intracranial pressure (ICP) monitors [12,13]. Life-saving healthcare systems involve telemetry and supervision of the vital human body parts by the basic essential indicators for the evaluation, diagnosis, stimulation, and treatment process [14,15]. RFEH antennae with wide operational BW and improved gain are desirable for an efficient RF harvester [16]. RF-spectral data from various research studies that have recently been reported shows a practical amount of energy for harvesting at GSM1800, UMTS2100, ISM2.4-Wi-Fi2. 45, and LTE2600 spectrum [5]. It is a challenging task to design an antenna that can operate over a specific broad and compact spectrum for a particular application [17,18].
Researchers have recently focused on developing ultrawideband antennae with band rejection capabilities to minimize interference from narrow-band for wireless applications [19,20]. To attain the desired goal, several designs approach, such as inserting slits and slots of varying diameters, are being applied to the radiating components, feed line, and ground planes [5,19]. e use of half (λ/2) [5] and a quarter (λ/4) [21] wavelength, open ended slits, and DGS [22,23] are also reported for various wireless communications applications. e authors in [24] introduce a circular patch monopole antenna with an annular-ring structure at 5.80 GHz. e antenna demonstrates a 12.8% BW increment, and a gain of 5.70 dBi at a relatively high frequency compares to a typical monopole antenna. A monopolar broadband antenna is reported by the authors in [25]. With the introduction of metallic Vias, the antenna attains a BW of 18%, resulting in a peak realized gain of 6 dBi between 2.15 and 2.35 GHz.
Incorporating a feeding-loop results in about 65% BW as demonstrated by the authors in [26]. e antenna realized a peak gain in the span of 3 to 7.7 dBi between 1.320 and 2.60 GHz operating frequencies (f o ). Besides, the deployment of a complex feeding technique in the 3D structural model, a CPW broadband antenna with a square-slotted pattern is demonstrated by the authors in [27]. e design realized a FBW of 17.2% at 2.440 GHz. e concept of introducing slots and slits on the feed line and the bottom ground of the CPW antenna is reported to improve the FBW by about 45% by the authors in [28,29]. e authors in [30] explore the analysis of corner truncated antennae comprising U and L slots. A single feed probe is used in the design study where the various dimensions of the substrate height (h) were examined. e design attained a peak FBW of 14.0% at 4.05 and 4.15 GHz. e authors [31,32] demonstrated the concept of integrating the feed line with slots and slits. e designs, respectively, realized a FBW of 4.70% and 17.70% for worldwide interoperability for microwave access (WiMax) and radio frequency identification (RFID) applications. e technique of fractal architectures deployed by the authors in [33][34][35] obtained a FBW of 22.50%, 0.78%, and 2.00%, respectively. Hence, the broadband antennae reported by the authors in [28,29,31] and [36] are generally designed with a complicated geometry that is difficult to actualize.
is work targets a wideband circular-slot antenna with an improved gain, having a simple and inexpensive geometry structure. e authors in [34,37] demonstrated the concept of an electromagnetic coupling using a single feed line to maintain simple antenna geometry. A tilted rectangular and triangular wideband monopole printed antennae are exploited by the authors in [38,39]. e designs reported a FBW of 51.40% and 62.00%, respectively. e designs also recorded a relatively low gain across the f o , which renders them unsuitable for low-power RFEH systems. A low-profile source antenna with broad BW and enhanced gain is required for better RFEH in ambient terrain [1,10]. A broadband antenna for RFEH is presented by the authors in [40]. e antenna achieved a FBW of 23% at 2.45 GHz. e authors in [41] reported a narrow-band antenna for the RFEH application. e source antenna achieved a 10 dB BW of 30 MHz with a peak gain of 3.360 dBi at 2.45 GHz. Printed patch antennae are regaining recognition in the design of RF harvesters due to their conformability to planar and nonplanar surfaces, compactness, low-profile, light, and cheap manufacturing cost [1,8,11]. Patch antennae are also preferred due to their adaptability in terms of f o , gain, radiation pattern, polarization, and matching BW. erefore, the proposed design in this work maintains a trade-off between simple geometry structures, compact size, affordability, and improve performance.
In this study, a circular-slot wideband antenna is reported.
e antenna is suitable for harvesting RF signals across GSM1800, UMTS2100, ISM2.4-Wi-Fi2. 45, and LTE2600 spectrum. e proposed design achieved a total dimension of 50 mm × 56 mm matched through a 50 Ω transmission line (TL). e proposed design offers a wide f o of 1.640 GHz to 3.150 GHz with an improved gain applicable for RFEH systems. e remaining sections of this work are divided into the following. Section 2 outlines the proposed design antenna configuration. e findings are addressed in Section 3. e concluding remark is presented in Section 4.

Antenna Design
RF spectrum measurements were performed before coming out with the antenna design to determine the availability of RF ambient power in the environment. Figure 1  cross section of the received ambient RF power levels. e survey highlights the significance of five major spectrums with reasonable power levels for RFEH. As a result, the operational frequency for the proposed antenna is specified within 1.640 to 3.150 GHz, which covers GSM1800, UMTS2100, ISM2.4Wi-Fi2.45, and LTE2600 frequency bands. e antenna presented in this paper is designed on a double layer of 1.6 mm height (h) FR-4 substrates, having 4.7 dielectric constant (ϵ r ), with 0.02 tangent loss (tanδ). e material is adopted because it is inexpensive, available, and simple to fabricate. e proposed antenna comprises a circular-ring radiating element integrated with two additional circular and rectangular slots. Figure 2 presents design architecture and the parameters of the proposed wideband circular-slot antenna. Firstly, antenna architecture was a model using a circular microstrip planar antenna based on a closed-form equation as expressed in the following equation [42]: where f lw represents the lower cutoff f o . p is the gap of the feed line in (cm). a provides the radius of the circular patch in (cm) over a constant value k � 1.15. us, "a" can be expressed as Solving for "a" at f lw � 1.6 GHz, and p � 0.5 cm, a is computed to be 1.6 cm (16 mm). e width of TL is initially evaluated at 2.7 mm from the Wheeler's closed-form equation [10] and then optimized at 2.8 mm. e calculated values from the model equation are transferred into a high-frequency structure simulator (HFSS) from ANSYS for further parametric tuning and optimization.
is section investigates the impact of various critical dimensional elements on the antenna's performance, notably its radiation pattern, impedance BW, and gain. All simulations are conducted through HFSS. e first circular antenna structure (Design-#1) resonates at 2.2 GHz with an unsatisfactory |S 11 |. Circular structures tend to provide a steady flow of currents [15,33]. A DGS is introduced into the antenna structure to achieve a broader impedance BW between 1.600 and 3.100 GHz as depicted in Design-#2. A circular-ring structure is realized by introducing a 23 mm circular slot into the radiator, as shown in Design-#3. Two circular slots are added to the orbital section of the radiator to achieve a broader resonance across 2.320 to 2.910 GHz f o . A good impedance matching is realized by extending rectangular slots from the bottom of the radiator. e DGS is also incorporated with a resonating circular parasitic patch to enhance the proposed antenna's gain. e embedded circular patch on the partial ground resonates with the corresponding pair of circular slots counterpart on the lefthand side (LHS) of the radiator. Additionally, a pair of rectangular slots and a semirectangular-circle slit are carved on the bottom ground for a broader impedance BW as described in Design-#4. e addition of the slots and slits into the orbital sides of the structure is realized through considerable parametric analysis to maintain the antenna wideband characteristics with a reasonable gain. Hence, Figure 3 illustrates the procedures used to achieve the desired wideband circular-slot antenna. International Journal of Antennas and Propagation e wideband circular-slot radiator is first excited with an upper circular slot along the LHS corner with a radius of r1. e diameter of the slot was tuned at λ/4 of the medium resonance mode of 2.1 GHz. r1 was then varied to investigate the effects of the slots on the antenna performance. Adjusting r1 introduces a noticeable impedance mismatched along the 2.00 to 2.400 GHz f o . e diameter of the slots was then optimized at 16 mm, as shown in Figure 4(a). e lower circular slot with a radius of r2 is integrated between the radiator and the feed line to further improve the upper resonance mode f u between 2.320 and 2.910 GHz and also reduce the impedance mismatch. e diameter of the lower orbital slot were then gradually tuned at λ/8 of f u at 2.45 GHz. Hence, r2 of the slot demonstrates an improved antenna performance at 10 mm diameter, as shown in Figure 4(b). e incorporation of the upper and lower circular slots to the radiator introduces an impedance mismatch to f lw . As such, a vertical rectangular slot is raised from the center of the radiator to enhance the impedance matching across the wide f o . Rectangular stubs have been frequently employed in various literature to increase the resonance performance depending on their orientation [43][44][45][46]. Varying the length of the slot t has an impact on the impedance BW, which    distorts the antenna peak achievable gain. e impedance matching tends to improve as t slowly increases from 6.5 to 9 mm and deteriorates beyond 10 mm. A good impedance matching is realized by extending the length t of the rectangular slot from the bottom of the radiator at 9.30 mm, as demonstrated in Figure 5(a). Additionally, a circular parasitic patch is embedded into the partial ground to resonate with their corresponding pair of the radiator orbital circular slot to enhance the proposed antenna's gain. A comparison of the circular parasitic patch with radius r3 and the corresponding length t is illustrated in Figures 5(a) and 5(b). e antenna's measured |S 11 | is in close agreement with the simulated data. e slight variation from the measured data is attributed to fabrication tolerance between the top and bottom view, the SMA source or connection loss, and soldering lead loss. e proposed design achieved -10 dB simulated and measured BW of 1.51 GHz over a frequency span of 1.640 to 3.150 GHz and 1.590 GHz between 1.550 and 3.140 GHz. e results findings cover a target f o , amounting to 68% and 73% of the simulated and measured FBW, respectively. Figure 6(b) depicts the peak gain variation as a function of frequency. A maximum peak measured and simulated realized gain of 3.1 dBi and 3.2 dBi is attained by the antenna at 2.600 GHz over a range of the targeted f o . A peak realized simulated and measured gain of (1.93 dBi, 2.6 dBi, and 3.3 dBi) and (1.8 dBi, 2.1 dBi, and 2.7 dBi) is also achieved at 1.800, 2.100, and 2.400 GHz, respectively.  Table 1 summarizes the antenna's outcomes, which are compared with the related work in terms of their   International Journal of Antennas and Propagation description, electrical dimensions, FBW, and gain. e authors in [18,24] realized a peak gain when compared to this work at the expense of a larger electrical dimension, lower FBW, and relatively higher f o . Besides, the proposed wideband circular-slot antenna demonstrates a good improvement with a broader f o that covers four major RFEH spectrums. Furthermore, the antenna achieved a high gain across f o , which is important in energy harvesting applications. As a result, the antenna provides a good trade-off between FBW, compact size, gain, and cost. e performance of the proposed wideband circular-slot antenna in an ambiance terrain is investigated in the Multimedia University, Cyberjaya campus. Figure 1 highlights the capability of the terrain for harvesting RF signals. e amplitudes of receiving RF signals vary based on the ambient circumstances. After evaluating the antenna parameters, a broadband RF-rectifier with a wide scale of input power is introduced to make a rectenna system. e RFrectifier also operates between 1.780 and 2.620 GHz. e two components are connected by a straight-through SMA-male to a SMA-male RF adapter. e broadband rectenna is then put to the test in an ambiance setting. e proposed wideband circular-slot antenna is subjected to a variety of tests at different locations in the campus through the rectenna system. Generally, the locations are marked between 30 and 200 meters from a nearby position or BS. e measurements location is around 1 to 2 m above the surface level. As demonstrated in Figure 8, the proposed circularslot antenna produces an output dc voltage V dc of 0.313 V via the wideband RF-rectifier output terminal.

Conclusion
Source antenna architecture presented in this paper is composed of a circular-ring radiating element loaded with two orbital circular and rectangular slots. A defected ground integrated with a pair of rectangular and semirectangularcircle slits is modeled at the bottom plane for enhancing the impedance matching BW. A circular parasitic patch resonating with its respective pair of radiator orbital slots is further added into DGS to improve the gain of the proposed antenna.
e proposed wideband circular-slot antenna achieved a measured and simulated operational BW of 1.59 and 1.510 GHz, resulting to a 73% and 68% FBW, respectively. A peak measured and simulated realized gain of (1.8 dBi, 2.1 dBi, 2.7 dBi, and 3.1 dBi) and (1.93 dBi, 2.6 dBi, 3.4 dBi, and 3.2 dBi) is attained by the antenna at 1.800, 2.100,  International Journal of Antennas and Propagation 7 2.450, and 2.650 GHz, in that order. e source antenna is implemented on the FR-4 board covering a size of 0.61 λ g × 0.70 λ g [47].

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.