A Single-Feed Wideband Circularly Polarized Dielectric Resonator Antenna Using Hybrid Technique With a Thin Metasurface

A compact metasurface-based circularly polarized (CP) dielectric resonator antenna (DRA) is proposed with wideband characteristics. The antenna forms a very simple structure, composed of a rectangular DR, a single coaxial probe, and plus-shaped unit cells-based metasurface. The metasurface is realized on a grounded FR-4 substrate. Next, a rectangular DR is loaded centrally over the metasurface. The DR is fed with a perturbed probe feed at an appropriate angle of (<inline-formula> <tex-math notation="LaTeX">$\theta $ </tex-math></inline-formula>=29°), along the diagonal line. Thus, a novel hybrid technique involving the angle of feed location from the center of DR, and the <italic>N</italic> <inline-formula> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> <italic>N</italic> unit cells-based metasurface is utilized for generating a wideband CP radiation. The resonance from the rectangular DR and surface waves along the <inline-formula> <tex-math notation="LaTeX">$7\times $ </tex-math></inline-formula> 7 plus-shaped unit cells-based metasurface is exploited to achieve a wide 3-dB axial ratio (AR) and impedance matching bandwidth. The fabricated antenna prototype used for the validation of predicted results confirms the successful implementation of the proposed technique. Measured results demonstrate a wide impedance bandwidth of 32% (3.6 GHz - 7.0 GHz) and an overlapping 3-dB AR bandwidth of 20.4% (4.2 GHz - 5.2 GHz). Moreover, the antenna adopts a left-hand circular polarization (LHCP) with 6–7 dBic measured gain within the operational frequency range. Overall, the proposed antenna offers low-profile, simplicity, ease of design, and high performance.


I. INTRODUCTION
With the rapid deployment of 5G at our doorstep, forthcoming wireless solutions incorporate smart technologies and pose stringent expectations. The demand for compact, efficient, lightweight, wideband, and high-gain antennas is on the rise. Besides, polarization is considered one of the most The associate editor coordinating the review of this manuscript and approving it for publication was Derek Abbott . significant performance features for antenna integration with modern communication systems [1]. Compared to linearly polarized antennas, circularly polarized (CP) antennas are gaining more recognition, owing to their salient advantages of low-cross talk, improved multipath rejections, and better mobility [2]. These features are extremely useful and have great significance in implementing various telecommunication systems and satellite applications. This includes radio frequency identification (RFID), radar technology, global positioning systems, healthcare, and weather forecast. Furthermore, with ever-growing requirements of higher data rates and insensitivity to the transmitter and receiver orientation, low-profile wideband CP antennas are desired [3], [4]. Microstrip patch antennas offer polarization flexibility, planar structure, and ease of fabrication. They have long remained a suitable candidate for various wireless applications. Despite many advantages, patch antennas are constrained by their narrow bandwidth and low gain performance [5], [6]. Dielectric resonator antenna (DRA), on the other hand, offers an edge over microstrip antenna in multiple ways, such as small size, easily fed, metallic lossless, high-quality factor, higher radiation efficiency, and no surface wave excitation [7], [8].
The realization of circular polarization typically requires the generation of dual orthogonal modes with a 90 • phase shift. In CP DRAs, this 90 • phase shift is generated by making use of either single-point or multi-point feeding techniques [9], [10], [11], [12]. Dual and quadrature feeds are demonstrated to achieve wide impedance matching and 3-dB axial ratio (AR) bandwidth [11], [13], [14]. However, the implementation of these feeds requires power dividers or external hybrid couplers, which come with in [29]-creased complexity, and bigger antenna size. On the other hand, the requirement of CP radiation is particularly challenging to achieve over a wide range of frequencies, for designing a single-feed wideband CP DRA. Considering this, an increased amount of interest resulted in the design of various single-feed CP DRAs. These DRAs are demonstrated to enhance the CP radiation bandwidth such as CP radiation was generated by the perturbation method in [15], by a circular-sector-slot in a circular cylinder DR [16], square-slots in rectangular DR [17], hexagonal-shaped DR [18], and a slotted-square DR with parasitic strips [19] for omnidirectional radiation. In addition, various researchers have expanded the AR bandwidth of DRAs by using a high-order mode approach. For example, in the case of a cylindrical DRA [20], the fundamental mode HEM111 and high-order mode HEM113 were excited by designing two feed strips for achieving broadband CP radiation. While three orthogonal modes were excited simultaneously by employing a trapezoidal DRA in [21]. Numerous methodologies have also been reported to improve the performance of CP DRAs with array configurations. Some of these approaches include probe fed DRA array [22], rectangular DRA array [23], elliptical DRA array [24], and a 2 × 4 sequentially rotated DRA array [25]. One of the major drawbacks of these configurations is the requirement of a feeding network, which may lead to design complexities.
Recently, efforts have been dedicated to improving the performance parameters of antennas by employing metasurface structures [26]. Metasurface, owing to their property of manipulating the electromagnetic waves, are placed either above or beneath the patch/radiators for achieving wideband and high gain operation [27], [28], and more recently on the single layer. CP microstrip single feed antennas have been miniaturized with improved bandwidth by using metasurface structures. These include a reactive impedance surface (RIS) [29], [30], an artificial ground plane/metamaterialsurface based single dipole antenna [31], a metasurface based slotted-rectangular microstrip antenna [32], and a stacked asymmetric metasurface above the rectangular patch [33] for wide 3-dB AR bandwidth, and high gain, respectively. More recently, a wide selection of antennas combining patches and metasurface of various shapes and sizes are investigated [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. However, to the best of the author's research, literature review, and knowledge, no rectangular DR antenna is yet reported over a metasurface for achieving wideband CP characteristics.
In this paper, a metasurface-based rectangular DR is demonstrated by utilizing a single coaxial feed at a particular angle for wideband CP radiation. The rectangular DR is placed over a metasurface consisting of 7 × 7 unit cells etched on a grounded substrate and a coaxial feed. The DR is excited along the diagonal at a particular angle using a modified upper probe of coaxial feed to investigate a wide 3-dB AR bandwidth. The proposed hybrid technique combining the feed angle and well-matched metasurface demonstrates a wide 3-dB AR bandwidth within the 2-VSWR-frequency range. Also, a wider 20.4% 3-dB AR bandwidth with 6-8 dBic gain and 1-1.5 dB gain variation is demonstrated over the 3-dB AR bandwidth for the proposed   antenna. This antenna is designed and analyzed rigorously using CST microwave studio [44]. It can be utilized in defense and satellite communication applications such as weather radar systems or terrestrial microwave links etc.

II. ANTENNA STRUCTURE AND CONFIGURATIONS
This section presents a detailed design of the metasurface DR antenna based on the proposed hybrid technique. It also explains the antenna radiation mechanism by considering predicted results and E-Field characteristics. Later, a comparison of predicted and measured results is carried out for practical validation.

A. DESIGN PROCESS
Based on the analogy presented in [45], an exploded view of the proposed wideband CP DRA is depicted in Fig. 1 (a). The antenna design is comprised of rectangular DR, metasurface, a single substrate layer, and a ground plane. Initially, it follows the design procedure by having rectangular DR without any metasurface and later introducing periodic patches of N × N unit cell array metasurface, as illustrated in Fig. 1 (b). The rectangular DR is designed using TMM10i dielectric substrate (h 2 = 6.5 mm, 2 = 10.2, tanδ = 0.002) with the following dimensions (length, l = 14.9 mm and width, w = 12.6 mm) on the middle of a FR4 substrate. A N × N unit cell metasurface (where N = 7) is realized on a grounded FR4 dielectric substrate (h 1 = 3.2 mm, 1 = 4.2, and tanδ = 0.022), as shown in Fig. 1 (c). The overall size of this FR4 substrate is 55 mm × 76 mm. Whereas, the metasurface unit cell takes the shape of a plus with dimensions S w = 2.5 mm, W p = 6.0 mm, L p = 8.76 mm, each separated by distance dx (in normal plane) and dy (in azimuth plane), respectively, for maintaining the periodicity. Generally, square, and rectangular-shaped metasurfaces have been utilized by various researchers previously with limitations of the narrow band. Therefore, a plus-shaped metasurface is considered here because of its potential wideband response. From the design perspective, a unit cell should have a reflection phase response of (±90 • ) exceeding the required operating frequency band [32]. To demonstrate this, the reflection phase characteristics of the proposed metasurface are investigated and shown in Fig. 2. The zero-degree reflection phase is located at 5.75 GHz with the reflection phase bandwidth (±90 • ) ranging from 4.9 GHz to 7 GHz, demonstrating the broadband response of the proposed metasurface. The metasurface unit cells are separated by 1.46 mm and 1.5 mm in both normal and azimuth plane. The antenna structure is then excited with a single coaxial feed positioned the center of DR. The coaxial feed location is optimized along the diagonal line direction on the rectangular DR for a wideband CP radiation [32]. Lastly, the metallic part around the feed location from the metasurface is removed with a radius of 2.0 mm, to avoid any deterioration of the results due to the contact between the coaxial feed and unit cells.

B. OPERATING PRINCIPLE AND NUMERICAL STUDIES
A metasurface being the 2-D equivalent of 3-D metamaterial is considered a surface distribution of electrically small scatterers, which can be utilized to control and manipulate the electromagnetic waves and the radiation mechanism of primary radiators [28]. The extra resonance for the radiation structure can be yielded by combining the primary radiator with a finite-sized metasurface [46]. The surface waves which propagate on the metasurface structure produce these extra resonances. The surface wave resonances generated on a finite metasurface structure depends on the number of unit cells and periodicity among them, which can be qualitatively calculated by he cavity model and transmission line model, as presented in [28] as: where, β SW denotes the propagation constant of the surface wave resonances, and L MS is the metasurface structure length, as given by: where, N is the number of metasurface unit-cells, and P is the periodicity among them. Replacing value of L MS in (1), we get: From [47], the propagation constant of the surface waves travelling and decaying away from the metasurface are related to the decay constant (α) and the frequency (ω), using the following equation: The propagation constant of the transverse magnetic (TM) and transverse electric (TE) can be computed by the following expressions: where, ω is the angular frequency, c is the speed of light, η is the intrinsic impedance, and Z Metasurface represents the surface impedance of the metasurface structure.  Considering the above, the surface wave resonance property of the metasurface structure is exploited here for the design of the proposed CP DRA. To serve the purpose, a hybrid technique for the DRA using diagonal probe feed along with a well-matched metasurface is investigated and presented in this article. This method is demonstrated to achieve a wideband CP performance. By utilization of a plus-shaped unit cells-based metasurface extra resonances both in |S 11 | and AR can be generated, which is then fine-tuned by perturbing the upper probe (UP) of the coaxial feed, as demonstrated in Fig. 3 (a). Fig. 3 (b) shows that a very good impedance matching of 29.4% (3.6 GHz -6.6 GHz) is achieved by optimizing UP to 1.1mm. Furthermore, it also improved the antenna's AR performance. Fig. 4 depicts the |S 11 | characteristics of standalone DRA and DRA with 3 × 3, 5 × 5, and 7 × 7 metasurface, respectively. This comparison shows that DRA with 7 × 7 metasurface achieves multiple resonance responses with significant input matching. The reflection characteristics and AR performance of the optimized DRA before and after the metasurface is compared in Fig. 5 (a), 5 (b), and 5 (c), for |S 11 |, VSWR, AR, and realized gain, respectively. The predicted 2:1-VSWR bandwidth of the antenna is shown in Fig. 5 (a), depicting higher VSWR bandwidth of DRA with metasurface as compared to DRA without metasurface. For DRA without metasurface, a single resonance occurs around 4.82 GHz with AR = 4.7 dB, indicating that the DRA without metasurface is not a CP (AR more than 5 dB) as shown in Fig. 5 (b). However, the DRA with metasurface achieves CP radiation depending upon the number of unit cells (N ). Fig. 5 (b) also demonstrates the impact on the predicted AR range for a different number of unit cells (N ). For N = 3 and N = 5, the multi-resonance response is observed which shifts first resonance towards the lower frequency range at 4.35 GHz (with AR = 0.8 dB and AR = 1.23 dB), while second resonance towards the higher frequency at 5.2GHz (with AR = 6.4 dB and AR = 0.66 dB), respectively. It is noteworthy that N = 5 also yields a third frequency resonance in the higher frequency range at 5.8 GHz (with AR = 2.4 dB), however, the overall trend does not satisfy the minimum AR requirement of <3-dB between 4.5 GHz and 5.1 GHz, resulting in a narrow 3-dB AR bandwidth. Finally, the optimized performance and desired CP performance are achieved by incrementing N = 7 with the two lowest AR values at 0.45 dB and 0.5 dB, discretely. It is also shown to achieve a wide 3-dB AR bandwidth of 19% ranging from 4.45 GHz to 5.39 GHz within the desired frequency range (2-VSWR).
The addition of metasurface on the ground extends the effective aperture of the antenna which leads to gain enhancement. Fig. 5 (c) provides a comparison of the realized gain. After placing DRA over the 7 × 7 metasurface, a gain improvement of 2.5 dB can be observed at the lower frequency band. Moreover, the DRA with metasurface shows a flat gain of 6 -6.5 dBic across the frequency range from 3.8 GHz to 5.05 GHz, with a maximum gain value reaching up to 7.2 dBic at 4.75 GHz. Although a comparable performance is achieved in terms of |S 11 | and gain, the AR response of 7 × 7 metasurface stands out to be the best among these. Thus 7 × 7 metasurface is preferred over 3 × 3 and 5 × 5 metasurface.
To further understand the CP wave's generation phenomenon, the distributions of the electric fields are investigated and presented with cone visualizations. These time-dependent field distributions on the DRA, at 5.0 GHz for ωt = 0 • , 45 • , 90 • , and 180 • , respectively are presented in Fig. 6. The circularly rotated behavior of these fields as observed from the DR with metasurface clearly recommends that the proposed DRA over metasurface undergoes a wideband CP radiation. Thus, it satisfies the requirement of the equal amplitude excitation of two orthogonal modes and a phase shift of 90 • across a wide frequency range. Furthermore, the clockwise rotation of electric fields is visible from Fig. 6, which confirms that the proposed antenna holds the characteristics of a left-hand circularly polarized (LHCP) radiation.

III. FABRICATION OF THE PROPOSED ANTENNA A. PROTOTYPE AND MEASUREMENTS
For practical realization, the proposed DR antenna prototype was fabricated and is shown in Fig. 7 (a), 7 (b) and 7 (c), respectively. The metasurface is printed on top of a grounded FR4 substrate availing a standard etching technique. On the other hand, the rectangular DR is cut out of a TMM10i dielectric slab using the precision cutting facility. The prototype antenna is fed with a standard 50-probe feed connector. The required diameter of the upper probe UP of the feed is achieved by bonding a copper tape, referring to Fig. 7 (a).
Careful consideration is carried out to measure the probe thickness with Vernier caliper, before placing the DR on top of the metasurface, to minimize the fabrication losses. Once placed, the DR is fixed with the help of a thin adhesive tape, which has an insignificant impact on the radiation performance of the antenna. A fabricated prototype was then tested with PNA Vector Network Analyzer and an anechoic chamber to compute the antenna's impedance characteristics and its radiation performance, as demonstrated in Fig. 8 (a) and 8 (b).

B. |S 11 | CHARACTERISTICS
The predicted and measured |S 11 | results of the prototyped antenna is depicted in Fig. 9. These results are measured using an Agilent N5242A Vector Network Analyzer and are shown to be in good agreement with the predicted results. However, a slight shift can be observed from the |S 11 | plot. The second resonance takes its minimum value at 6 GHz against the predicted 5.5 GHz, which can be primarily ascribed to the fabrication tolerances (either bonded copper tape thickness or the hole drilled in the DR for probe feed). Due to smaller changes in physical length, the effective/radiating length changes are more significant on higher frequencies. It could be due to inconsistency of the bit used for the drilling of the feed hole, thus impacting the feed diameter, which leads to the difference. The measured |S 11 | offers a wide impedance bandwidth of 32% ranging from 3.6 GHz to 7 GHz. Similarly, the measured 2:1-VSWR bandwidth ranges 34% from 3.4 GHz to 7 GHz, complementing a wide impedance matching within the operational frequency range.

C. AR, GAIN AND RADIATION PATTERNS
The AR, gain, and radiation patterns of the fabricated metasurface-based DR antenna were measured by using the NSI-700S-50 spherical near-field chamber, available at the Australian Antenna Measurement Facility (AusAMF). These parameters are presented in Fig. 10, and Fig. 11, separately. Fig. 10 shows the antenna's predicted and measured AR.   The AR measurements are carried out by fixing the linearly polarized transmitting antenna and measuring the received signals of the CP DRA with 360 • rotation method. Measured values for the AR are collected across all frequencies to get a frequency versus AR plot. A wide 3-dB AR bandwidth of 20.4% ranging from 4.2 GHz to 5.2 GHz is achieved, with a 0.5 dB increment and a slightly shifted response as compared to the predicted 19% 3-dB AR bandwidth ranging from 4.45 GHz to 5.39 GHz. This shift is similar to the measured |S 11 | plot and it can be attributed either by the fabrication/material tolerances or measurement errors due to the antenna's alignment and rotation in the anechoic chamber. Fig. 10 also provides a comparison of the predicted and measured CP gain at the boresight. It can be observed that measured gain varies between 6-7 dBic across the operating frequency band. The measured gain values remain 0.5-1 dB

D. FAR-FIELD RADIATION PATTERNS
The antenna's 3-D radiation patterns are presented in Fig. 11 at three different frequencies of 4.8 GHz, 5.0 GHz, and 5.4 GHz. The normalized predicted and measured radiation patterns are taken in two principal planes. LHCP radiation patterns are compared at these frequencies as plotted in Fig. 12, respectively. The antenna depicts quite stable VOLUME 10, 2022 radiation patterns in the broadside direction; however, a slight beam squinting has occurred at higher frequencies. Overall, the predicted and measured results show good agreement among them.

IV. CONCLUSION
A low-profile, lightweight and single-fed wideband CP DR antenna over a metasurface has been presented and demonstrated experimentally. It uses a novel hybrid technique to realize wideband CP radiation characteristics. The technique is investigated to exploit the multi-resonance characteristics of a plus-shaped unit cell-based metasurface, with a strategic fed rectangular DR placed over it for performance improvement. The antenna radiation with the introduction of the metasurface is demonstrated to realize a wide bandwidth performance. The fabricated antenna with a compact size of 0.93λ • × 1.3λ • × 0.16λ • exhibits a measured wide impedance bandwidth of 32% (3.6 GHz -7 GHz) and an overlapping 3-dB AR bandwidth of 20.04% (4.2 GHz -5.2 GHz), respectively. The novel yet simple design tends to reduce the complexity of staked layers, and it can be easily extendable for multi-band and high-gain CP array configurations.  He is currently the Distinguished Professor in electromagnetic and antenna engineering with the University of Technology Sydney and a Visiting Professor with Macquarie University, Sydney.