Circularly Polarized Modified Minkowski Metasurface-Based Hybrid Dielectric Resonator Antenna for 5G n79 Wireless Applications

Abstract

This paper presents a circularly polarized hybrid cylindrical dielectric resonator antenna (HCDRA) over a modified Minkowski unit-cell-based metasurface for 5G n79 band (4.4–5 GHz) and IEEE 802.11n WLAN (5 GHz) applications. The location of the perturbed probe feed mechanism and the asymmetric nature of the metasurface are the factors that influence the circularly polarized (CP) radiation within the DR element. The magnitude of E-field distribution and parametric study of the antenna to obtain the optimized feed location are the pieces of evidence of CP radiation. The return loss (RL) and axial ratio (AR) bandwidths produced by the proposed antenna are 1.837 GHz and 750 MHz with a peak gain of 7.04 dBic. The gain obtained is more than 5 dBic across the offered bandwidth of the proposed antenna. The proposed antenna is fabricated and tested in an anechoic chamber for measured results, and these results closely match with the simulation results.

1. Introduction

Bringing advanced technologies and high standards to upcoming wireless solutions, 5G is quickly becoming a reality. The 5G n79 band is a New Radio (NR) band within the Frequency Range 1 (FR1) with frequencies below 6 GHz. The center frequency of the 5G n79 band is 4700 MHz with an operating range of 4400 to 5000 MHz [1,2]. There is a growing demand for wideband, high-gain antennas due to their compact size, light weight, and efficiency. Antenna polarization is also an important performance factor in modern communication systems. Dielectric resonator antennas (DRAs) are widely used because of their excellent features such as wide bandwidth and better radiation. These antennas have low losses and are more efficient than metallic antennas. DRAs come in different shape geometries, including cylindrical, rectangular, spherical, half-split cylindrical, disk, and hemispherical [3].

Circularly polarized antennas play essential roles in RFID, radar, WLAN, and satellite communication as they reduce multipath fading and have better weather adaptability. CP antennas are best for long-distance communications. With the rapid increase in usage and deployment of wireless networks, low wideband circularly polarized antennas are desired. Circularly polarized antennas are better than linearly polarized antennas because of their advantage of low cross-talk and better mobility [4]. Various wideband and multiband hybrid DR antennas are reviewed to identify the techniques used to generate CP [5]. An electromagnetic coupled hybrid DRA with a hexagonal split ring slot is presented in [6] for CP radiation.

For wideband CP radiation, a rectangular DRA coupled with orthogonal slots and excited with a microstrip circular ring has been investigated [7]. Multi-functional CP antennas are essential for satellite communication, GPS, and other wireless systems that require a high degree of polarization purity and immunity to signal fading [8]. CP antennas often reduce multipath interferences and ease alignment between transmitting and receiving antennas. For CP radiation, a low-profile patch antenna inspired by metamaterials is proposed and studied [9]. The 2D metasurface has unique properties, including blocking and absorbing EM waves [10,11]. Nowadays, planar metasurfaces are used for radio frequency and energy harvesting because of their advantages of negative permeability and refractive index [12,13]. Loading metasurfaces (MTSs) is one of the effective strategies for increasing antenna bandwidth [14] and gain. It is possible to generate CP radiation from DR elements by changing the aperture feeding mechanism through an open-ended slot ground [15] and combining a conformal E-shaped patch with a probe feed [16]. A quasi-self-complimentary metasurface for CP radiation [17], a mushroom-like metasurface, and a DR element are combined in the first step to create a miniaturized and broadband response [18], and a rectangular-shaped DRA is designed and constructed using metasurface lens loading for enhanced gain [19]. For dual CP radiation applications, a wideband rectangular DRA with a penetrated probe feed loaded over a rectangular metasurface with a slot has been reported [20]. A similar approach is reported with a different perturbed probe feed at the edge of the rectangular DRA loaded on a plus-shaped unit-cell-based metasurface for wideband CP radiation applications [21].

This paper proposes a hybrid cylindrical DRA over a modified Minkowski fractal−shaped metasurface for broad CP radiation and provides a consistent gain of more than 5 dBic across the offered bandwidth. The proposed hybrid structure consists of a cylindrical dielectric resonator excited by a perturbed coaxial probe feeding mechanism and a metasurface designed to provide circularly polarized wideband radiation. A cylindrical DRA is placed over a modified Minkowski fractal−shaped metasurface, which consists of unequal rectangular−shaped cells with different indentation widths and lengths. The design of the metasurface is crucial for achieving a wide operating CP bandwidth with low loss and steady gain across the bandwidth. The antenna’s performance is typically simulated using the electromagnetic simulation software Ansys-HFSS 18.0.

2. Hybrid Cylindrical DRA over Modified Minkowski-Shaped Metasurface

The proposed wideband circularly polarized DRA configuration consists of a cylindrical DR of height (D_h) 10.16 mm and diameter (Dr) 15 mm. It is designed with a Rogers RT/Duroid 6010 (${ϵ}_r$ = 10.2, tanδ = 0.001) and a dielectric substrate of Rogers RT/Duroid 5880 (S_h = 3.2 mm, ${ϵ}_r$ = 2.2, tanδ = 0.001). A metasurface with a 7 × 7 unit cell array is designed with a ground plane size of 54 mm × 75 mm. DRA is placed over the metasurface, which consists of unsymmetrical rectangular Minkowski-shaped unit cells with a size of X × Y = 8.5 × 6 mm². The distance between the unit cells in the ‘x’ direction is 1.4 mm, and the distance between unit cells in the ‘y’ direction is 1.6 mm in the 7 × 7 unit−cells configuration-based metasurface. This non−uniform arrangement of the rectangular Minkowski-shaped unit cells is responsible for CP radiation, unlike the case of a DR antenna without a metasurface. A coaxial feed location from the center of the cylindrical DR along the diagonal line direction, i.e., (−Dr/2,−Dr/2), is optimized for impedance matching to offer a wideband circularly polarized radiation. This HDRA−1 over a metasurface uses a dielectric resonator as the main radiating element and a metasurface as a reflector to enhance the antenna performance. The dielectric resonator antenna provides a wideband operation; the metasurface, feed location, and the diameter of the DR element are responsible for CP radiation. The dielectric resonator antenna and metasurface combination in Figure 1 lead to a compact and efficient antenna design, which is attractive for various data−link and wireless LAN applications.

Researchers have used rectangular and square-shaped metasurfaces in the past, but their narrow band response limited their use. The possibility of a broadband reaction makes a Minkowski−shaped metasurface an ideal candidate here. It is recommended that a unit cell have a reflection phase response (±90°) surpassing the frequency range required by the metasurface design. Figure 2 demonstrates the investigation of the reflection phase analysis of the proposed metasurface. The reflection phase at zero degrees is located at 4.85 GHz, with the reflection-phase bandwidth (±90°) offered from 4.2 to 5.55 GHz, indicating the wideband response of the proposed modified Minkowski metasurface. Metasurface unit cells are spaced by 1.4 mm and 1.6 mm in the normal and azimuth planes.

A cylindrical DR is optimized for broadband CP radiation and uses a coaxial feed located along the diagonal of the substrate. The distributions of the electric fields are examined and visualized to gain a better understanding of the CP wave generation phenomenon. The magnitude of the E-field in Figure 3 illustrates four time-dependent field distributions (θ = 0°, 45°, 90°, and 135°) for the DRA at 5.0 GHz. From the DR with a metasurface, we can observe the circular rotation of these fields, which suggests a CP radiation for the proposed DRA. This attains the amplitude and phase shift requirements for orthogonal modes of CP radiation. In addition, the clockwise rotation of the electric fields is evident in Figure 3, which indicates it is a left-hand circularly polarized antenna. To avoid any deterioration of the results due to contact between the coaxial feed and unit cells, the metallic part around the feed location is removed with a radius of 2.5 mm.

3. Parametric Analysis and Simulation Results

The performance of the proposed antenna is evaluated through parametric analysis by taking various antenna configurations, using a penetrated probe−fed DRA without a metasurface and later introducing 3 × 3, 5 × 5, and finally, 7 × 7 unit−cell arrays of metasurface structures, as shown in Figure 4. The corresponding S₁₁ plots of Figure 4 are plotted together in Figure 5 for various N × N metasurface configurations. It is observed from the S₁₁ plot with and without metasurface configurations in Figure 5 that the return loss obtained at 6.3 GHz for a DRA without a metasurface is −35 dB, and the bandwidth is 1.05 GHz. Adding a metasurface shifts the higher frequency band to the lower frequency band, i.e., at 5 GHz. The return loss obtained with a metasurface configuration at 5 GHz is −35 dB, and the bandwidth is 1.5 GHz, which gives the enhanced fractional bandwidth of 450 MHz and also results in the size reduction in the antenna as the resonance frequency is shifted from higher to lower by utilizing the property of the metasurface. It can also be observed from Figure 6 that the peak gain of 3.94 dBi is obtained without a metasurface configuration, and it is less when compared to the peak gain of 6.92 dBi with a metasurface configuration. It is also observed that as the array of unit cells increased, the consistency in gain across the offered bandwidth also increased. It is shown in Figure 7 that the diameter (Dr) of the DR is varied to optimize the resonance bandwidth at 5 GHz with circular polarization (which is also correlated with Figure 3). Based on a parametric and optimetric study of an HFSS EM solver, the RL bandwidth obtained for Dr = 15 mm and ph = 5 mm is 1.5 GHz, which is higher than the bandwidth for the other Dr and ph values shown in Figure 7 and Figure 8.

4. Prototype, Measurements, Discussion and Assessment of Results

The prototype of the proposed design before and after alignment with necessary stacking and bonding is shown in Figure 9a. The measurement of S₁₁ using VNA (Vector Network Analyzer) and other radiation parameters (gain, AR, and radiation patterns) are took place in an anechoic chamber, as shown in Figure 9b,c. The deviations in measured results compared to the simulation results are due to the manual soldering of the connector to the ground and the bonding losses of the DR layers with the substrate. The comparisons of the measured and simulated S₁₁ and AR plots are presented in Figure 10 and Figure 11, respectively. The measured broad bandwidth of 1.837 GHz (4.46–6.297 GHz) and a wide 3 dB axial ratio bandwidth of 750 MHz (4.72−5.49 GHz) are achieved with the proposed Minkowski−shaped metasurface and an optimized feed location for the DR configuration.

Figure 12 provides the gain versus frequency plot of the proposed HCDRA, where the measured gain obtained is 7.04 dBic across the offered frequency range from 4.46 to 6.297 GHz. The patterns in the XZ and YZ planes are similar and in close agreement with the simulated results, as shown in Figure 13, Figure 14 and Figure 15. The change in patterns with respect to the frequency has been given in Figure 13, Figure 14 and Figure 15, which includes 3D radiation patterns, and hence it is observed from the patterns that the gains are consistent across the bandwidth obtained. The lower part of the measured radiation deteriorates due to the substrate’s two-layer bonding (with NITTO tape) and soldering losses. The simulation and measured results are summarized in

Table 1. Summary of simulation and measured results of proposed HCDRA.

Parameter	Simulated Results	Measured Results
Operating frequency band	4.24–5.7525 GHz	4.46–6.297 GHz
RL bandwidth	1.5125 GHz	1.837 GHz
AR bandwidth	0.56 GHz	0.75 GHz
Gain	6.92 dBi	7.04 dBi

.

The performance of the proposed HCDRA is compared with the existing literature, which is given in

Table 2. Performance comparison with existing literature.

[Ref.]	Feed Mechanism and Shape of the DRA	CP is Achieved by	Volume (λ03) at Center Frequency	RL Bandwidth (GHz)	Gain (dBic)	Polarization Conversion Due to Metasurface
[15]	Parasitic aperture coupled feed and rectangular DR	Combination of stair-shaped DR and open-ended slot on the ground plane with an offset feed	0.46 × 0.46 × 0.07	3.844–8.146	3.9	No
[16]	A combination of conformal metal strip and probe feed and rectangular DR	Employment of parasitic patch at an optimized distance beside the conformal metal strip of the two identical rectangular DRAs to generate CP	0.46 × 0.46 × 0.34	3.50–4.95	6.2	No
[17]	Penetrated probe feed and cylindrical DR	Quasi-self-complementary characteristic of the metasurface	6.08 × 4 × 0.06	24.65–26.06	6.03	Yes
[20]	Penetrated probe feed and rectangular DR	Due to the metasurface and stacking of two types DR elements	0.93 × 1.29 × 0.16	4.2 to 5.4 and 7.62 to 8 (dual bands)	8.4	Yes
[21]	Perturbed probe feed and rectangular DR	Due to plus-shaped unit-cells-based metasurface and rectangular DR	0.93 × 1.29 × 0.16	3.6–6.6 GHz	6–7.2	Yes
Proposed Work	Disturbed or perturbed probe feed and cylindrical DR	Variation in feed location of DRA and asymmetric nature modified Minkowski-shaped metasurface	0.92 × 1.25 × 0.22	4.46–6.297 GHz	7.04	Yes

. The methodology used to generate CP in [15,16] is due to the feed and the feed mechanism combination, i.e., the DR elements’ placement. The quasi-self-complimentary nature of the metasurface is accountable for CP radiation [17], and the slotted rectangular array of unit cells given in [20] is responsible for splitting the CP radiation into dual CP bands, respectively. The operating frequencies are within the range of millimeter wave (5G-NR-FR2) and sub 6 GHz (5G-NR-FR1) applications. The type and size of the substrate, metasurface, size, and shape of the proposed DRA differ from the work reported in [21]. The asymmetric nature and location of the probe feed (indirectly, it relates to the diameter of the DR element) on the metasurface are accountable for CP radiation. The gain is steady across the bandwidth of the proposed work compared to the other reported works presented in Table 2.

5. Conclusions

This paper proposes a cylindrical DRA over a modified Minkowski-shaped metasurface for broadband CP radiation at 5 GHz wireless applications. The evolution and configuration of the proposed antenna are designed based on parametric analysis, and the equivalent results are presented. With the metasurface, the design configurations shift its higher resonance to the lower resonance, showing the size reduction in terms of its overall size. The wide return-loss bandwidth of 1.837 GHz and axial ratio bandwidth of 750 MHz with a peak gain of 7.04 dBi are obtained. The proposed design exhibits CP radiation around 5 GHz, which is suitable for 5G n79 NR band (FR1), data-link, and IEEE 802.11n WLAN applications.