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Circularly Polarized Multiple Layer Dielectric Resonator Antennas

A sequence of anisotropic and isotropic materials of dielectric constant 12 and 10 respectively have been stacked alternatively to form a four-layer stack structure with aperture coupled feed mechanism for excitation. Applying this excitation, orthogonal mode pair TEδ21 and TE y 2δ1 has been excited at frequencies 7.54 GHz and 7.8 GHz, respectively in Y Z and ZX planes to generate circular polarization. A circularly polarized bandwidth in the region (7.54 GHz–7.92 GHz) in conjunction with impedance bandwidth in the region (5.23 GHz–5.52 GHz) with a gain of 5.2 dBi has been accomplished. The designed antenna is appropriate for C-band and weather radar applications. The design assessment has been done using Ansys HFSS. The three stages of antenna design are examined. Further, the design is investigated with a 6-layer structure and an 8-layer structure.


INTRODUCTION
The fields of dielectric resonator antennas (DRAs) have been progressing immensely because of their pluses over conventional antennas such as wider bandwidth, radiation efficiency, and evasion of surface waves. The ever-increasing challenge in the world of wireless communication has led to the design and development of such antennas that can offer multifunctionality with maximum advantages at minimum cost and compact design. Since dielectric resonator antennas have come into the picture, only linearly polarized (LP) DRAs have been explored and operated in depth. In recent times, LP-DRAs are seen to be blended with many losses and multipath interface. These shortcomings of LP-DRA have laid path to the effort in the direction of circularly polarized (CP) DRA. CP-DRA has potential to surmount all the downsides of LP-DRA. It also overpowers the effect on the orientation of transmitting and receiving antennas [1][2][3]. Most of the circularly polarized antenna have dual-feeding mechanism which increases the size and complexity of the circuit. Numerous procedures like generation of higher order modes to generate circular polarisation [4] have been considered in literature to overcome the need of multiple feeds. Practices like altering the shape of DRA are also used to produce circular polarization. Various geometries like trapezoidal-shaped DR [5], zonal slot/DRA [6], stair shape slot DRA [7], and triangular ring shape aperture [8] are considered to accomplish circular polarization. The difficulty in a design adds to the complexity in manufacture process. Stacked antenna structure can be used to enhance gain [9,10]. Circular polarization is also achieved through structures discussed in [11][12][13][14] and cross slot DRA [15]. Generally, DRA is produced from plastic material like polyvinyl chloride (PVC). The antenna created from this stuff is not appropriate for robust and extended-distance communication applications and consequently requests the need of new tough materials to connect the later generation antenna commitments. Hence, sapphire, being stable, can be used [16]. It has a number of benefits such as light transmission characteristics and thermal insulation. As per the Literature assessment very few works have been done employing anisotropic material [17,18].
It is the latent option for upcoming smart phones and mobile communication.
A sequence of anisotropic and isotropic materials of dielectric constant 12 and 10 respectively have been stacked In this document, a design of multiband DRA consisting of anisotropic materials of dielectric constants 12 and 10, respectively, which are stacked alternatively to form a four-layer stack structure, is proposed with aperture coupled feed mechanism for excitation has been presented. The literature indicates that analysis of isotropic DRA has been done intensively as compared to anisotropic material.
The paper further talks about the antenna structure and design in Section 2, supported by an explanation on different designs in Section 3. Further, Sections 4 and 5 examine results along with a parametric study in Section 6. Section 7 presents the conclusion. Figure 1 shows the proposed Rectangular DRA (RDRA). The dielectric resonator structure has been placed on a rectangular FR-4 epoxy substrate having length and width measurements 50 mm × 50 mm along with the thickness of 0.8 mm and dielectric constant of 4.4. Anisotropic material of size 10 mm × 13 mm × 2.5 mm and isotropic material of size 8 mm × 12 mm × 2.5 mm are placed alternatively to form a four-layer layout. The dielectric constants of alternate material are 12 and 10 correspondingly. In addition to this, a patch of size 40 mm × 20 mm with corners trimmed and a cross-slot of dimensions (10 mm × 2.6 mm) cut have been positioned to achieve circular polarization. The method used to excite the antenna structure is aperture coupled technique.

DESIGN PATTERN
Three design configurations are evaluated as shown in Fig. 2. The three are reviewed below. The final design (design 3 as shown in Fig. 2(c)) is endorsed based on improved axial ratio and bandwidth.
Design 1 contains a basic structure that has stacked RDRA, a feedline, and a slot, with specifications of the aperture slot as 1.5 mm × 9 mm and feedline 1.2 mm × 25 mm. This configuration is linearly polarised.
In design 2, all the stacked RDRA parameters are kept constant, and an addition in design is done. Additionally, a patch of dimensions 40 mm × 20 mm has been etched with corner trimmed to provide circular polarisation along with stacked RDRA with feedline and slot. This design change delivers axial ratio less than 3 dB, but less bandwidth is obtained in this design. In design 3, which is the final structure design, the structure has been constructed by an extra addition in design 2. Two cross slots of dimension 10 mm × 2.6 are added to it. The insertion of slots results in reducing the quality factor, boosts the response of antenna, and plus consequently improves the bandwidth of antenna.

RESULTS AND DISCUSSION
Design 1 does not show any circular polarization. It displays twin 10 dB impedance bandwidths in the frequency regions of (5.29 GHz-5.84 GHz) and (6.92 GHz-7.29 GHz). In design 2, orthogonal mode pair TE x δ11 and TE y 1δ1 as shown in Fig. 3 has been excited at frequencies 5.95 GHz and 6.05 GHz correspondingly in Y Z and XZ planes which is responsible for circular polarization. The E and H plane radiation patterns at frequencies 5.95 GHz and 6.05 GHz are displayed in Fig. 4. Axial ratio is often quoted for antennas in which the desired polarization is circular; however, Axial Ratio: < 3 dB is acceptable. Design 2 bestow three regions of operation (5.26 GHz-5.54 GHz), (6.76 GHz-6.94 GHz), and (7.73 GHz-8.02 GHz). For the frequency range (5.95 GHz-6.05 GHz) at Phi = 0 degrees and theta = 150 degrees, the axial ratio is beneath 3 dB for design 2, displaying circular polarisation.  In Fig. 5, S 11 and axial ratio bandwidth of design 2 are exhibited. At frequency 6 GHz, the LHCP and RHCP radiation patterns at both the planes are shown in Fig. 6. The antenna displays LHCP with cross-polarization separation > 18 dB, which is reasonable for workable application.
In design 2, the antenna was examined lacking of slot. The addition of the slot in design 3 lowers the quality factor, boosts the response of the antenna, and consequently improves the bandwidth of design 3 [19][20][21]. The field distribution at various resonating frequencies and generation of orthogonal   mode TE x δ21 and TE y 2δ1 at frequencies 7.54 GHz and 7.8 GHz respectively in Y Z and XZ planes are displayed in Fig. 7. The impedance and axial ratio bandwidth of design 3 are displayed in Fig. 9. In this configuration, for the frequency range (7.54 GHz-7.92 GHz) axial ratio is below 3 dB at Phi = 0 degrees and theta 100 degrees. The E and H plane variations of radiation patterns at frequency 5.36 GHz and 7.6 GHz are exhibited in Fig. 8. At frequencies 7.6 GHz and 7.9 GHz, the LHCP and RHCP radiation patterns are demonstrated in Fig. 10. It shows that the antenna exhibits RHCP with cross-polarization separation > 18 dB, which is satisfactory for practical application. The gains of three designs is demonstrated in Table 1. All the explored outcomes of three designs are also encapsulated in Table 1. A reasonable analysis of results with other suggested designs is presented in Table 2.
Further, the investigations have been done on stacking materials of dielectric constants 12 and 10 respectively to form six-and eight-layered stacked structures as shown in Figs. 11(a)-(b). As the height of structure increases, higher order modes are generated. The generation of higher order mode TE x 1&3 in   Table 3.

MEASURED RESULTS
The prototype of design 3 under measurement is revealed in Fig. 15. The S 11 parameter is assessed using a vector network analyzer while axial ratio is measured using the Star Lab system. A closeness between simulated and measured S 11 is revealed in Fig. 16. The ranges of simulated impedance bandwidths are (5.23 GHz-5.52 GHz) and (7.53 GHz-7.99 GHz), respectively, and the measured impedance bandwidth ranges are (5.27 GHz-5.58 GHz) and (7.29 GHz-7.7 GHz). The simulated and measured axial ratio bandwidth regions of (7.53 GHz-7.99 GHz) and (7.29-7.70 GHz) respectively are displayed in Fig. 17.

PARAMETRIC ANALYSIS
To scrutinize optimized execution, a parametric study has been carried out. During the conduction of parametric analysis, one parameter is differed while others are kept steady. The analysis is carried out on two cross slot dimensions. The cross-slot length is diverse from 9.6 mm to 10.2 mm. The impact of       bandwidth is acquired at 2.6 mm. Further, the assessment has been accomplished for aperture slot length. The aperture slot length is differed from 8.7 mm to 9.3 mm. The impact of modification of length on S 11 and axial ratio is exhibited in Figs. 20(a)-(b). The optimized length is 9 mm. The evaluation has been conducted for aperture slot widths 1.4 mm, 1.5 mm, and 1.6 mm as displayed in Figs. 21(a)-(b). It has been observed that the impedance bandwidth of antenna remains nearly the same in all cases. The parametric analysis on the increasing height of 6 layered stacked geometry shown in Fig. 11(a) has been performed. The height of the uppermost 6th layer is varied from 2 mm to 2.6 mm. Not much variation in operating frequency bands is observed, but S 11 becomes better at height of 2.

CONCLUSION
A layout using novel materials, i.e., anisotropic material and isotropic material stacked alternatively to form a four-layer stack structure with aperture coupled feed process for excitation, has been employed. The orthogonal mode pair TE x δ21 and TE y 2δ1 has been excited at frequencies 7.54 GHz and 7.8 GHz respectively in Y Z and XZ planes to deliver circular polarization. A circularly polarized bandwidth in the region (7.54 GHz-7.92 GHz) in combination with impedance bandwidth in the region (5.23 GHz-5.52 GHz) has been accomplished. A gain of 5.2 dBi has been attained. The proposed antenna is excellent for the C band and weather radar application.