A New Wideband Circularly Polarized Dielectric Resonator Antenna Loaded with Strips

This paper proposes a novel wideband circularly polarized (CP) dielectric resonator antenna (DRA) loaded with strips. The CP DRA comprises a circular DR, two pairs of driven L-like DR strips with diﬀerent lengths, and a square ground-plane. To couple the electromagnetic energy to circular DR, an orthogonal cross-slot is used, and a stepped microstrip-line is also used to adjust the impedance matching. Inﬂuenced by the concept of deformed DR, two pairs of L-like DR strips of diﬀerent lengths are inserted into the circular DR as part of the driven element to excite a new axial ratio (AR) resonant point. An antenna prototype is simulated, manufactured, and measured to validate the unique design. The measured results show that the designed antenna has broadband characteristic with a − 10dB IBW of 54% (1.91–3.32 GHz) and 3dB ARBW of 42.1% (2.10–3.22GHz).


Introduction
Recently, DRAs [1, 2] have been intensively studied due to the attractive features of high efficiency, low cost, and wideband operation. e basic characteristics of DRAs in higher-order mode, CP dual-function and transparent designs, and a glass decoration DRA are described in [1] for the first time. In [2], a novel high-gain mushroom-shaped DRA was designed to obtain −10 dB impedance bandwidth (IBW) of 65%, which comprises a hollow cylindrical resonator and a top-mount spherical-cap lens used to increase the gain. Compared with linearly polarized DRAs, CP DRAs [3,4] with the advantages of mitigating multipath fading and polarization mismatch have become increasingly common in wireless communication systems. However, the bandwidth of CP DRAs is typically insufficient to satisfy the needs of broadband communication.
e CP DRAs are summarized as multiple-feeding and single-feeding structures. Although the multiple-feeding DRAs can easily achieve a wideband CP compared with the single-feeding DRAs, it requires additional hybrid-couplers or power-dividers [5][6][7][8][9], which inevitably increases the antenna dimension and structure complexity. erefore, studying broadband CP DRAs with single-feeding characteristics is a great challenge and potentiality recently.
Influenced by the concept of deformed DR, in this study, two pairs of driven L-like DR strips of different lengths are introduced into a circular DR as part of the driven element for enhancing the CPBW. An orthogonal cross-slot is used to couple the electromagnetic energy to circular DR, and a stepped microstrip-line is used to adjust the impedance matching. First, the circular DR is used to achieve broad CP operation through a cross-slot coupled method. Second, two pairs of driven L-like DR strips of different lengths are placed next to the circular DR, which can jointly stimulate a highfrequency additional AR resonant. ird, by optimizing and using these structures, the proposed DRA's IBW and ARBW are widened. Finally, to demonstrate the wideband CP design concept, an optimized antenna prototype is simulated, fabricated, and measured. e measured results show that the designed CP DRA exhibits a wide characteristic of 54%− (1.91-3.32 GHz−) 10 dB IBW, and 42.1% (2.10-3.22 GHz) 3 dB ARBW, which has potential application values in wireless local area network (WLAN, 2.4-2.48 GHz) and wireless broadband access service (WiBro, 2.3-2.39 GHz) communication systems. e corresponding content is summarized in four sections to explain the operated mechanism. Section 1 shows the introduction; Section 2 is the description of the antenna structure and the principle analysis of designed process; Section 3 compares measured and simulated results, and Section 4 concludes the feature of the proposed antenna. Figure 1 shows the structure and dimensions of the designed CP DRA. As shown, the antenna consists of a circular DR (R1), two pairs of driven L-like DR strips (R2 × W5, R3 × W6), a square ground-plane (L × W), and a stepped microstrip-line (L1 × W1, L2 × W2). e circular DR and DR strips with the same height (H2 � 19 mm) and dielectric constant (εr 2 � 10.2) are placed on top of an FR4-substrate with permittivity (εr 1 � 4.4) and height (H1 � 0.8 mm). e stepped microstrip-line is placed on the bottom surface of the FR4substrate to improve impedance matching. Additionally, an orthogonal cross-slot of different sizes (L3 × W3, L4 × W4) is etched into the square ground to provide the original CP mode. Because of the introduction of the L-like DRs, the CPBW can be expanded, and a new AR passband in high frequency is excited. Table 1 shows the detailed parameter values optimized by using an electromagnetic simulator (ANSYS HFSS). Figure 2 shows four step-by-step antenna's prototypes used to elaborate the broadband CP operating principle. Ant. 1 is a typical circular DR with crossslot coupled feeding; Ant. 2 inserts a pair of driven L-like DR strips with an arc of θ3; Ant. 3 introduces a pair of driven L-like DR strips with an arc of θ2. By integrating these driven DR strips, the CP DRA is designed. e corresponding variations of IBW and ARBW with four evolutionary prototypes are plotted in Figure 3. Two CP resonant modes of Ant. 1 are excited at 2.20 and 2.86 GHz, respectively. Generally, when an orthogonal cross-slot is used as a coupled feeding structure and is placed in a circular or square DR, two pairs of orthogonal modes can be excited, as discussed and analyzed in [26]. e low-frequency AR point (2.20 GHz) comes from the interaction between TE 111 (x1) and TE 111 (y1) modes of DRA, whereas the high-frequency AR point (2.86 GHz) is excited by the x'-polarized mode of short slot and the TE 111 (y1) mode of DRA, as shown in Figure 3(b). Without adding any DR strips, the optimal bandwidth of Ant. 1 is too narrow to meet the communication requirements. Two pairs of driven L-like DR strips with different arcs of θ2 and θ3 are introduced into Ant. 2 and Ant. 3 to increase the CPBW. As shown, these driven L-like DR strips can not only improve IBW but also reduce the AR values. It is noticed that the L-like DR strips with an arc of θ2 have a significant impact on the CPBW than L-like DR strips with an arc of θ3. When two pairs of driven L-like DR strips are integrated and placed next to the circular DR, an AR resonant point at 3.16 GHz can be stimulated. Furthermore, the ARBW shifts toward higher frequencies and the IBW and ARBW in lower frequencies could also be improved. After simulating and optimizing, the presented antenna exhibits a −10 dB IBW of 49.6% (2.00-3.32 GHz) and 3 dB ARBW of 43.6% (2.08-3.24 GHz).

Electric Field Distribution.
e vector electric filed distributions on the upper surface of circular DR and L-like DR strips are simulated at 2.18, 2.72, and 3.16 GHz to clarify the DRA's CP operating principle, as shown in Figure 4. All the simulated results are produced by commercial software ANSYS Electronics Desktop. e input power is 1 W, and a 50 Ω lumped port is also used to excite the proposed CP DRA. As shown in Figures 4(a) and 4(b), the electric field mainly concentrates on the circular DR at 2.18 and 2.72 GHz. e electric filed direction of the circular DR and L-like DR strips is the same, and the vector electric fields at phase � 0°and 90°are orthogonal. However, at 3.16 GHz, the electric field of L-like DR strips and the partial electric field of the circular DR are reverse and cancel each other out. In addition, the vector electric field at phase � 0°and 90°is also orthogonal at 3.16 GHz. With the different phases (0°and 90°), the direction of the vector electric field rotates clockwise at three frequency points, which means the proposed CP DRA can radiate left-hand CP waves. Finally, the designed DRA's CPBW can be significantly enhanced by combining these CP resonant points.

Antenna Parameter Analysis.
To study the influence of antenna dimensions on CPBW, some parameters are scanned and analyzed in this section. e arcs of L-like DR strips (θ2 and θ3), the width of the stepped microstrip-line (W1), and the width of the orthogonal cross-slot (W3) are analyzed using simulated soft ANSYS HFSS. e corresponding 3 dB ARBW and −10 dB IBW with different sizes are shown in Figure 5. Note that the IBW varies significantly with different W3 and W1 values across the whole operating bandwidth, whereas the AR values barely vary. is means that the two parameters are crucial in controlling the proposed antenna's impedance matching. However, compared with W1 and W3, the AR values change significantly with different arcs of θ2 and θ3. Furthermore, the |S 11 | values vary in the whole frequency band. erefore, the L-like DR strips also participate in the excitation of CP mode. e best |S 11 | and AR values were obtained when W1 � 1.8 mm, W3 � 2.6 mm, θ2 � 150°, and θ3 � 60°. Additionally, Table 2 compares the previously published and presented CP DRAs, revealing that the designed antenna with modified DR has an excellent CPBW. In addition, although the [26][27][28] have more compact and higher CPBW than the proposed antenna, the proposed CP DRA design is unique, and their bandwidth enhancement methods are completely different. e bandwidths of [26][27][28] are increased by introducing parasitic elements, whereas the proposed antenna is enhanced by loading modified DR.

Experimental Results
In this section, an optimal antenna's prototype is manufactured and measured to verify the design accuracy. e −10 dB reflection coefficients (|S 11 |) are tested using vector network analyser (Agilent N5224A), and an anechoic chamber is used to measure the gains, ARs, and radiation patterns. To show a clear comparison, the correspondingly tested and simulated results are shown in Figures 6-8. As shown, the measured and simulated −10 dB IBW is 54% (1.91-3.32 GHz) and 49.6% (2.00-3.32 GHz), whereas the 3 dB ARBW is 42.1% (2.10-3.22 GHz) and 43.6% (2.08-3.24 GHz), separately. Note that the measured −10 dB IBW completely covers the whole 3 dB ARBW, implying that usable overlapping bandwidth is more than 42%. Furthermore, within the CPBW, the tested and simulated peak gains are approximately 5.92 and 6.18 dBic, respectively. e simulated radiation efficiency is greater than 95%. A notch appears at 2.4 GHz in the |S11| and gain curves, which could be caused by the interaction between cross-slot and microstrip-line. Furthermore, Figure 7 includes measured photograph of the designed antenna. e normalized radiation patterns of the prototype in two orthogonal planes (phi � 0°and 90°) with different frequencies (2.18, 2.72, and 3.16 GHz) are plotted in Figure 8. As depicted, left-hand CP fields are stronger than the right-hand CP parts by more than 25 dB, which means the LHCP waves can be radiated at three frequency points in +z-direction. At certain      International Journal of Antennas and Propagation  International Journal of Antennas and Propagation frequencies, the front-to-back ratios of the proposed antenna are below −20 dB, indicating that the antenna produces a higher front-to-back ratio. ere is a slight mismatch between the simulated and measured results because of the imperfect fabricated and tested environment. Finally, the rationality of the prototype can be verified by combining these simulated and tested results.

Conclusion
is paper proposes a new wideband CP DRA loaded with strips. e DRA comprises a circular DR, two pairs of driven L-like DR strips of different lengths, and a square groundplane. An orthogonal cross-slot is used to couple the electromagnetic energy to the circular DR, and a stepped microstrip-line is used to improve IBW. Influenced by the concept of deformed DR, two pairs of L-like DR strips of different lengths are inserted into the circular DR as part of the driven element to excite a new AR resonant point. To verify the design, a prototype of the designed DRA is fabricated and measured. e measured results show that the CP DRA has a wider 3 dB ARBW of 42.1% (2.10-3.22 GHz) and a broader −10 dB IBW of 54% (1.91-3.32 GHz), indicating that the CP DRA has potential applications for wireless local area network (WLAN, 2.4-2.48 GHz), wireless broadband access service (WiBro, 2.3-2.39 GHz), and world interoperability for microwave access (WiMAX, 2.5-2.7 GHz) communication systems.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.    International Journal of Antennas and Propagation