IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 70, NO. 7, JULY 2022 5971 Communication 3-D-Printed Wideband Circularly Polarized Dielectric Resonator Antenna With Two Printing Materials Zhen-Xing Xia and Kwok Wa Leung Abstract — A new 3-D-printed wideband circularly polarized (CP) dielectric resonator antenna (DRA) with two printing materials is investigated. The DRA has a multilayer comb-shaped structure at its top for generating CP fields. Three dielectric strips are embedded inside the DRA body to support multiple transverse-electric (TE) DRA modes to widen the DRA bandwidth. To slightly improve the axial-ratio (AR) performance, the DRA is twisted in the horizontal plane. For demon- stration, a prototype operating in C-band was designed and printed with two different materials. The reflection coefficient, AR, radiation pattern, antenna gain, and efficiency of the antenna are measured, and reasonable agreement between the measured and simulated results is observed. The prototype has a measured 10 dB impedance bandwidth of 69.7% (4.80–9.94 GHz) and 3 dB AR bandwidth of 68.6% (4.52–9.24 GHz), achieving a wide overlapping bandwidth of 63.2% (4.80–9.24 GHz). Both the AR and overlapping bandwidths are record-high for a single-fed CP DRA. The prototype has a measured peak antenna gain of 8.3 dBic inside the overlapped passband. Index Terms — 3-D printing, circular polarization, dielectric resonator antenna (DRA), wideband antenna. I. I NTRODUCTION Circularly polarized (CP) antennas have been extensively employed in wireless communication systems because they allow more flexible orientations between the receiver and the transmitter. Also, they can alleviate the multipath problem caused by reflections from the ground surface and building walls [1]. The dielectric resonator (DR) antenna (DRA) has a number of advantages, such as compact size, ease of excitation, different radiation patterns, and avoidance of conductor loss [2]–[5]. Thus far, numerous CP DRAs have been reported (e.g., [6]–[12]). Basically, these CP antennas can be categorized into the single-fed [6]–[8] and dual-/multifed CP types [9]–[12]. The former is simple but generally suffers from a narrower 3 dB axial-ratio (AR) bandwidth of only 6% or less, whereas the latter has a wider 3 dB AR bandwidth of 20% or more at the cost of having a more complex feed network and larger overall antenna size. To obtain a compact CP DRA, the single-fed type is considered in this communication, but some design techniques are introduced to substantially increase its AR bandwidth. Different irregular geometries have been introduced to broaden the AR bandwidth of a single-fed CP DRA. They include the Manuscript received 2 August 2021; revised 15 February 2022; accepted 14 March 2022. Date of publication 1 April 2022; date of current version 26 July 2022. This work was supported in part by the General Research Fund (GRF) Grant from the Research Grants Council of Hong Kong, SAR, China, under Project CityU 11217018; and in part by the Fundamental Research Program of Shenzhen City, under Grant JCYJ20170818094814530. (Corresponding author: Kwok Wa Leung.) The authors are with the State Key Laboratory of Terahertz and Mil- limeter Waves, Department of Electrical Engineering, City University of Hong Kong, Hong Kong, SAR, China, and also with the Shenzhen Key Laboratory of Millimeter Wave and Wideband Wireless Communica- tions, CityU Shenzhen Research Institute, Shenzhen 518057, China (e-mail: zxxia2-c@my.cityu.edu.hk; eekleung@cityu.edu.hk). Color versions of one or more figures in this communication are available at https://doi.org/10.1109/TAP.2022.3161434. Digital Object Identifier 10.1109/TAP.2022.3161434 stair-shaped [13], [14], notched trapezoidal [15], semieccentric annu- lar [16], fractal-shaped [17], and bowtie-shaped [18] structures. Also, the rotated-stacked approach [19], [20] has been shown to be effective in widening the AR bandwidth. The AR bandwidth can be further enhanced by using a multisegment DRA [21], [22]. For instance, a design that combines two half-split cylindrical DRs and a rectan- gular DR can provide a wide AR bandwidth of 41%, but its boresight antenna gain is reduced to 1.5–2.1 dBi [21]. Recently, a single-fed multilayered CP DRA with an AR bandwidth of 22.8% has been reported [23]. It composes of three dielectric layers. The middle rectangular low- ε r layer is sandwiched by two high- ε r ceramic films, where ε r denotes the dielectric constant. In assembling this multilayered DRA, it is easy to introduce air gaps between two adjacent dielectric layers, shifting the resonant frequency away from the desired value. All CP DRAs, as mentioned above, are fabricated using traditional mechanical approaches. As compared with conventional fabrication methods, the 3-D printing method has the merits of allowing a more flexible design, requiring a shorter lead time, being more eco-friendly, and incurring a lower development cost [24]–[27]. Since the 3-D printing technology was proposed in the late 1980s [28], several 3-D printing tech- niques, such as fused deposition modeling (FDM), stereolithography apparatus, polymer jetting, and selective laser melting, have been developed. Various 3-D-printed passive components operating at fre- quencies from gigahertz to terahertz have been reported, such as lens antennas [29]–[31], reflectarrays [32], [33], horn antennas [34], [35], waveguides [36], [37], and electromagnetic bandgap structures [38]. However, only limited efforts have been made on the 3-D-printed DRA thus far [39], [40]. In this communication, a new 3-D-printed single-fed inhomoge- neous CP DRA with a record-high 3 dB AR bandwidth of 68.6% is studied for the first time. The DRA comprises a twisted DR and interlaced dielectric slabs of different dielectric constants at its top. The twisted DR is embedded with three dielectric strips with a lower dielectric constant, supporting multiple broadside radiating modes with their resonant frequencies close to one another. The interlaced dielectric slabs at the top have a decreasing height from the center to adjust the magnitude ratio and phase difference of the two degenerate modes. The DRA is rotated by 45 ◦ with respect to the excitation slot to facilitate the excitation of the two degenerate modes. A prototype that operates in C-band was designed and fabricated using the FDM multimaterial 3-D printing technology. The reflection coefficient, AR, radiation pattern, antenna gain, and efficiency of the antenna are measured. Reasonable agreement between the measured and simulated results is obtained. II. A NTENNA D ESIGN A. Original Configuration Fig. 1 shows the original antenna design. The DRA is mounted on a rectangular substrate with a width of W g , a length of L g , and a 0018-926X © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: University of Sydney. Downloaded on August 11,2023 at 08:14:26 UTC from IEEE Xplore. Restrictions apply. 5972 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 70, NO. 7, JULY 2022 Fig. 1. Configuration of the original design. (a) Perspective view. (b) Side view of DRs. (c) Top view of substrate showing the feed network. (d) Top view (embedded dielectric strips are not shown for clarity). thickness of t . The substrate has a dielectric constant of 3.55 and a loss tangent of 0.0027. With reference to Fig. 1(a), the DRA consists of the upper and lower dielectric blocks. The upper block has interlacing dielectric layers with two different dielectric constants of ε r 1 and ε r 2 ( ε r 1 > ε r 2 ) , with thicknesses of t 1 and t 2 , respectively. Their heights vary linearly; the middle layer has the highest height of h 1 , whereas the ones at the two edges have the lowest height of h 2 . This upper block is a variant of the comb structure of a CP DRA design [7]. The comb structure can be interpreted as a polarizer that converts linearly polarized (LP) fields into CP fields. The lower part Fig. 2. Twisted configuration (embedded dielectric strips are not shown for clarity). Fig. 3. Configurations of reference DRAs. is a dielectric block with a higher dielectric constant of ε r 1 . It has three embedded dielectric strips with the lower dielectric constant of ε r 2 . Its design parameters are given in Fig. 1(b). A rectangular slot with two open stubs is etched in the ground plane to excite the DRA, as shown in Fig. 1(c). The inset shows a small conductive rectangular ring that feeds the excitation slot. To help obtain the orthogonal degenerate modes for generating CP fields, the DRA is placed at 45 ◦ with respect to the excitation slot, as shown in Fig. 1(d). B. Improved Configuration It was found that the AR bandwidth can be slightly improved by horizontally twisting the DRA through an angle of α , with the bottom face remaining at the original position, that is, making 45 ◦ with the excitation slot. Fig. 2 shows the twisted configuration. This final design is used in the following parts of this communication. Table I shows the optimized values of the antenna parameters. C. Working Principle To understand the operating principle of the CP DRA, four reference DRAs (Ant. I–IV) were simulated and compared with our final design. With reference to Fig. 3, Ant. I is the basic design with a homogenous trapezoidal DR, from which Ant. II is obtained by embedding three dielectric strips into the DR body. Ant. III is obtained by loading the top of Ant. II with a set of height-varying dielectric slabs having a higher dielectric constant of 10. Ant. IV is obtained from Ant. III by loading the second height-varying dielectric slabs with a lower dielectric constant of 3. Twisting Ant. IV gives our current DRA (final design), as shown in Fig. 2. Fig. 4 shows the simulated reflection coefficients and ARs of the reference and current DRAs. With reference to the figure, Ant. I has multiple resonant modes that are far away from each other, leading to a narrow operating bandwidth. After inserting three dielectric strips (obtaining Ant. II), the resonant frequencies of Ant. I shift upward because Authorized licensed use limited to: University of Sydney. Downloaded on August 11,2023 at 08:14:26 UTC from IEEE Xplore. Restrictions apply. IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 70, NO. 7, JULY 2022 5973 Fig. 4. Simulated reflection coefficients and ARs of reference and current DRAs. the inserted dielectric strips have a lower dielectric constant of 3, decreasing the effective dielectric constant of the DRA. As a result, Ant. II has a lower quality factor (Q-factor), thus giving a wider impedance bandwidth. Also, as can be observed from Fig. 4, the embedded dielectric strips can improve the AR performance in the lower frequency band. This is because the embedded dielectric strips act as a multistage linearly polarized (LP)-to-CP converter [22] that alters the phase velocities of the two orthogonal E -field components ( E u and E v ) to different extents, helping them to have an equal magnitude and a quadrature phase difference. By loading the top of Ant. II with the dielectric slabs (obtaining Ant. III), several adjacent resonant modes in the upper frequency band can be merged to widen the impedance bandwidth, as shown in Fig. 4. Also, Ant. III has better AR performance in the upper frequency band. This can be expected because the loaded dielectric slabs with low heights serve as a high-frequency polarizer that converts LP fields into CP fields. With reference to Fig. 4, by further loading the Ant. III with the second dielectric slabs having a low dielectric constant (obtaining Ant. IV), multiple resonant modes of the Ant. IV are merged because the overall antenna Q-factor is reduced. Additionally, these low- ε r dielectric slabs help to fine-tune the magnitude ratio and phase difference of the two orthogonal E -field components ( E u and E v ) , thus giving a wide 3 dB AR bandwidth. It is noted that directly loading the top of Ant. I with interlaced dielectric slabs can obtain a wide 10 dB impedance bandwidth but narrow 3 dB AR bandwidth, thus giving a very limited usable CP bandwidth. It is worth mentioning that by twisting Ant. IV (obtaining current design), the antenna AR performance can be slightly improved, as shown in Fig. 4. With reference to the figure, the current DRA has multiple resonant modes that merge together, giving a wide 10 dB impedance bandwidth of 66.5% (4.87–9.73 GHz). Five AR minima at 4.6, 6.3, 7.5, 8.5, and 9.1 GHz can be observed in the AR passband, leading to a very wide 3 dB AR bandwidth of 73.1% (4.34–9.34 GHz). The overlapping impedance and AR bandwidths are 62.9% (4.87–9.34 GHz), which is the operating bandwidth of the CP antenna. The various CP modes of our current design are studied. Fig. 5 shows the simulated E -field distributions at orthogonal phases for each CP mode. For ease of discussion, a reference xy -plane at the middle height of the DRA is used, as shown in Fig. 5(a). With reference to Fig. 5(b)–(f), for each CP mode, the xy -plane E t -field (vector sum of the major E -field distribution) rotates in a clockwise direction at orthogonal phases ( ω t = 0 ◦ and 90 ◦ ) as the wave propagates in the z -direction, showing that it is a left-hand circular polarization. Conversely, a right-hand circular polarization can be obtained by clockwise rotating the DR by 90 ◦ . With reference to Fig. 5. Simulated E -field distributions of the CP DRA at orthogonal phases for each CP mode ( E t represents the vector sum of the major E -field distribution). (a) Reference coordinate system. (b) 4.6 GHz. (c) 6.3 GHz. (d) 7.5 GHz. (e) 8.5 GHz. (f) 9.1 GHz. Fig. 5(b), at the first CP mode (4.6 GHz), the pattern of the x z -plane E -field is similar to that of the TE y 1 δ 1 mode of a conventional rectangular DRA [5], verifying that the DRA is excited in the modified TE y 1 δ 1 mode. In addition, the y z -plane E -field distribution shown in Fig. 5(b) indicates that the DRA is also excited in the TE x δ 11 mode. Hence, two degenerate fundamental modes of TE y 1 δ 1 and TE x δ 11 are simultaneously excited and contribute to the first CP mode. Similarly, it can be inferred that the second CP mode at 6.3 GHz is caused by the degenerate high-order TE y 1 δ 3 and TE x δ 13 modes, as shown in Fig. 5(c). As can be observed from Fig. 5(d)–(f), the DRA cannot produce degenerate modes at the frequencies of the third, fourth, and fifth CP modes. It was found that these three CP Authorized licensed use limited to: University of Sydney. Downloaded on August 11,2023 at 08:14:26 UTC from IEEE Xplore. Restrictions apply. 5974 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 70, NO. 7, JULY 2022 Fig. 6. Simulated reflection coefficient and AR of the CP DRA as a function of frequency for different embedded strip numbers of N = 2, 3, and 4. Fig. 7. Simulated reflection coefficient and AR of the CP DRA as a function of frequency for different lower DR dielectric constants of ε r 1 = 8, 10, and 12. modes are mainly due to the combined effects of high-order DR modes and coupling slot modes. Note that the DRA is excited in the hybrid mode of TE y 1 δ 3 and TE y 3 δ 3 in the x z -plane at 7.5 GHz, as shown in Fig. 5(d). It is worth mentioning that the CP modes in the upper band can be fine-tuned by changing the height of the loaded interlaced dielectric slabs. D. Parametric Study To characterize our CP DRA, a parametric study was carried out using ANSYS HFSS [41]. In the parametric study, only one parameter was varied at one time, with the values of other parameters given in Table I. To begin with, the effect of the number N of the embedded dielectric strips is studied. Fig. 6 shows the simulated reflection coefficient and AR for different strip numbers of N = 2, 3, and 4. With reference to the figure, N has relatively mild effects on the impedance matching but significant effects on the AR. This is a favorable result because N can be used to fine-tune the AR after the antenna is matched. The AR bandwidth is optimum at N = 3 when the 3 dB criterion is used. Also, the effect of the dielectric constant ε r 2 of the embedded dielectric strip is investigated. It was found that as ε r 2 increases from 1 to 5, the antenna matching level can be remarkably improved, with the 10 dB impedance bandwidth slightly reduced. For the AR, it is strongly affected by ε r 2 . When ε r 2 increases from 1 to 3, two separate AR passbands can be merged, thus giving a wide AR passband of 73.1% (4.34–9.34 GHz). However, further increasing ε r 2 to 5 will conversely worsen the AR performance. Hence, the optimum dielectric constant of the embedded strip is given by ε r 2 = 3. Next, the effect of the lower DR dielectric constant ε r 1 is inves- tigated. Fig. 7 shows the simulated reflection coefficient and AR for different dielectric constants of ε r 1 = 8, 10, and 12. With reference to Fig. 8. Measured and simulated reflection coefficients and ARs of 3-D-printed CP DRA. the figure, the antenna resonant frequency obviously shifts downward with an increase in ε r 1 . This can be expected because the DRA with a higher dielectric constant should have a lower resonant frequency. It can be seen from the figure that the AR performance is strongly affected by ε r 1 . When ε r 1 increases from 8 to 10, three separate AR passbands can be merged, thus giving a wide AR passband of 73.1% (4.34–9.34 GHz). However, further increasing ε r 1 to 12 will conversely worsen the AR performance. Hence, the optimum dielectric constant of the lower DR is given by ε r 1 = 10. The effects of the bottom length l 0 and width w 0 of the lower DR were also studied. It was found that the reflection coefficient does not change too much when the l 0 is slightly varied, but it will obviously shift downward with an increase in w 0 . Furthermore, both parameters have remarkable effects on the AR. Using 3 dB criterion, the optimum AR is found at l 0 = 29 mm and w 0 = 9 mm. III. S IMULATED AND M EASURED R ESULTS To verify the idea, a CP DRA operating in C-band was designed and 3-D printed. Two different 3-D printing materials of ε r 1 = 10 and ε r 2 = 3 were used to reduce the antenna Q -factor and maintain a relatively compact size. Both printing materials have loss tangents of ∼ 0.003 at 6 GHz. Our 3-D printer has a printing resolution and tolerance of 0.05 and 0.1 mm, respectively. It took about 100 min to print the DRA at a material cost of less than USD 15. In our experiment, the reflection coefficient was measured using an Agilent vector network analyzer N5230A, whereas the AR, radiation pattern, antenna gain, and antenna efficiency were measured using a Satimo StarLab system. Fig. 8 shows the measured and simulated reflection coefficients of the CP DRA. The inset of the figure shows the photographs of the prototype. With reference to the figure, the measured | S 11 | has a 10 dB impedance bandwidth ( | S 11 | ≤ − 10 dB) of 69.7% (4.80–9.94 GHz), reasonably agreeing with the simulated band- width of 66.5% (4.87–9.73 GHz). Both impedance bandwidths are sufficient for the current 5 GHz WLAN bands (5.15–5.35 and 5.725–5.875 GHz) and the new WiFi-6E band (5.925–7.125 GHz). The discrepancy between the measured and simulated results is caused by fabrication and experimental tolerances. Fig. 8 also shows the measured and simulated ARs of the CP DRA in the boresight direction ( θ = 0 ◦ ) . As can be observed from the figure, the measured and simulated 3 dB AR bandwidths (AR < 3) are given by 68.6% (4.52–9.24 GHz) and 73.1% (4.34–9.34 GHz), respectively. The measured usable CP bandwidth is 63.2% (4.80–9.24 GHz). Fig. 9 shows the measured and simulated radiation patterns of the CP DRA at 5.0, 7.0, and 9.0 GHz. With reference to the figure, stable broadside radiation patterns are obtained for both the Authorized licensed use limited to: University of Sydney. Downloaded on August 11,2023 at 08:14:26 UTC from IEEE Xplore. Restrictions apply. IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 70, NO. 7, JULY 2022 5975 TABLE I D ESIGN P ARAMETERS OF P ROPOSED CP DRA Fig. 9. Measured and simulated radiation patterns of 3-D-printed CP DRA. (a) 5.0 GHz. (b) 7.0 GHz. (c) 9.0 GHz. Fig. 10. Measured and simulated antenna gains of 3-D-printed CP DRA in boresight direction ( θ = 0 ◦ ) as a function of frequency. The measured total antenna efficiency is also shown. x z- and y z-planes. In the boresight direction ( θ = 0 ◦ ) , the measured copolar left-hand CP (LHCP) fields are stronger than the cross-polar right-hand CP (RHCP) fields by more than 18 dB, being sufficient for practical applications. It is noted that the relatively high cross- polar fields of RHCP in the upper band are mainly caused by the high-order DRA modes. Fig. 10 shows the measured and simulated realized gains (mis- match included) of the CP DRA in the boresight direction, with a reasonable agreement between them. As can be seen from the TABLE II C OMPARISON B ETWEEN C URRENT AND E XISTING S INGLE -F ED CP DRA S figure, the measured gain is generally lower than the simulated counterpart, as expected. The measured peak CP gain is 8.3 dBic at 6.96 GHz. Fig. 10 also shows the measured total antenna efficiency that has considered impedance mismatch. With reference to the figure, the efficiency is higher than 82% across the usable CP passband (4.80–9.24 GHz), being acceptable for practical applications. Table II compares our 3-D-printed CP DRA with other single- fed CP DRAs available in the literature. With reference to the table, although our CP DRA has the highest profile, it features a compact footprint with the widest usable CP bandwidth and highest peak gain. It is worth mentioning that a wide CP DRA can be obtained by adding four metallic plates around the DR [43]. However, this design method will inevitably lead to a bulky CP antenna. Also, by using a 3-D printer with two independent printing heads, our DRA can be conveniently fabricated in one go at a reasonable cost. A suggested design guideline of the CP DRA is given as fol- lows. First, design a homogenous trapezoidal DRA [15] with its resonant frequency at the target band. It is followed by embedding several dielectric strips with a low dielectric constant ( ε r 1 ) into the DR body. Considering the commercially available low-loss printing material, the value of ε r 1 can be chosen as 3. Our experience shows that the number of embedded strips can start from 2. Next, design the interlaced dielectric slabs and load them on top of the DR to give a good AR. The initial height of the slabs can be chosen as about 0.15 λ 0 , where λ 0 is the wavelength in the air at the center frequency. Finally, tune the coupling slot size to optimize the frequency and impedance matching level and thus maximize the overlapping bandwidth between the impedance and AR. IV. C ONCLUSION A 3-D-printed single-fed wideband CP DRA with two different printing materials has been investigated for the first time. The DRA consists of a twisted DR embedded with low- ε r dielectric strips and loaded by interlaced height-varying dielectric slabs. It was excited by a slot with two open stubs. The slot is fed with a small conducting ring. By making use of the multiple DRA modes and slot modes, five CP modes have been obtained and merged together to give a very wide 3 dB AR bandwidth. To demonstrate the idea, a prototype operating in C-band was designed, 3-D printed, and tested. It has been found that the prototype has the measured and simulated 10 dB impedance bandwidths of 69.7% and 66.5%, and measured and simulated AR bandwidths of 68.6% and 73.1%, respectively. A wide measured overlapping bandwidth between the impedance and AR bandwidths has been found to be 63.2%, which is a record-high Authorized licensed use limited to: University of Sydney. Downloaded on August 11,2023 at 08:14:26 UTC from IEEE Xplore. Restrictions apply. 5976 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 70, NO. 7, JULY 2022 value for the DRA. Also, stable radiation patterns have been observed across the overlapped passband, with the peak measured antenna gain given by 8.3 dBic. Finally, it is worth mentioning that with advanced multimaterial 3-D printing techniques, innovative DRA structures that were deemed not practical before can now be fabricated conveniently at a reasonable cost, greatly advancing the development of DRA. 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