I. Introduction
The rapid development of wireless communication technology in recent years has led to an increase in the significance of and demand for high-gain antennas with a wide bandwidth [1–5]. Notably, many effective solutions have been applied to achieve these characteristics. For instance, in [6–8], wide bandwidth and high gain were achieved using a stacked metasurface layout. Meanwhile, Weily et al. [9] employed an operating frequency reconfigurable antenna to improve the antenna bandwidth. Awan et al. [10] sought to achieve high gain characteristics in an ultra-wideband wireless communication system by modifying a conventional rectangular monopole antenna using slots and stubs and placing a frequency-selective surface under the antenna as a reflector to enhance its gain. An empty high-gain, substrate-integrated, cavity-backed waveguide slot antenna with multiple high-order hybrid modes was proposed in [11] to broader the bandwidth. Furthermore, Yang et al. [12] introduced a wideband pattern reconfigurable antenna that uses seawater, operating within the frequency band of 1.66–2.78 GHz, with S11 < −10 dB. Meanwhile, Ferreira-Gomes et al. [13] proposed chiral dielectric metasurfaces for a highly integrated, circularly polarized broadband antenna, whose impedance bandwidth and gain were measured to be 22.6% (25.3–31.6 GHz) and 10.4 dBi, respectively. Moreover, a circularly polarized antenna array based on hybrid couplers for 5G devices, which was introduced in [14], achieved an operating frequency band of 3.35–4.35 GHz, with a peak gain of 10.73 dBi.
A 3D printing technology has gained significant attention owing to the flexibility it offers in the choice of materials and its capability to fabricate complex structures while also saving time [15–18]. Some examples of 3D printing technology include binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and stereo lithography [19]. Compared to traditional mechanical machining, 3D printing technology can be employed to manufacture arbitrary 3D objects, thus significantly reducing fabrication costs [20, 21]. In addition, since it is characterized by high resolution and accuracy, 3D printing can be applied in the fields of microfluidics, aerospace, and bioengineering [22, 23]. With recent remarkable progress in 3D printing materials and technology, one can even print using polymer materials, metals, ceramics, and their mixtures [24–26]. Along these lines, Cuevas et al. [27] proposed an efficient knowledge-based artificial neural network for designing a circularly polarized lens antenna and then fabricated it using 3D printing technology. In [28], a wide-bandwidth and high-gain D-band antenna was introduced and fabricated using commercially available 3D printing techniques. The proposed antenna concept was successfully prototyped and verified by experimental results. Furthermore, a fully 3D-printed hemispherical dielectric resonator antenna with a metallic cap was proposed in [29]—the overall weight of the topology was 22% of the nominal weight. Meanwhile, Zhang et al. [30] proposed a 3D-printed, wideband, circularly polarized, parallel-plate Luneburg lens antenna, where 3D printing technology was employed to fabricate complex structures consisting of various dielectric posts. The designed antenna not only realized the Luneburg lens function but also behaved as a wideband polarizer, providing a broadband, circularly polarized beam. In [31], a high-efficiency and high-gain antenna using low-loss and temperature-stable Li2Ti1−x(Cu1/3Nb2/3)xO3 microwave dielectric ceramics was introduced, offering a lightweight and low-cost antenna for potential application in Beidou navigation satellite systems.
In this work, a novel high-gain wideband antenna based on split-ring resonators and a back cavity structure is proposed. The proposed antenna comprises a bowl-shaped reflector, split-ring resonators, and a circular-fractal, multilayered, stacked microstrip antenna. The bowl-shaped reflector is fabricated using 3D printing, with its cambered internal surface painted with sliver conductive adhesive. The simulated and measured results demonstrate that the proposed structure can successfully enhance both gain and bandwidth. Moreover, the antenna offers the advantages of low cost, low cross-polarization, and easy fabrication. The measurement results indicate that the proposed antenna achieved a fractional bandwidth of 18.5% (5.63–6.78 GHz) and reflection coefficients lower than −10 dB, while its gain ranged between 10.9 dBi and 14.6 dBi (a peak gain of 14.6 dBi was attained at 5.7 GHz). Furthermore, good agreement was observed between the simulation and measurement results.
This article is structured as follows: the design theory and structure of the antenna are described in Section II, the measurement results and performance of the antenna are presented and investigated in Section III, and a conclusion is provided in Section IV.
II. Antenna Design and Analysis
1. Operational Principle
A fractal antenna features stable radiation characteristics along with high self-similarity and space-filling ability. Notably, the space self-filling abilities of a fractal antenna allow for improved space utilization, which serves to extend its surface current path inside a radiation structure, thereby letting it achieve wider bandwidths than other same-sized antennas.
The structure chart and equivalent circuit diagram of the split-ring resonator are presented in Fig. 1. Notably, a split-ring resonator is a microstructural element capable of generating a magnetic response and achieving negative permeability. Its structure usually comprises two concentric gapped rings. When an electromagnetic wave strikes a split-ring resonator, the surface-induced current is generated by the varying magnetic fields of the two rings. Therefore, the radiation characteristics and bandwidth of an antenna can be enhanced by loading it with split-ring resonators. Notably, the gap between the two metal rings form the capacitance C, while the metal rings themselves are equivalent to the total inductance L. Therefore, a split-ring resonator can be considered approximately equivalent to an LC resonant circuit [32]. The resonant frequency can be expressed as follows:
In the above equations, g refers to the width of the gap between the two metal rings, w is the width of the metal rings, and t denotes the thickness of the dielectric plate. In this work, the initial size of the unit cells was designed based on these formulas, and the model for a single element was simulated. Fig. 1(c) depicts the structure and configuration of the split-ring resonator.
Since rays of light can be converged using a convex lens, the principle of converging lens is utilized in antenna and microwave technologies. The radiation patterns of an antenna are usually modified by reflectors, and a parabolic reflector can be used to enhance antenna gain. Ray tracing of a parabolic reflector is depicted in Fig. 2.
The above discussion implies that the gain and bandwidth of a circular-fractal, multilayered, and stacked microstrip antenna can be improved by loading it with split-ring resonators and a parabolic reflector. The proposed antenna was designed based on these design principles.
2. Antenna Design
Before designing the integral antenna, it had to be developed in such a way that it could feed the antenna system. In this work, the antenna system was electromagnetically fed by a circular-fractal, multilayered, and stacked microstrip antenna. The design formula for circular patch microstrip antennas has already been introduced in [16], where ɛr refers to the effective dielectric constant, h is the thickness of the dielectric plate, f0 (GHz) is the resonant frequency, and r (dm) indicates the radius of the circular radiating patch. A preliminary calculation was conducted using this formula. A slotting method was employed to construct a four-order iterative ring fractal structure, and a multilayered stacked structure was adopted to enhance the bandwidth. The circular-fractal, multilayered-stacked microstrip antenna was fed using an SMA connector of 50 Ω. The inner and outer conductors passed through the two dielectric plates, with the inner conductor connected to the upper radiation patch. The GND was printed at the bottom of the dielectric plate and then connected to the flange plate of the SMA contact. The thickness of the two dielectric plates was 1.6 mm, the dielectric constant was 2.25 mm, and the distance between the two dielectric plates was 2.05 mm:
Based on the above design principles and formulas, the initial size of the antenna was calculated and optimized. Subsequently, the bowl-shaped back cavity structure was fabricated using a commercial Formlabs printer based on stereo lithography apparatus technology, which offers a high-resolution printing accuracy of 0.025 mm. Some of the setup parameters for the printing process were as follows: the temperature was fixed at 35°C, the infill density was set to 100%, and the laser wavelength was 405 nm. The dimensions of the 3D-printed bowl-shaped back cavity structure are 147.2 mm × 147.2 mm × 28.7 mm. Its cambered internal surface was coated with sliver conductive adhesive to be used as a parabolic reflector. Furthermore, four circular double-reverse split-ring resonators were printed on a Wangling F4B225 dielectric substrate of thickness 1.6 mm and dielectric constant 2.25, and then loaded onto the bowl-shaped structure. The dimensions and geometry of the antenna are shown in Table 1 and Fig. 3, respectively.
3. Antenna Analysis
To demonstrate the structure of the proposed antenna and its contributions to endfire gain, the evolution process and simulated results of its different versions are shown in Fig. 4.
The structures of a circular-fractal microstrip antenna and a circular-fractal, multilayered, and stacked microstrip antenna are illustrated in (i) and (ii) of Fig. 4(a), respectively. The cambered internal surface of the bowl-shaped back cavity structure was coated with sliver conductive adhesive to form a parabolic reflector, the structure of which is shown in (iii) of Fig. 4(a). Furthermore, in (iv) and (v) of Fig. 4(a), to demonstrate the effects of the parabolic reflector and split-ring resonators on gain improvement, a contrast is maintained in the proposed antenna’s structure before and after removing the parabolic reflector and split-ring resonators, respectively. The final structure of the proposed antenna is shown in (iv) of Fig. 4(a). Notably, the construction process is detailed in Section II-2 of this study. For convenience, these five antennas are named ANT1, ANT2, ANT3, ANT4, ANT5, and ANT6.
The radiating field and radiation pattern at the center frequency (5.8 GHz) are demonstrated in Fig. 4(b) and 4(c), respectively. It is evident that ANT1 achieved a wider beamwidth than the other antennas, while the proposed antenna attained the narrowest beamwidth. Furthermore, endfire gains of 4.3 dBi, 9.8 dBi, 12.5 dBi, 9.6 dBi, 9.4 dBi, and 14.6 dBi were attained by ANT1, ANT2, ANT3, ANT4, ANT5, and ANT6, respectively, indicating that the maximum gain increased by 239.5% after loading the multilayered-stacked antenna, the parabolic reflector, and the split-ring resonators. Overall, the proposed structure for the antenna succeeded in effectively improving its radiation characteristics.
III. Results and Discussion
The fabricated high-gain wideband antenna proposed in this work is presented in Fig. 5. The input reflection coefficient was measured using a vector network analyzer (Agilent 5230A), and radiation patterns were measured in a microwave anechoic chamber. The simulated and measured results for the reflection coefficient and gain are presented in Fig. 6, showing good agreement. The proposed antenna operated within the frequency range of 5.63–6.78 GHz (reflection coefficient ≤ −10 dB), achieving gains ranging from 10.9 to 14.6 dBi (the maximum gain is 14.6 dBi at 5.7 GHz). Furthermore, the measured and simulated radiation patterns at 5.8 GHz and 6.7 GHz are illustrated in Fig. 7, indicating slightly different results, possibly due to errors in measurement and fabrication.
Finally, a comparison between the performance of the proposed antenna and those reported in previous works is presented in Table 2. The factors that are compared include bandwidth, gain, and size of the antenna. Table 2 clarifies that the proposed antenna achieved high gain and suitable wideband performance, considering λ0 to be the wavelength in free space at the center frequency of the reference antennas. In the case of the antenna proposed in [33], although its size and gain are better than those of the proposed antenna, its fractional bandwidth is just 7.6%. As for the antenna reported in [34], its maximum gain is less than that of the proposed antenna. Furthermore, the antennas reported in [34, 35] are smaller in size than the proposed antenna, but their fractional bandwidths are just 16.7% and 14.0%, respectively. In [36], the size of the back surface and profile of the antenna are approximately 10 and 18 times larger than the proposed antenna, respectively, but its bandwidth and maximum gain are less than those of the proposed antenna. Furthermore, the profile and area of the back surface in [37, 38] are relatively small, and the bandwidth in [38] is greater than that of the proposed antenna, but the maximum gains they achieved are only 11.8 dBi and 12.9 dBi, respectively.
IV. Conclusion
With the development of wireless communication technology, the demand for high-gain antennas with a wide bandwidth has increased manifold. Hence, it is of practical significance to design a wide-bandwidth, high-gain antenna suitable for use in practical engineering applications. Therefore, in this paper, a novel high-gain wideband antenna fabricated using 3D printing is proposed. To achieve high gain and a wide bandwidth, split-ring resonators and a bowl-shaped reflector were employed to adjust the radiation pattern of the circular-fractal, multilayered, stacked microstrip antenna. The antenna’s operating frequency extended from 5.63 GHz to 6.78 GHz (reflection coefficient ≤ −10 dB), and it achieved a gain ranging from 10.9 dBi to 14.6 dBi, with a peak gain of 14.6 dBi at 5.7 GHz. Furthermore, the measured and simulated results were observed to be in good agreement.
The proposed antenna not only offers high gain and wide bandwidth but is also low cost and can be easily fabricated. Overall, the 3D-printed, wide bandwidth, and high-gain antenna proposed in this paper presents a novel method for designing high-gain wideband antennas.
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