I. Introduction
With both academics and industries expressing rising interest in millimeter-wave (mmW) communication, a growing demand for wideband antenna arrays that offer high gain at low costs has emerged [1, 2]. While large-scale arrays are usually preferred for high-gain applications, a low-loss feeding network is crucial to achieving extremely high gain.
In this context, two commonly used feeding networks used in antenna array design are series feed [3–5] and corporate feed [6–8]. Series feed offers the advantages of a small size and reduced spurious feed radiation. However, it usually suffers from significant drawbacks when the feedline length covers a considerable portion of the wavelength. Such a situation must be treated as a transmission line with significant distributed constants, leading to standing waves and increased losses, which negatively influence the matching performance and gain of antennas [3]. In contrast, the corporate feed is capable of achieving higher gain, but it is accompanied by drawbacks, such as higher feedline radiation and increased array size or substrate layer. To address this, a corporate-feed 2 × 4 substrate integrated waveguide (SIW) cavity-backed antenna array with a broad bandwidth was proposed by employing wide radiating slots [6]. However, since the feeding network was located on the same substrate as the radiating elements it resulted in a limited array scale. In this context, it is worth mentioning that SIW-based feeding networks are characterized by low radiation loss, which in turn allows for lower insertion loss. However, they require a larger area compared to a microstrip alternative, which hinders the implementation of large-scale arrays. To achieve low loss, previous studies have proposed using a compact feeding network featuring low design complexity, as well as a SIW-based series-parallel antenna array with simplified power distribution networks [9, 10]. While Jin et al. [9] proposed an mmW 4 × 4 patch antenna array capable of good radiation performance using a compact SIW-based series-parallel differential feeding network, Yang et al. [10] presented a compact 4 × 4 slot antenna array equipped with a simplified 1-to-4 SIW power distribution network that can be easily scaled up to form a larger array. In addition, the gain of an antenna can be improved by increasing the radiation aperture. However, an element spacing of more than one wave-length usually results in high grating lobes. Therefore, in [11], a high-gain 16 × 16 planar integrated antenna array with element spacing larger than a wavelength and an acceptable sidelobe level was accomplished by employing grooves as secondary radiation sources. The large element spacing also facilitated the design of the feeding network.
This paper presents a wideband, high-gain mmW series-parallel 4 × 4 antenna array composed of eight series-fed 2 × 1 subarrays integrated with a simplified 4-way parallel feeding network. A wide impedance bandwidth (IBW) is accomplished using a SIW cavity-backed patch operating at TE210 SIW cavity mode and TM10 patch mode. Furthermore, to increase the gain, a stepped waveguide horn with a large aperture is vertically integrated with the SIW cavity-backed patch. In addition, two 4 × 1 series-parallel array layouts—a center-fed one and an edge-fed one—are studied to find that the center-fed layout contributes to a low sidelobe level. Finally, a prototype of the proposed 4 × 4 array is fabricated and measured, demonstrating a 14.7% −10 dB IBW and 22.05 dBi peak gain. Moreover, owing to its large element spacing and simplified feeding network, the proposed array can be scaled up into a large-scale array.
II. Wideband and High-Gain mmW Antenna Array Design
1. Principles of the FMCW Radar
Fig. 1(a) presents the configuration of the proposed 2 × 1 subarray, which is vertically integrated with a series-fed 2 × 1 SIW cavity-backed patch subarray, along with an aluminum stepped waveguide horn on each element. To obtain a wide operating bandwidth, the SIW cavity and patch were designed to resonate in the TE210 and TM10 modes, respectively. In this context, the initial size of the TE210-mode SIW cavity can be calculated using the following formulas [12, 13]:
where ɛr is the relative permittivity of the substrate, c is the light velocity of free space, d is the diameter of the metallic via, dp is the center-to-center spacing between adjacent vias, and Lc and Wc are the length and width of the TE210-mode SIW cavity, respectively.
Fig. 1(b) shows the simulated |S11| and realized gain curves of the proposed 2 × 1 antenna subarray. It exhibits two resonant poles at 24.3 GHz and 26.9 GHz, which contributed to a wide −10 dB IBW of 20.2%. Furthermore, the gain curve displays a 3-dB gain bandwidth of 14%, along with a high peak gain of 13.7 dBi. Fig. 2 presents the electric field distribution in the substrate at 24.3 GHz and 26.9 GHz. It is observed that upon exciting the TE210 SIW cavity mode and the TM10 patch mode, which have similar field distribution characteristics, the first and second resonant poles are obtained, respectively.
To provide a detailed illustration of the high-gain operation, Fig. 3 illustrates the design evolution of the proposed 2 × 1 subarray. Subarray 1 is a normal series 2 × 1 SIW cavity-backed patch subarray with large element spacing. Building upon Subarray 1, Subarray 2 is created by incorporating a uniform waveguide horn onto each antenna element, with both horns designed on the same aluminum plate. Subarray 3 replaces the two uniform waveguide horns with two stepped waveguide horns to achieve better impedance matching. Finally, Subarray 4, which is also the proposed configuration, includes a groove placed between the two stepped waveguide horns for sidelobe suppression and gain improvement.
A comparison of the |S11|, gain, E-plane (xoz-plane) and H-plane (yoz-plane) radiation patterns of Subarrays 1–4 are presented in Fig. 4. Subarray 1, featuring a large element spacing of approximately 1.1 λ0 (λ0 is the free-space wavelength of the center frequency of 25.5 GHz), achieved a peak gain of only 8.9 dBi and a high sidelobe level of −0.2 dB. In the case of Subarray 2, the introduction of uniform waveguide horns resulted in a significant increase in peak gain and suppression of the sidelobe level, albeit with a degradation in |S11|. Subarray 3, featuring stepped waveguide horns, attained improved |S11| characteristic, but it was accompanied by a slight degradation in the peak gain and sidelobe level. Since the stepped waveguide horns enabled smooth energy transmission, Subarray 3 achieved excellent impedance matching. To further enhance sidelobe suppression, the final design incorporated a groove between the horns, which acted as a secondary radiation source. With the addition of the groove, the surface wave was radiated to achieve improved gain. Moreover, the coupling between adjacent antenna elements caused by the surface wave was efficiently suppressed, thus enhancing sidelobe suppression [11]. Consequently, the proposed subarray simultaneously achieved high peak gain, a low sidelobe level, and a wide −10 dB IBW.
2. The Series-Parallel 4 × 4 Array
To compose a series-parallel 4 × 4 antenna array, four series-parallel 4 × 1 subarrays were arranged. As shown in Fig. 5, such an antenna array can be arranged into two types of layouts—an edge-fed one (Layout A) and a center-fed one (Layout B)— with the subarray configuration and element spacing being the same. The only difference between these two layouts is their port settings. The two input ports in Layout A are located on the two edges of the subarray, while those of Layout B are situated at the center of the subarray. The radiation patterns of these two layouts at 26 GHz are presented in Fig. 5. It is observed that the co-polarization pattern in the E-plane for Layout A suffers from an undesirably high sidelobe level of about −7.1 dB. In contrast, the sidelobe level for Layout B is about −13.4 dB. Therefore, Layout B was applied to our design to achieve a low sidelobe level. The underlying mechanism for this observation can be explained using the electric field distributions shown in Fig. 2. In a series 2 × 1 subarray, the antenna elements located closer to the input port are allocated more energy. Therefore, in Layout A, two elements situated at the center have lower radiation energy than two elements at the two edges. In contrast, two elements located at the center in Layout B have higher radiation energy than two elements at the two edges. Therefore, a center-fed 4 × 1 antenna array (Layout B) generates more concentrated radiation, ultimately generating a low sidelobe level.
Along with the application of the proposed series-fed 2 × 1 subarray, a simplified 4-way power divider was introduced in the 4 × 4 array. As shown in Fig. 6(a) and 6(b), the feeding network consists of a 4-way power divider and a rectangular waveguide (RWG)-SIW transition structure. The 4-way power divider is responsible for exciting the eight series-fed 2 × 1 antenna subarrays through four coupling slots. Furthermore, the RWG-SIW transition structure mainly consists of a SIW cavity, a central aperture, an off-center patch, and four matching vias. An off-center patch embedded in the aperture is added to improve the operating bandwidth. In addition, there is a pair of arc-shaped indentations on the front side of the aperture to further improve impedance matching performance. Fig. 6(c) presents the simulated S-parameters of the feeding network, which shows that the −10 dB IBW exceeded 20%, attaining an average insertion loss of 0.8 dB, while the input signal was equally divided into eight output ports.
The geometry of the proposed 4 × 4 antenna array, comprising an aluminum horn sheet, an aluminum fixture, and two laminated substrates (Sub 1 and Sub 2), is depicted in Fig. 7. Sub 1, located at the bottom, is placed along with the RWG-SIW transition structure and the 4-way SIW power divider. For Sub 2, four of the proposed side-fed 4 × 1 antenna subarrays were designed. In this context, it should be noted that the aluminum fixture at the bottom was added for test convenience and, therefore, is not required for practical applications.
III. Measurement and Discussion
The proposed 4 × 4 antenna array was fabricated for experimental verification. Photographs of the fabricated prototype are presented in Fig. 8(a). The overall size of the antenna array was 61 mm × 61 mm, with the two aluminum sheets having the same thickness of 4 mm. Sub 1 was composed of Rogers RT/duroid 5880 substrate with a thickness of 0.787 mm, while Sub 2 was made of Rogers RO4003C substrate with a thickness of 0.813 mm. The simulated and measured reflection coefficients and gains are illustrated in Fig. 8(b), showing good agreement. Notably, there is a slight degradation at the lower band edge, which may have been caused by manufacturing errors. The fabricated array attained a −10 dB IBW of 14.7% in the 24–27.8 GHz frequency range, and a 3-dB gain bandwidth of 11.4% in the 24.1–27 GHz range. The measured peak gain reached up to 22.05 dBi—only 0.17 dB less than the simulated value. Furthermore, Fig. 9 shows the simulated and measured normalized radiation patterns of the proposed array at 24 GHz, 25.5 GHz, and 26.9 GHz. Symmetrical patterns were obtained, with the cross-polarization levels being lower than −25 dB in the boresight direction across the entire operating band.
Table 1 presents a comparison of the proposed mmW antenna array and previously reported SIW-based antenna arrays. It is evident that the proposed array offers extremely high gain, a wide IBW, and a compact feeding network. In contrast, although [6] and [7] proposed a single-layer low-cost slot array and a multilayer wideband patch, respectively, their feeding networks were too complicated. To simplify the feeding network, [9] employed a series-parallel feeding structure, and [10] used high-order SIW modes. Furthermore, large element spacing was adopted in [11] to obtain high gain and facilitate the design of the feeding network. The design proposed in this study employed both a series-parallel feeding structure and large element spacing to simplify the feeding network and achieve high gain.
IV. Conclusion
In this work, an mmW series-parallel 4 × 4 antenna array characterized by high gain and a wide operating bandwidth is proposed. Furthermore, to realize high gain and simplify the feeding network, a series-fed 2 × 1 SIW cavity-backed patch subarray integrated with stepped waveguide horns was investigated. Subsequently, a series-parallel 4 × 1 array with low sidelobe characteristics was developed to be incorporated into a 4 × 4 array. For verification, the proposed 4 × 4 array with a simplified feeding network was fabricated and measured. The measured −10 dB IBW and 3-dB gain bandwidth reached 14.7% and 11.4%, respectively. Moreover, the fabricated array achieved a high peak gain of 22.05 dBi at 26 GHz. Overall, owing to its wide band, high gain, low cost, and compact feeding network, the proposed array is considered suitable for various mmW wireless applications.
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