Abstract
A Ka-band high-power waveguide wide-slit antenna array with high efficiency and high-power capacity is proposed in this paper. The antenna uses a waveguide vertical transition structure as a part of the feed network, which improves the compactness and power capacity of the antenna. A 2 × 8 antenna array was simulated, fabricated by 3D printed technology, and measured. The measured reflection coefficient is less than −10 dB in the range of 30.4–30.9 GHz, the tested gain at 30.8 GHz is 25.8 dB, and the corresponding aperture efficiency is larger than 80%. This antenna can be applied in high-power millimeter-wave systems.
1. Introduction
High-power microwave (HPM) is generally defined as electromagnetic waves with peak power higher than 100 MW in the frequency range of 1 to 300 GHz [1–3]. Recently, high-power technologies have been guided by the demand for microwave sources in communication systems, long-range radars, and new gas pedals to higher frequencies. According to published literature, Ka-band high-power microwave sources [4] have reached 500 MW. Horn antennas are often used in high-power microwave systems, but their low gain and large size are not conducive to practical scenarios. To improve the practicality and broaden the application range, the development of miniaturized and high-efficiency Ka-band high-power millimeter-wave antennas has become an urgent problem.
Traditional HPM antennas, such as Vlasov antennas [5], COBRA antennas [6–8], and large-aperture parabolic antennas [9], meet the needs of experiments and applications to some extent, but their large size, complex configuration, and low-radiation efficiency limit their applications. Later, radial spiral line array antennas [10], radial line slot arrays [11], and transmit-array antennas [12–16] were developed. These types of antennas have good performance but are particularly difficult to process when the wavelength of high-frequency electromagnetic waves becomes shorter. In addition, the feed network is complex and it is difficult to form a large array. Meanwhile, many Ka-band planar array antennas [17–19] have been reported in recent years, but no definitive study of their power capacity has been made.
Therefore, we propose a wide-slit antenna array consisting of a waveguide resonant cavity and a waveguide vertical transition structure. As a traditional transmission line structure, the rectangular waveguide has the advantages of easy processing, low transmission loss, and high-power capacity. It has been a commonly used large transmission structure in the field of high-power microwaves. Most traditional waveguide slot array antennas [20] use longitudinal offset slots with narrow slot widths. When it is applied to high-power microwaves, the electric field of the slot is too concentrated, which reduces the power capacity of the system. The slot feed is also chosen for the feed network, which further reduces the power capacity of the system. Therefore, this paper innovatively adopts the waveguide vertical conversion structure in the design process to avoid the slit structure, thus increasing the power capacity. The radiation structure takes the form of 2 × 2 radiating wide slits. The wide slit has a higher power capacity, and the closely arranged antenna surface has a more uniform current distribution, which can improve the aperture efficiency of the antenna, reduce the size of the antenna, and facilitate the planarization and miniaturization design of the antenna. The antenna is fabricated by 3D printed technology. Recently, metal 3D printed technology [21–23] has been widely used in the fields of aerospace, automotive, medical applications, and manufacturing, as it shows great promise to complete almost any structural part of computer-aided drafting (CAD) and to increase processing speed while reducing production costs.
2. The Antenna Unit Design
Figure 1(a) shows the model of the antenna unit, which consists of a resonant cavity and a rectangular waveguide. There is a 2 × 2 radiating wide slit on the top of the resonant cavity, and the bottom of the resonant cavity is connected vertically with the waveguide to form a feed structure. The microwave is fed vertically into the resonant cavity through the rectangular waveguide and then radiated outward through the 2 × 2 radiating wide slits on the top of the resonant cavity. The whole structure is simple, without superfluous structure, which can greatly increase the power capacity of the antenna unit. As shown in Figure 1(b), the maximum electric field is 10418 V/m when the input power is 1 W. According to a study [24], the metal breakdown threshold under vacuum conditions is known to be 750 kV/cm and the pressure is 10−3 Pa, then the power capacity of the unit under vacuum conditions is calculated to be 51.8 MW. Figures 1(c) and 1(d) show the top and front views of the antenna unit and the antenna unit’s dimensional parameters. The wall thickness t of the structure is 1 mm.
The simulation results of the reflection coefficient and gain of the antenna unit obtained by using ANSYS HFSS full-wave simulation software are shown in Figure 2(a). In the required 30∼31 GHz band, the reflection coefficient is less than −18 dB, and the aperture efficiency of the antenna is calculated as follows:
(a)
(b)
So, the aperture efficiency of the antenna unit by calculated as 74%. Figure 2(b) shows the radiation pattern of the antenna unit at 30.5 GHz.
3. Power Divider and 2 × 8 Array Design
For high-power applications, the power capacity should be considered. Figure 3 shows the electric field distribution in a rectangular waveguide of 8.636 mm × 4.318 mm. The maximum electric field is 7150.1 V/m when the input power is 1 W. Therefore, if we choose a breakdown threshold of 750 kV/cm for the metal, the power capacity is about 100 MW in a vacuum. Thus, the design goal is to reduce the field electric field strength as much as possible to make the antenna array reach the power limit of the waveguide.
3.1. Power Divider Design
To expand the designed antenna unit to a larger array, we need to redesign the feed network and at the same time, we must keep the power capacity of the antenna at a high level. Therefore, we choose the usual waveguide power divider to form the feed network, which has a higher power capacity. The divider divides into E-plane and H-plane, as shown in Figure 4, and the power capacity is about 56 MW.
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Figures 5(a) and 5(b) show the reflection and transmission coefficients of the E-plane and H-plane power dividers, respectively. The reflection coefficients of both structures are less than −10 dB at 30–31 GHz, while the transmission coefficients are about −3 dB. The simulated S-parameter results prove that the structures have good transmission performance.
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3.2. 2 × 8 Array Design
Using the mentioned antenna unit and power dividers, we get a 2 × 8 antenna array with a size of 30 mm × 117 mm as shown in Figure 6(a). Two 8-way waveguide power dividers feed 16 cavities, each with four radiating slits. Figure 6(b) shows the electric field distribution at 30.5 GHz, the maximum electric field is 10365 V/m, so the power capacity in a vacuum is about 50 MW.
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Figure 7(a) shows the simulated results of this array. The reflection coefficient is less than –10 dB at 30–30.75 GHz, which proves the good performance of the array, and the gain of the antenna is 25.9 dB at 30.5 GHz, so its aperture efficiency is 85.3%. Figure 7(b) shows the radiation pattern of the array. The 2 × 8 array can be used as a subarray unit with 26 dB gain and 50 MW power capacity. Multiple subarrays can be combined into a larger array to meet the power capacity and gain requirements of antenna arrays in different scenarios.
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4. Measurement Results
Figure 8 shows the fabricated antenna array, which is made of aluminum by 3D printed technology with a wall thickness of 1 mm. The reflection coefficients, S11 are shown in Figure 9. Compared with the simulated result, the measured results have a frequency shift. The center frequency is shifted by about 0.25 GHz, the offset is less than 1%, and the antenna’s reflection coefficient is below −10 dB in the frequency band of 30.4–30.9 GHz. Compared to the simulated results, the bandwidth is much narrower and there is a huge difference at 30 GHz. The value of the H-plane power divider, shown in Figure 4(b), has a relatively large impact on the performance of the device, and it has the highest sensitivity. A sweeping analysis was performed for values from 7 mm to 7.6 mm, as shown in Figure 10. At 7.1 mm, the reflection coefficient of the antenna array is close to 0 at 30 GHz, which is very different from that at 7.4 mm, but in good agreement with the test results. Therefore, the values of the fabricated antennas may shift to around 7.1 mm, which, together with the effects of other performance parameters, leads to large differences between the simulated and tested results of the antennas.
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The simulated and measured radiation patterns of the antenna at the center frequency, 30.5 GHz, in the E-plane and H-plane are shown in Figure 11. The simulated and measured are in good agreement. The simulated cross-polarization level is −24.57 dB. Figures 12(a) and 12(b) show the measured radiation patterns at 30.25, 30.5, 30.75, and 31 GHz for E-plane and H-plane, respectively. The radiation patterns of the antenna are very consistent at each frequency point.
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Figure 13 shows the measured and simulated frequency characteristics of the gain. The dashed lines indicate the gain at aperture efficiency of 90, 80, 70, and 60%. The gain of the antenna array is greater than 25 dB at 30.5–30.9 GHz, and the measured aperture efficiency is higher than 70%, with a maximum efficiency greater than 80%.
Finally, a comparison with previously reported Ka-band antenna arrays is shown in Table 1, and the proposed antenna array shows a high aperture efficiency.
5. Conclusions
In summary, a novel waveguide wide-slit antenna array based on 3D printed technology with high-power capacity and high efficiency is presented in this paper. A vertical transition structure with high transmission efficiency is used to connect the feed network and the radiating structure. This vertical transition structure not only increases the power capacity of the array, but also makes it more compact, and the tight arrangement of the radiating structure improves the aperture efficiency of the antenna as well as enables the planarization and miniaturization of the antenna. At 30.5–30.9 GHz, the measured gain of the antenna array is higher than 25 dB, the aperture efficiency is larger than 70%, and the highest efficiency is greater than 80% in the operating band. The simulated power capacity of the 2 × 8 array is about 50 MW in a vacuum, it can be used in high-power millimeter-wave systems.
Data Availability
No data were used to support this study.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was in part supported by the Science and Technology on High Power Microwave Laboratory Fund under grant no. 6142605190201.