# Development of a Wideband Slotted Antenna Array with Low Profile and Low Sidelobe
## 1 Introduction  
With the development of wireless communication systems, high-frequency bands are gaining increasing attention. For instance, in recent years, the K-band (i.e., 18-27 GHz) has been used in many radar and satellite communication systems [1; 2; 3; 4]. To reduce external interference and improve communication quality, high-performance antennas, which have high gain (\(>\)25 dBi), low profile, and low sidelobe level (SLL \(\leq-\)20 dB), are often required [5; 6; 7]. Due to the easy fabrication and integration with other circuits, planar antennas, including microstrip-based [8; 9; 10] and substrate-integrated waveguide (SIW)-based [11; 12] structures, have been widely used. However, these planar antennas are obviously affected by the dielectric loss of the planar substrates at high frequencies [13; 14]. To solve this problem, waveguide slot arrays with low insertion loss have been studied for achieving high-gain and low-profile engineering solutions [15; 16; 17; 18; 19]. Typically, a slotted waveguide antenna array is fed with a series structure [20], which is widely used due to its simpleness. However, when the size of the arrays becomes large, its long-line effect can dramatically affect its working bandwidth [21]. To solve this problem, some multi-layered slotted waveguide antenna arrays have been proposed [22; 23; 24], where a full-corporate-feed network is used. Generally, there are at least four layers, including a radiation layer, a cavity layer, a coupling layer, and a feeding layer. However, in the back cavity between the radiation layer and the coupling layer, there are usually large metal walls for power division, which can significantly affect the working bandwidth and the total profile of the multi-layered antenna arrays. Moreover, the multi-layered antennaarrays are mainly fed with a uniform-amplitude distribution, which leads to high SLLs. For purpose of obtaining low sidelobe performance, 45\({}^{\circ}\) linearly polarized radiation units are developed. This method can optimize the radiation pattern without affecting the gain and the beam-width [25]. However, the overall SLL reduction is still limited, and moreover, this method usually requires an additional layer to adjust the radiation direction, making it difficult to be miniaturized. The power division for the antenna array, which is mainly realized by the feeding network, can also dramatically affect the performance of the antenna array, especially its SLL [26]. Using unequal amplitude distribution can help to reduce the SLL. For instance, in complex antenna arrays, by using the Taylor distribution, the SLL can be suppressed due to the monotonic amplitude variation, realizing a good SLL [27]. However, it is more recommended to be used in a large antenna array where a large number of radiation elements is required [28]. For smaller antenna arrays, such as an 8-unit radiation structure, the Chebyshev distribution [29] can provide a similar power ratio to the Taylor distribution. While, for the same array size, the Chebyshev distribution can usually provide a lower SLL with the same beam-width. In order to realize the unequal power distribution by using either the Taylor or the Chebyshev distribution, T-junctions are necessarily designed for a waveguide-based feeding network. To reduce the unequal phases caused by the unequal power distribution in the T-junctions, some methods are developed. For instance, the width and the wall thickness at one of the outputs are often adjusted to compensate for the phase difference. Nevertheless, the adjustment range is very limited, especially for short or small T-junctions. Moreover, it is difficult to realize the impedance matching by using such structures for a complex full-corporate-feed network. For instance, the output port of the T-junction in the middle of the H-junction is often short [27]. As a result, a large width is required for high power distribution ratio, which is obviously difficult to achieve.