# SIW Cavity-Backed Antenna Array Based on Double Slots for mmWave Communications
## 1 Introduction  
Substrate integrated waveguide (SIW) is a technology enabling the design and manufacture of miniaturized waveguides. SIW technology mimics a dielectric waveguide that is implemented in a planar form due to the use of metallized vias, which form a waveguide in the employed substrate [1]. In contrast with microstrip technology, SIW can be used to design radio-frequency devices with millimeter-wave frequencies (mmWave) [2]. The main reason allowing this is the avoidance of radiation losses when the frequency increases, since the propagated wave is strongly confined in the substrate integrated waveguide. Therefore, losses in SIW devices are produced in the metal and mainly in the dielectric material, as is the case for microstrip technology where these losses are more significant. Apart from the above characteristics of SIW, it has the powerful advantage compared with conventional waveguide technology of enabling the cost-effective design of waveguide circuits. In recent years, due to the mentioned advantages offered by SIW, many antennas and radio-frequency components such as filters or phase shifters have been reported using this technology [3]. SIW antennas are of special interest due to the ease of integrating them as radiating elements in a complete system [4]. Due to the low profile of SIW technology, a broad variety of SIW antennas have been conceived with both single and multiple layers. For instance, various researchers [5; 6] have employed several SIW layers to create an antenna with a progressive aperture, where the substrate is drilled to improve the bandwidth and the gain. In [7], a 60 GHz antenna array is presented where multiple SIW layers were fabricated on low-temperature co-fired ceramic (LTCC). In this case, the radiating element is an aperture backed by a cavity. Other antennas have been designedwith layers of different types. For example, in [8], the feeding layer is based on microstrip, while the radiating elements in the antenna array in [9] were implemented in a metallic plate. Regarding SIW antennas with a single layer, examples can be found in [10,11], where the former used a corporate feeding network, and the latter did not. Moreover, SIW antennas in the literature apply different geometries and strategies to radiate the electric field that arrives at the antenna elements. Slotted antennas are preferred due to their easy implementation and the possibility of tuning the radiation in the design. In [12], the impedance bandwidth of a cavity-backed slot antenna was improved by introducing a via hole. Nevertheless, the antenna operates at low frequency and it consists of a single antenna with low gain. Arrays of slot antennas that have also been reported are generally based on a backed cavity. There are examples where higher-order modes were excited in the backed cavity in order to operate in distinct frequency bands [13,14] or, conversely, the excited modes were close enough to a provide wide operating bandwidth [15,16]. Additionally, the relative orientations between slots can be adjusted in order to achieve the desired polarization [17]. In this paper, we propose a cavity-backed antenna array whose radiating elements are double slots at millimeter-wave frequencies. We study the impact of introducing a single or a double slot in a cavity-backed antenna, finding that the double slot provides more control in the operating frequency band of the antenna array. A preliminary work on a cavity-backed antenna with two slots is presented in [18]; however, the slots were used there to tune two very narrow bands around a center frequency of 10 GHz and the backed cavities were fed by cascade power dividers. In our work, a compact corporate feeding network is used and each backed cavity contains a subarray of 2 \(\times\) 2 double slots, increasing the directivity of the complete antenna array. Additionally, the double slots are tuned to enhance the operating frequency band and achieve the desired directivity at mmWave bands.