# Review of Substrate Integrated Waveguide Circuits for Beam-Forming Networks Working in X-Band
## 4 Substrate Integrated Waveguide (SIW) Antennas  
The use of SIW technology appears to be very interesting in producing low profile, light, and low-cost antennas. However, the design of high gain, squint-less, and very wide band SIW antennas can be quite challenging. There are some methods to achieve this, for example, by using slot arrays [49] or by using cavity-backed patches [50; 51; 52; 53]. In most cases, the performances obtained by SIW antennas are lower compared to their rectangular waveguide counterparts [54; 55; 56; 57; 58]. However, substrate integrated waveguides are gaining a lot of attention for their fabrication of low profile and low-cost antennas for 5G applications [59; 60], where standard waveguides are not practical. In particular, the gain of the antenna is a trade-off since it increases linearly as the area of the antenna increases, while it decreases exponentially with the length of the transmission lines as a result of losses. In this view, for a given substrate, it is not convenient to make very big antennas because, above a certain size, the gain actually decreases with increasing dimensions. On the other hand, when the radiating slots and the BFN are on the same substrate layer, the crossing between them must be avoided. The radiating elements must  
Figure 16: Experimental isolation of the single-layer Magic Tee prototype shown in Figure 14.  
Figure 15: Experimental response of the single-layer Magic Tee prototype shown in Figure 14.  
**(a)** Reflection coefficients; **(b)** Transmission coefficients. be far enough from the discontinuities in order to avoid asymmetries in the radiation pattern. In this view, very good results can be obtained with the example illustrated in the following [61]. Since the band of the antenna was in the interval from 10.7 GHz to 12.7 GHz, the antenna must be squint-less over a very wide range of frequencies. This means that traveling wave antennas cannot be used, since they are affected by the variation of the squint of the main lobe as the frequency changes. Also, resonating subarrays made by several radiating elements cannot be used, since their band is limited. The basic idea to satisfy the specifications was the use of special radiating slots that offered a wideband response that was connected using a SIW BFN. The basic brick of the antenna was the 4 \(\times\) 2 element shown in Figure 17. The relatively high dielectric permittivity of the Taconic RF-35 substrate was exploited to shrink the network in such a way that the distance between two adjacent slots was less than half the wavelength (in the free space), thus avoiding the grating lobes. In order to obtain good radiation over a very wide band, the single radiating element was composed of two slots and a cylindrical metallic post (Figure 17). The two radiating slots were called \"active\" and \"passive\". The active slot resonant length depended on the offset with respect to the center of the waveguide [62]. The passive slot was positioned at the center of the waveguide. It could radiate thanks to the active slot and the metallic post, since they perturbed the field inside the SIW. The lengths of the slots were similar, but their resonant frequency was different because of the different offset from the center line [63; 64], thus widening the band with respect to the single radiating slot. The position of the small metallic post was then used to enhance the radiation from both slots. The 4 \(\times\) 2 arrays were then connected by using a single layer SIW network in order to realize a uniform array, thus generating a broadside main lobe. Figure 18 shows the built array that was composed of 32 \(\times\) 16 radiating slots. The final size of the antenna was 352 \(\times\) 352 mm\({}^{2}\). The antenna was designed using a standard a commercial simulator (Ansys HFSS) for the SIW network and a home-made software for the external part [65]. The latter takes into account the radiation from all the slots into the free space and the mutual coupling among them.