# Review of Substrate Integrated Waveguide Circuits for Beam-Forming Networks Working in X-Band
## 5 New Trends (Empty SIW)  
One of the recent trends in SIW technology is the so-called empty substrate integrated waveguide (ESIW) or air-filled SIW [66; 67]. In practice, because of the high losses of the dielectric, even when a very expensive one was used, this solution created an empty waveguide inside a PCB in order to reduce losses and maintain the other characteristics of SIWs. There were some attempts to build empty SIWs in the literature. For example, in [68], the structure was made by stacking printed boards between two metallic covers, but this solution was not advantageous over a device made by a machined block of aluminum or copper. In [69], another solution was proposed. However, a part of the EM field was inside a dielectric material, so that the dielectric losses were not negligible and the advantage of the solution was limited. Very recently [70; 71; 72; 73; 74; 75] other solutions have been proposed, but they used screws to close the final prototype. For example, in [71] a sandwich of three substrates was proposed, where the middle one was machined in order to create the empty waveguide. The upper and lower substrates can be made of low-cost material, such as FR4, whereas the middle layer was made of more expensive material (Rogers RT/duroid 6002(r)). Moreover, the authors did not specify how the sandwich of layers was glued. Also, many transitions between planar waveguides (i.e., microstrips or coplanar waveguides) and empty SIW have been designed [76; 77; 78; 79; 80; 81]. The devices obtained by those methods were produced by using different processes, so they were not suitable for mass production. Moreover, the use of screws did not ensure the perfect planarity of the device and increased the size and weight, thus weakening the advantages of SIWs. Figure 19: Measured radiation pattern of the antenna shown in Figure 18 [61]: E-plane (blue line); H-plane (green line). Figure 20: Squint of the main lobe of the antenna shown in Figure 18 as a function of the frequency [61]: (a) E-plane; (b) H-plane. In order to overcome these problems, a solution to build empty waveguides embedded in a single dielectric layer has been presented in [82]. With this solution, low loss and low-cost devices can be made, while maintaining the compatibility with standard PCB processes. The waveguide was built by using a stack of two substrates (Figure 21). The main substrate was thicker (2 mm in this case) and it was milled to build the waveguide. Then, a layer of copper was deposited by using chemical deposition or electroplating, depending on the thickness needed. Of course, the width of the milled waveguide depended on the thickness of the deposited copper in order to obtain the desired cutoff frequency. Above the main substrate a thin layer of substrate (250 um thick), metalized on both sides, was attached by using a no-flow prepreg. The prepreg itself was worked so as to open some apertures of the same size of the waveguides in such a way to avoid the field inside the dielectric and then to reduce losses. This gluing process was done at high pressure and high temperature, and it was exactly the same used in the production of multi-layer PCBs. It is worth noting that the dimensions of the holes in the main substrate and in the prepreg were chosen in a suitable way in order to compensate any variations as a result of the gluing process. This reduced the dielectric inside the waveguides, thus reducing losses and improving the repeatability. Since the prepreg was non-conductive, there was a discontinuity between the main substrate and the cover. The electrical continuity between lower and upper substrates was then guaranteed by using two rows of metalized via holes along the guide, similar to SIWs (Figure 22). Looking at Figure 22, it was apparent that the EM field was mostly inside the empty guide with only a minimal part inside the prepreg. Moreover, the field was not inside the dielectric of the main and secondary layers.