# Review of Substrate Integrated Waveguide Circuits for Beam-Forming Networks Working in X-Band
## 1 Introduction  
In recent years, substrate integrated waveguide (SIW) [1; 2; 3] technology has gained more and more attention in the microwave community for its flexibility in the realization of microwave circuits [4].  
The propagation and dispersion characteristics of SIWs are very similar to those of rectangular waveguides so that the skills required for hollow waveguides can be used for SIWs. As a matter of fact, many devices designed for waveguide technology can easily be built using substrate integrated waveguides. Moreover, SIW devices are manufactured using standard printed circuit board (PCB) procedures, making them simpler to build, lighter, and less expensive than a waveguide device built with a standard process such as milling. The fact that SIW devices are realized using a dielectric substrate allows for an easy integration with other planar circuits, for example based on microstrip technology. The presence of the dielectric also allows for a shrinking of the dimensions with a scale factor proportional to the square root of the relative dielectric permittivity.  
Of course, the presence of a dielectric inside the waveguide increases the losses, depending on the loss tangent of the material used. Losses are also affected by the relatively low height of the substrate waveguides, which increases the conductor losses.  
Dielectric losses can be reduced by using substrates with low loss tangent, though these substrates are more expensive. Conductor losses can also be decreased by using thicker substrates, thus affecting the size, weight, and cost of the device. These problems are mitigated for wide-band devices and signal-level circuits, where additional losses can be compensated by the use of amplifiers.  
Another drawback of SIW devices compared to standard waveguide ones is the limited power handling resulting from the reduced height of the waveguide.  
In this contribution, a review of devices designed and built for X-band applications is presented. More specifically, the devices have been designed to be used in satellite digital video broadcast (DVB-S) applications as an alternative to existing dish antennas. The goal is to have a series of devices that arethinner, lighter, and have less visual impact, compared to dish antennas. In order to keep the cost of the appliances low, particular attention must be devoted to the choice of the substrates. Very low-cost FR4 substrates are not suitable for microwave devices, since there are high losses and big uncertainties about its dielectric permittivity. Other substrates, such as Rogers RT/duroid@5870/5880, are very good at microwave frequencies and exhibit very low dispersions of characteristics over different samples. However, they are very expensive, and they are also not suitable for the application under examination. There are some mid-level substrates, such as the Taconic RF-35, used in nearly all the prototypes illustrated in this review that are particularly suited for big antennas. They are relatively low-cost, and they show very good behavior in terms of their losses and their uniformity in electrical parameters over samples.  
This situation can change in the future by the advent of new technologies, such as metamaterials [5; 6; 7; 8; 9; 10], that promise better characteristics thanks to artificial materials.  
The manuscript is divided as follows. In Section 2 some fundamental theories about SIWs are presented, and in Section 3 a number of devices for beam-forming networks are illustrated. In Section 4 an array antenna for DVB-S application is designed and measured. Finally, in Section 5 a new fabrication process for empty SIWs is proposed in order to build devices that are not affected by dielectric losses.