**1. Introduction**

Microwave devices are generally designed using metallic waveguides and planar technology. Traditional metallic waveguide is low insertion loss (IL), and high-quality factor (Q value), but the components are bulky and nonplanar, which cannot be integrated into RF integrated circuits. Planar technology (microstrip transmission lines or coplanar waveguides) could be easily integrated, but the insertion loss and Q value are worse. At higher frequencies (>30 GHz), planar structures are prevented to apply due to high transmission losses [1].

To tackle the above-mentioned problems, the concepts of SIW are proposed [2, 3]. The SIW is considered a quasi-waveguide, developed by two parallel copper sheets with rows of conducting cylinders embedded in a dielectric substrate on each side to connect the two plates, as shown in **Figure 1**. When the parameters of the design conform following rules, SIW can be equivalent to a conventional rectangular waveguide [4].

$$s > d \tag{1}$$

$$
\kappa/\lambda\_{\epsilon} < 0.25 \tag{2}
$$

$$l/k\_o < 1 \times 10^{-4} \tag{3}$$

$$s/\lambda\_{\epsilon} > 0.05\tag{4}$$

Where s is the gap between the adjacent vias, d is the diameter of a via, *λ<sup>c</sup>* is the cutoff wavelength, *l* is the total loss, and *ko* is the wave number in a vacuum.

**Figure 1.** *Three-dimensional view of SIW.*

In this way, a rectangular metallic waveguide filled with a dielectric material is constructed in planar form, the most key advantage, thus making a complete integration with other planar transmission-line circuits on the same substrate. Meanwhile, SIW structures maintain most of the benefits of classical metallic waveguides. Thus, SIW is a suitable choice for less lossy, compact, and simple microwave and mm-wave systems.

After 20 years of development, SIW has been used to design all kinds of passive circuits, such as filters, power dividers, switches, phase shifters, diplexers, and crossovers, and presents high performance. At the same time, many techniques have been developed in recent times. This chapter provides a brief, yet necessary, understanding of current research on the SIW components.
