# A Review on SIW and Its Applications to Microwave Components
## 5 Losses in Substrate-Integrated Waveguides  
The key losses linked to SIW components are associated with three loss mechanisms including conductor loss, dielectric loss, and radiation loss [25; 26; 27; 28]. The conductor loss is linked to the metallic walls limited conductivity; the dielectric loss is linked to the substrate material loss tangent; while the radiation loss is linked to the outflow of EM energy over the regular openings in the dual paths of metallic vias [15; 29]. The conductor loss and the dielectric loss that take place in substrate-integrated waveguide components are like those that occur in rectangular waveguide components. The radiation loss, conversely, is particularly related to the substrate-integrated waveguide and is because of the regular openings in the dual paths of metal vias inserted in the dielectric substrate. One main issue in the design of a substrate-integrated waveguide component is the capability to efficiently diminish the losses, especially if the component is designed to function over the millimetre-wave band [29]. The substrate-integrated waveguide conductor loss is comparable to the conductor loss in the traditional waveguide component, and this is because of the limited conductivity of the metal edges. In [25], it was shown that a substantial decline in losses linked to conductors may be accomplished through improving the substrate depth while maintaining the remaining sizes in the substrate-integrated waveguide structure. A decrease in pitch \(p\) may also lead to a decline in conductor loss owing to the improved metallic coat [25]. SIW conductor loss may be marginally reduced by increasing the size of the metal via diameter. Attenuation constant \(\alpha_{c}\), owing to the substrate-integrated waveguide conductor loss at a specific frequency, \(f\), may be investigated based on Equation (4) [30]; \(h\) is the dielectric substrate depth, \(\sigma_{c}\) is the metallic wall conductivity, \(f_{0}\) is the SIW cavity cut-off frequency, \(\varepsilon_{0}\) stands for the free space dielectric constant, \(\varepsilon_{r}\) stands for the substrate material dielectric constant, and \(w_{\mathit{off}}\) stands for the SIW cavity effective width. As Equation (4) was initially designed for evaluating the conductor loss attenuation constant in the traditional waveguide, the effective width given in Equations (2) or (3) is used as an alternative to  
Figure 5: Some popular transitions between planar transmission lines and substrate-integrated waveguides: (**a**) microstrip-to-SIW transition based on a simple taper; (**b**) CPW-to-SIW transition based on a current probe; (**c**) CPW-to-SIW transition based on a ninety-degree bend [14]. the original width of the substrate-integrated waveguide [30]. This is to ensure that the equation is accurately adapted in the design of SIW components. \[\alpha_{c}(f)=\frac{\sqrt{\pi f\epsilon_{0}\epsilon_{r}}}{h\sqrt{\sigma_{c}}} \quad\frac{1+2{(f_{0}/f)}^{2}h/w_{eff}}{\sqrt{1-{(f_{0}/f)}^{2}}} \tag{4}\]  
The dielectric loss in substrate-integrated waveguide devices is also similar to the dielectric loss in conventional waveguide devices. The loss is entirely reliant on the dielectric substrate material loss tangent and not the physical structure of the substrate-integrated waveguide [14]; provided the depth of the substrate, \(h\), is reduced in size when compared to a half-wavelength; \(h\) will have no impact on the dielectric loss [31]. The dielectric loss is usually the greatest provider of deficiencies in millimetre-wave components; therefore, a cautious choice when selecting the appropriate substrate material is of paramount significance. Mathematical formulation developed for the computing the dielectric loss attenuation constant in the traditional waveguide may be utilised in evaluating the dielectric loss attenuation constant \(\alpha_{d}\) in the substrate-integrated waveguide.