What is a Substrate-Integrated Waveguide?

A Substrate-integrated waveguide (SIW) is a dielectric-filled waveguide implemented by a set of two vias implanted within a dielectric substrate, that electrically connects the top and bottom walls. It is a modern-day transmission line that has created new possibilities to  design efficient circuits and components operating in the radio frequency (RF) and microwave frequency spectrum. It retains the benefits of microstrip, like compactness and ease of integration, while also retaining waveguide attributes, including minimal radiation loss, elevated Q-factor (operating point), and the increased power processing capacity. 

Figure 1. Substrate Integrated Waveguide (SIW) 

Construction 

The structure of the substrate-integrated waveguide is given in Figure 2 below. The physical geometry indicates the formation of an SIW by inserting a dielectric substrate in the middle of two opposite metal plates. The side walls of the SIW structure are achieved using two lines of regular metallic vias positioned adjacent to the lengths of the substrate.  

Figure 2. 
There are three fundamental design parameters that must be considered when designing the substrate-integrated waveguide to function at an assigned accuracy. They are, the width of the substrate-integrated waveguide w, the metal via diameter d,  and the separation length between the adjacent metal vias, i.e. the pitch p. The width relates to the cut-off frequency of the propagation mode of the substrate-integrated waveguide. Also, by reducing the value of p to d/2 reduces the SIW to a regular dielectric-filled waveguide.  

Selection of the substrate is considered vital for the structural and electrical performance in comparison to other types of transmission line structures including microstrips, coplanar, waveguides, etc. Therefore, it becomes important to select the right substrate for the application. 

Applications of SIW

Substrate-Integrated waveguides are used in various applications where high-frequency signal frequency signal transmission is required. They’re used in planar antennas with SIW feed networks. They’re also used in Bandpass and bandstop filters to pass or reject a specific band of frequencies. They’re used in couplers and power dividers to split or combine signals. 

SIW vs Traditional Waveguides 

Substrate-Integrated waveguides are typically smaller and lighter as compared to the traditional waveguides. They offer superior integration with other planar components and can be manufactured using standard PCB processes, while the traditional waveguides require more complex and costly fabrication methods. A downside of SIWs is that they exhibit higher losses than the traditional metal waveguides but lower losses than the other planer transmission lines.