A substrate-integrated waveguide (SIW) (also known as post-wall waveguide or laminated waveguide) is a synthetic rectangular electromagnetic waveguide formed in a dielectric substrate by densely arraying metallized posts or via holes that connect the upper and lower metal plates of the substrate. The waveguide can be easily fabricated with low-cost mass-production using through-hole techniques, where the post walls consist of via fences. SIW is known to have similar guided wave and mode characteristics to conventional rectangular waveguide with equivalent guide wavelength. Since the emergence of new communication technologies in the 1990s, there has been an increasing need for high-performance millimeter-wave systems. These need to be reliable, low-cost, compact, and compatible with high-frequencies. Unfortunately, above 10 GHz, the well known microstrip and coplanar lines technologies cannot be used because they have high insertion and radiation losses at these frequencies. The rectangular waveguide topology can overcome these issues, as it offers an excellent immunity against radiation losses and presents low insertion losses. But in their classical form, rectangular waveguide is not compatible with the miniaturization required by modern applications. The concept of SIW was developed in the early 2000s by Ke Wu to reconcile those requirements. The authors presented a platform for integrating all the components of a microwave circuit inside a single substrate, with a rectangular cross-section. Using a single substrate guarantees a limited volume and a simplicity of manufacture, while the rectangular cross-section of the line provides the advantages of the waveguide topology in terms of losses.
Principles of SIW
Geometry A SIW is composed of a thin dielectric substrate covered on both faces by a metallic layer. The substrate embeds two parallel rows of metallic via holes delimiting the wave propagation area. The organization of the vias and the geometric parameters are described in the attached figure. The width of a SIW is the distance a {\displaystyle a} between its two vias rows, which is defined from center to center. An effective width a e {\displaystyle a_{e}} may be used to characterize more precisely the wave propagation. The distance between two successive vias of the same row is s {\displaystyle s} , and the vias diameter is denoted by d {\displaystyle d} .
Transverse magnetic propagation modes In classical solid-walled rectangular waveguide, the general formulation of propagation involves a superposition of transverse electric (TE) and transverse magnetic (TM) modes. Each of these is associated with particular fields and currents. In the case of TM modes, the current in the vertical walls is longitudinal, i.e. parallel to the propagation axis, usually denoted as z {\displaystyle z} . Then, given the vertical geometry of the vias, it is impossible for such modes to appear in SIWs: the electrical current cannot propagate from via to via. Only TE modes are able to propagate through SIW. Each mode appears above a precise cut-off frequency determined by the waveguide dimensions and the filling medium. For TM modes, decreasing the waveguide height (usually denoted as b {\displaystyle b} ) increases the cut-off frequency with 1 / b {\displaystyle 1/b} . In the case of SIW, the height is the thickness of the substrate, which is so low that the cut-off frequency of TM modes is much higher than the dominant mode.
Effective width One of the objectives of the SIW geometry is to reproduce the characteristic propagation modes of rectangular waveguides inside a thin template. The width a {\displaystyle a} of the waveguide is an essential parameter of those modes. In the typical SIW geometry, a {\displaystyle a} is the distance between the two vias rows from center to center (see figure). Due to the vias geometry, this distance cannot be used directly; because of the space between successive vias and their circular shape, the signal inside the guide does not behave exactly as it would in a perfectly rectangular waveguide of the same width. To apply waveguide theory to SIWs, an effective width a eff {\displaystyle a_{\text{eff}}} can be used. It takes into account the shape of the vias and the space in-between. Its value lies between a {\displaystyle a} and a − d {\displaystyle a-d} . A common simple definition is
a eff = a − d 2 0.95 s , {\displaystyle a_{\text{eff}}=a-{\frac {d^{2}}{0.95s}},}
and a more refined definition used for large values of d / a {\displaystyle d/a} is
a eff = a − 1.08 d 2 s + 0.1 d 2 a . {\displaystyle a_{\text{eff}}=a-1.08{\frac {d^{2}}{s}}+0.1{\frac {d^{2}}{a}}.}
… excerpt ends here. Continue reading the full article.
