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 which 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 consists 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.
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
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
Each mode appears above a precise cut-off frequency determined by the waveguide dimensions and the filling medium. For TM modes, decreasing the waveguide thickness (usually denoted as
One of the objectives of the SIW geometry is to reproduce the characteristic propagation modes of rectangular waveguides inside a thin template. The width
To apply waveguide theory to SIWs, an effective width
A common simple definition is[1][2]
and a more refined definition used for large values of
Using this effective width, the propagation constant of a SIW will be similar to that of a classical rectangular waveguide whose width is
SIWs are promising structures that can be used in complex microwave systems as interconnects, filters, etc. However, a problem may arise: the connection of the SIWs with other kinds of transmission lines (TL), mainly microstrip, coplanar and coaxial cable. The goal of such transitions between two different topologies of TL is to excite the correct transmission mode in the SIW cavity with the minimum loss of power and on the broadest possible frequency range.
Rapidly after the presentation of the concept of SIW by Ke Wu, two different transitions were mainly used.[4][5] First, the tapered transition allowing to convert a microstrip line into a SIW and secondly a transition between a coplanar line and a SIW (see attached figure). The tapered transition from microstrip to SIW is useful for thin substrates. In this case, the radiation losses associated with microstrip lines are not too significant. This transition is massively used and different optimizing process have been proposed.[6][7] But this is not applicable to thick substrates where leakages are important. In that situation, a coplanar excitation of the SIW is recommended. The drawback of the coplanar transition is the narrower bandwidth.
These two kinds of transitions involve lines that are embedded in the same substrate, which is not the case for coaxial lines. There exists no direct transition between a coaxial line and a SIW: an other planar line have to be used to convert properly the coaxial TEM propagation modes to the TE modes in SIW.
Several studies have been carried out to optimize the transition between topologies without being able to determine a universal rule making it possible to draw the absolute transition. The architecture, the frequency range, the used materials, etc. are examples of parameters that make specific the design procedure.[1][8][9][10]
The propagation constant of a transmission line is often decomposed as follow :
and the oscillating electric and magnetic fields in the guide have the form[11]
This decomposition is valid for all kind of transmission lines. However, for rectangular waveguides, the attenuation due to radiations and substrate conductivity is negligible. Indeed, usually, the substrate is an insulator such that
The SIWs show comparable or lower losses compared the other traditional planar structures like microstrip or coplanar lines especially at high frequencies.[1] If the substrate is thick enough, the losses are dominated by the dielectric behavior of the substrate.[13]
Part of the signal attenuation is due to the surface current density flowing through the metallic walls of the waveguide. These currents are induced by the propagating electromagnetic fields. These losses may also be named ohmic losses for obvious reasons. They are linked to the finite conductivity of the metals: the better the conduction, the lower the losses. The power lost per unit length
It can be shown that in a classical rectangular waveguide, the attenuation of the dominant mode
It is noticeable that
On the top and bottom horizontal metallic plates, the current is scaled with
Another key point of the conduction losses experienced by the SIWs is linked to the roughness of the surfaces that may appear due to the synthesis processes. This roughness decreases the effective conductivity of the metallic walls and subsequently increases the losses. This observation is of crucial importance for the design of SIWs as they are integrated on very thin substrates. In this case, the contribution of the conduction losses on the global attenuation is predominant.[1][13][15]
The attenuation due to the dielectric behavior of the filling medium can be determined directly from the propagation constant.[11] Indeed, it can be proven that, making use of a Taylor expansion of the function
The dielectric losses
Because the vertical walls of the SIW are not continuous, radiation leakages may flow between the vias. These leakages can significantly affect the global transmission quality if the vias geometry is not chosen carefully. Some studies have been conducted to describe, predict and reduce the radiation losses. They have resulted in some simple geometric rules that have to be satisfied in order to reduce the radiation losses.[3][4][14][16][17]
The geometric parameters of interest are the diameter
The content is sourced from: https://handwiki.org/wiki/Engineering:Substrate_integrated_waveguide