Substrate-integrated waveguide
The waveguide can be easily fabricated with low-cost mass-production using through-hole techniques, where the post walls consists of via fences.
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.
But in their classical form, rectangular waveguide is not compatible with the miniaturization required by modern applications.
[1] The concept of SIW was developed in the early 2000s by Ke Wu to reconcile those requirements.
[1][2] 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.
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.
One of the objectives of the SIW geometry is to reproduce the characteristic propagation modes of rectangular waveguides inside a thin template.
The formulas given above are empirical: they were established comparing the dispersion characteristics of different SIWs to those of rectangular waveguide filled with the same dielectric material.
[5] 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.
The most common terms are the following:[11][12] This decomposition is valid for all kinds of transmission lines.
However, for rectangular waveguides, the attenuation due to radiations and substrate conductivity is negligible.
In the same way, if the wall thickness is much thicker than the skin depth of the signal, no radiation will appear.
This is in fact one of the advantages of closed waveguides compared to open lines such as microstrips.
[13] Part of the signal attenuation is due to the surface current density flowing through the metallic walls of the waveguide.
This can be explained keeping in mind that this ohmic losses are determined by integrating the current density on a path enclosing the waveguide walls.
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.
[4][15][13] The attenuation due to the dielectric behavior of the filling medium can be determined directly from the propagation constant.
consists in choosing a template with better dielectric properties: the lower the loss tangent
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.
They have resulted in some simple geometric rules that have to be satisfied in order to reduce the radiation losses.
They must be tuned in such a way to approximate the behavior of a continuous metallic wall: the spacing of the vias has to remain small compared to their diameter, while the diameter must be small compared to the waveguide guided wavelength (
