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Substrate integrated waveguide
(SIW) is an new form of transmission line that has been popularized
in the past few years by some researchers. New transmission lines
only come along once in a lifetime, so pay attention, this is a
SIW is shown in the HFSS model
above. A rectangular guide is created within a substrate (usually
a soft board) by adding a top
metal over the ground plane and caging the structure with rows of
plated vias on either side. To an EM wave, if everything works out,
it looks like a dielectrically-filled rectangular waveguide, with
reduced height compared to the "normal" 2:1 width:height
ratio. Reduced height is no big deal, it just reduces the impedance
the wave sees (increases capacitance/length).
and disadvantages of SIW
The tradeoffs of any transmission
media line start with its attenuation characteristics. Read our
page on transmission line loss,
to become familiar with the four loss mechanisms C,
One attraction to SIW is that
the amount of metal that carries the signal is far greater than
it would be in microstrip or stripline. Therefore conductor loss
One potential disadvantage if
SIW is that leakage losses can be substantial. This is related to
how tight the vias are spaced. This means that R
is a non-zero term.
Another disadvantage is that
by introducing a dielectric into the guide (compared to air in "normal"
rectangular waveguide) you now have introduced dielectric losses
. This term is proportional to frequency, so the application of
SIW at millimeter-wave needs to look at this term carefully. Chances
are that losses due to conductivity of the substrate G
will be close to zero if you chose a good substrate. If you try
this trick on high-resistivity silicon,
let us know how much you lose due to conduction!
Because it is a waveguide, SIW
exhibits a lower cut-off frequency.
Update November 2010!
This new material came from Sinan, a Masters student at Bilkent
University in Ankara, Turkey. Thank you, sir!
Substrate Integrated Waveguide
In high frequency applications,
microstrip devices are not efficient, and because wavelength at
high frequencies are small, microstrip device manufacturing requires
very tight tolerances. At high frequencies waveguide devices are
preferred; however their manufacturing process is difficult. Therefore
a new concept emerged: substrate integrated waveguide. SIW is a transition
between microstrip and dielectric-filled waveguide (DFW). Dielectric
filled waveguide is converted to substrate integrated waveguide
(SIW) by the help of vias for the side walls of the waveguide.
Figure 1: (a) Air filled waveguide, (b) dielectric filled waveguide,
(c) substrate integrated waveguide
Because there are vias at the
sidewalls, transverse magnetic (TM) modes do not exist; TE10
therefore is the dominant mode. There are many articles for designing
substrate integrated waveguides, however there are missing parts
in many of them, they just give the known equations but they do
not carefully investigate those equations. In this report, the tolerances
of the published SIW design equations are inspected.
2. SIW Design Equations
SIW devices can
be thought as a form of dielectric filled waveguide (DFW), therefore
the starting point can be DFW. For TE10 mode, the dimension
"b" is not important as it does not affect the cut off
frequency of the waveguide. Therefore the substrate can be at any
thickness; it only affects the dielectric loss (thicker=lower loss).
Figure 2: Dimension
definition of rectangular waveguide
For a rectangular waveguide, cut off frequency of arbitrary mode
is found by the following formula:
c: speed of light
m, n: mode numbers
a, b: dimensions of the waveguide
For TE10 mode, the
much-simplified version of this formula is:
For DFW with same cut off frequency,
dimension "ad" is found by:
Having determined the dimension
"a" for the DFW, we can now pass to the design equations
d: diameter of the via
p: pitch (distance between the vias)
Figure 3: Dimensions
for DFW and SIW
In published articles about SIW design, the following two conditions
are required 
(guided wavelength) is: 
3. Investigation of the Equations
In this part, we
investigated (5) and (6) equations and the geometry in Figure 4
is used for testing. The substrate is Rogers 3003, 10 mil with =3.0.
We designed a SIW at Ka band. As seen from Figure 5, pitch and diameter
are measured from the centers.
Figure 4: Geometry
of SIW for testing
Figure 5: Diameter
Ka band is used at 26.5-40 GHz and for the maximum case (40 GHz),
we investigated the results. At 40 GHz, =
179 mil, therefore maximum d is 35.8 mils. Diameter sweep results
of CST are shown in Figure 6 and pitch sweep results are in Figure
Figure 6: Results for d=30, 40, 50 mil and p=1.5d
Figure 7: Results for p=20, 30, 40, 50, 60, 80 and d=20 mil
As seen from Figure 6, when the
diameter increases, the bandwidth narrows from the higher frequency
side. The result for d=50 mil can be inspected in this figure.
As seen from Figure 7, when the
pitch increases, the bandwidth narrows from the higher frequency
side and for higher values of p, the response is distorted. The
responses for p=60 and 80 should be inspected.
When we investigated the results
of Figures 6 and 7, it is clear that these design equations work
well, however they are not obligatory for designing SIW. They can
be used as initial design equations and after the first design they
can be optimized if the frequency range to be used is different.
These equations are necessary for finding the equivalent waveguide.
 K. Wu, D. Deslandes and Y.
Cassivi, "The Substrate Integrated Circuits - A New Concept for
High-Frequency Electronics and Optoelectronics," TELSKIS 2003,
Nis, Serbia and Montenegro, pp. Oct. 2003.
 J. E. Rayas-Sanchez and V.
Gutierrez-Ayala, "A General EM-Based Design Procedure for Single-Layer
Substrate Integrated Waveguide Interconnects with Microstrip Transitions", IEEE MTT-S Int. Microwave Symp. Dig., Atlanta, GA, Jun. 2008,