Substrate integrated waveguide

Updated September 23, 2006

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New for October 2006! 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 this is a big deal!

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).

Advantages and disadvantages of SIW

The tradeoffs of any transmission line start with its attenuation characteristics. Read our page on transmission line loss, to become familiar with the four loss mechanisms _C, _D , _G and _R .

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 _C is lower.

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 _D . 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.

Equations

Coming soon!

Transition to microstrip

Coming later!