Design of coupled-line components with the Suspended-Strip Gap Waveguide at mm-wave frequencies

The so-called, Gap Waveguides are based on a periodic structure of squared metal pins that generates a high impedance condition over the pins surface. This effect leads to obtain a stopband of propagation whenever a metal plate is placed at a distance ha<λ/4 from the pins. In the case of the Suspended-Strip Gap Waveguide (SSGW), a propagation path is allowed through a metal strip placed over the pins, using an auxiliary dielectric substrate, Fig. 1 a). The fundamental mode of this waveguide is a quasi-TEM mode which propagates in the air region (concentrated between the strip and the top metal plate). Consequently, dielectric losses are avoided, which is an interesting feature for mm-wave operation. Furthermore, the presence of the periodic structure avoids the undesired parallel-plate modes and resonances usual when the circuit is packaged. All these advantages with respect to typical planar transmission lines have motivated the interest in this structure during the last years.