Millimeter-wave broadband mode-transition between grounded coplanar waveguide and postwall waveguide

In this paper, a broadband mode-transition between grounded coplanar waveguide (GCPW) and post-wall waveguide (PWW) is proposed. The transition is composed of GCPW, microstrip line (MSL) and PWW. The transition is fabricated on liquid crystal polymer (LCP) substrate due to its low dielectricity and availability of roll-to-roll process for fabrication of devices with cost reduction. The GCPW GSG pad has two through-holes at longitudinal direction and a rectangular slot are added in each pattern of ground pads. Broadband impedance matching is obtained by these structures thanks to an addition of lumped inductance and capacitance to the mode-transition. In the mode-transition between MSL to PWW, a shape of microstirp line is tapered and dimension is optimized for broadband operation. A 24% impedance bandwidth for |S₁₁| less than -15 dB is achieved in measurement of fabricated mode-transition. Loss of GCPW GSG pad and MSL-PWW transition with 0.120 dB and 0.276 dB at 60 GHz are evaluated from measured results. This mode-transition is one of candidate for usage of low loss antenna-in-package with substrate at higher frequency band.