Design of an ultra-wideband coplanar strip-to-parallel stripline transition using an analytical model based on conformal mapping

A new design method of an ultra-wideband coplanar strip (CPS)-to-parallel stripline (PSL) transition is presented. The proposed design is based on an analytical transition model obtained by conformal mapping. The proposed transition consists of a CPS-to-asymmetric PSL transition and an asymmetric PSL-to-PSL transition. The conductor shapes of the transition at top and bottom sides of the substrate are adjusted to obtain the optimal impedance matching and smooth field transformation. For measurements, the proposed transition is designed in a back-to-back configuration. To verify the proposed design, two transitions with different substrates are designed and fabricated. The PSL impedance is set at 50 Ω, but the CPS impedances are set at 147 and 120 Ω, respectively. The measured insertion loss of <1 dB and return loss of >10 dB have been obtained for more than 30 and 20 GHz bandwidths, respectively.