Excitation of coupled slotline mode in finite-ground CPW with unequal ground-plane widths

The coupling between the desired coplanar-waveguide (CPW) mode and the unwanted coupled slotline mode is presented for finite-ground CPWs with unequal ground-plane widths. Measurements, quasi-static conformal mapping, and finite-difference time-domain analysis are performed to determine the dependence of the slotline-mode excitation on the physical dimensions of the finite-ground coplanar line and on the frequency range of operation. It is shown that the ratio of the slotline mode to the CPW mode can be as high as 10 dB. Air-bridges are shown to reduce the slotline mode by 15 dB immediately after the air-bridge, but the slotline mode fully reestablishes itself within 2000 mum of the air-bridge. Furthermore, these results are independent of frequency