Excitation of Coupled Slotline Mode in Finite Ground CPW with Unequal Ground Plane Widths

The coupling between the desired CPW mode and the unwanted coupled slotline mode is presented for finite ground coplanar waveguides 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 FGC 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 18 dB. The use of airbridges is shown to reduce the slotline mode by 15 dB, but that the slotline mode fully reestablishes itself after 2000 microns. Furthermore, these results are independent of frequency.