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Finite-ground coplanar (FGC) waveguide lines on top of polyimide layers are frequently used to construct three-dimensional Si-SiGe monolithic microwave/millimeter-wave integrated circuits on silicon substrates. Requirements for high-density, low-cost, and compact RF front ends on silicon can lead, however, to high crosstalk between FGC lines and overall circuit performance degradation. This paper presents theoretical and experimental results and associated design guidelines for FGC line coupling on both highand low-resistivity silicon wafers with a polyimide overlay. It is shown that a gap as small as 6 μm between two adjacent FGC lines can reduce crosstalk by at least 10 dB, that the nature of the coupling mechanism is not the same as with microstrip lines on polyimide layers, and that the coupling is not dependent on the Si resistivity. With careful layout design, isolation values of better than -30 dB can be achieved up to very high frequencies (50 GHz).