Microwave dispersion and attenuation of coplanar waveguides on high-resistivity silicon substrates with insulation layer are characterized by optoelectronic measurements directly on the wafer. Freely positionable photoconductive switches are employed for ultrashort electric pulse injection. The electric pulses on the coplanar waveguides are sampled electro-optically in time domain. Both, attenuation and dispersion of microwaves are significantly altered by the insulation layer over the whole investigated frequency range from 10 GHz to 600 GHz. We observe a significant increase in microwave absorption for bias conditions that lead to accumulation or inversion at the Si/insulator interface.