A study on the microwave performance of a metallic transmission line capable of simultaneously transmitting microwave and optical signals is presented targeting millimeter-long interconnects. Conventional analytical solution is used to find the optimal structure for a given characteristic impedance. Then, functionality of the link is validated through S-parameter measurements for 3–13 mm long lines. The waveguide parameters, such as resistance, inductance, capacitance, and conductance are extracted based on a lumped circuit model. The modeling enables structure optimization for interconnect bandwidth density of 1 Gb/s/μm and more.