Loss Reduction Technique of Printed Transmission Line at Millimeter-Wave Frequency

Since surfaces of dielectric substrates for printed boards are usually roughened to make tight copper-coating, it was found out that an effective conductivity of a surface of the copper foil, attaching on the dielectric substrate, more degraded than that of an opposite surface of the copper foil, facing an air region, at frequency range beyond centimeter waves. To overcome such difficulty, a technique to reduce transmission losses was devised for printed transmission lines at millimeter-wave frequencies. By symmetrically-loading metal patterns on both sides of the dielectric substrate and by biasing equi-voltage to both metal patterns, the current density on the under surface of the metal pattern decreases, and thus the transmission loss is relatively unaffected by the roughness of the dielectric surfaces. Based on this consideration, a bilaterally metal-loaded tri-plate strip transmission line was fabricated and its unloaded Q factor was measured at 60 GHz, compared with that of a unilaterally metal-loaded tri-plate transmission line. The Q factor of the bilaterally metal-loaded tri-plate transmission line was measured to be 700, while that of the unilaterally metal loaded tri-plate strip transmission line was 200. From the theoretical and experimental investigation, effectiveness of the proposed technique was confirmed.