Bilaterally Metal-Loaded Tri-Plate Transmission Line (BIT Line) As a Low-Loss Printed Transmission Line at Millimeter-Wave Frequencies

New guiding structure for millimeter-wave integrated circuits was proposed in this paper. Since surfaces of dielectric substrates for printed boards are usually roughened to make tight copper-coating, it was found out that effective conductivities of surfaces of the copper foils, attaching on the dielectric substrate, more degraded than those of the opposite surfaces of the copper foils, 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 currents on the under surfaces of the metal patterns decrease, and thus the transmission loss may be 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, namely conventional 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, and thus the capability to realize low-loss printed millimeter-wave ICs was confirmed by using the proposed transmission line.