Low-loss and low-cost solution for printed transmission lines at millimeter-wavelengths by using bilaterally metal-loaded tri-plate transmission line

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, beyond centimeter frequencies. The transmission loss therefore degrades due to the roughness of the copper foil surface. On the other side, it is confirmed that the current density on the under surface of the copper foil decreases by symmetrically-loading metal patterns on both sides of the dielectric substrate and by biasing an equi-voltage to both metal patterns, and thus the transmission loss is relatively unaffected by the roughness of the dielectric surfaces. Based on this consideration, a bi-laterally metal-loaded tri-plate transmission line was evaluated by using a cheap FR-4 substrate, being poor material for use as millimeter-wave lengths but being cost-effective.