Effects of Insulating Dielectric Layers in Millimeter-Wave Planar Transmission Lines

Insulating dielectric layers added to waveguiding structures to reduce the conductor losses at high frequencies complicate the structure and usually introduce dominant mode crossing and leakage. A qualitative study is used to derive a design constraint that ensures that insulated structures fully utilize the conductor loss reduction available from the insulating dielectric layers. A numerical example is provided in which it is shown that in certain cases arbitrary dielectrics cannot be used for insulating layers.