Two or three conductor coplanar transmission lines and lossy coplanar resistive films are formed on a surface of a substrate. The resistive film dimensions and resistivity are selected to suppress various spurious electromagnetic modes within and around the substrate. The resistive films may be positioned along the outer edges of the transmission lines or between the transmission line conductors. The resistive film may have regular spaced openings for producing an average resistivity different than that of a continuous resistive film. In one embodiment, a signal conductor has a serpentine shape and resistive film elements are positioned between adjacent sections of the signal conductor. In another embodiment, interdigitated resistive film elements extend between transmission line conductors.