FPC Directional Couplers Dielectric Laboratories / Knowles FPC Directional Couplers are high-frequency and surface-mountable. These devices incorporate DLI's high dielectric ceramic materials to provide a small size and minimal performance variation over temperature. These components are well-matched for monitoring incident and reflected power. Dielectric Laboratories / Knowles FPC couplers feature excellent repeatability and 50Ω characteristic impedance.