Abstract: A ceramic or other substrate is tested for dielectric constant K and loss tangent by placing it on a central transverse plane across a cylindrical waveguide. A swept-frequency signal is injected into the waveguide at an input coupling loop and is picked up at an output coupling loop. Maximum transmission through the dielectric substrate occurs at a frequency that depends on the waveguide radius, the substrate thickness, and the dielectric constant. The dielectric constant can be obtained from the resonant frequency of a predetermined transmission mode, e.g., the TE.sub.01 mode. The loss tangent can be calculated from the transmission bandwidth. The measurement of the dielectric constant is insensitive to the position of the substrate in the gap between waveguide sections, and thus intimate contact is not required.