Strip-Line Methods for Dielectric Measurements at Microwave Frequencies

Strip-line methods for measuring dielectric constants and loss tangent of sheet insulation, particularly of strip-line laminates, offer advantages for laboratory study and routine quality control of microwave dielectric properties at frequencies above 1 GHz. Theoretical and practical problems are discussed including possible strip circuit test patterns, effects of air gaps, correction for fringing field, adaptability to different dielectric thicknesses, and the relation of the measured loaded Q to both the unloaded Q of resonant strips and the dissipation factor of the dielectric period. Band-stop and band-pass filter test fixtures and measuring arrangements are described which are suitable for laboratory measurements or, when the filter circuit is made a permanent part of the fixture, for rapid and precise quality control testing. Dielectric properties measured at X-band (8-12 GHz) are given for a number of materials. Data are also included on the variability of copper-clad glass cloth-reinforced polytetrafluoroethylene (PTFE) and the dependence of apparent dielectric constant on clamping pressure and test temperature.