Coaxial Waveguide Methods for Shielding Effectiveness Measurement of Planar Materials Up to 18 GHz

The issue concerning the measurement of the shielding effectiveness (SE) of planar materials over a wide frequency range is of crucial relevance in several electromagnetic compatibility applications. This paper describes three different coaxial specimen holders for the measurement of the SE of thin metallic films over a nonconducting substrate or sandwiched between two insulating layers from a few kHz up to 18 GHz. Besides the well-known ASTM D4935 flanged coaxial cell, two novel versions of coaxial fixtures with an interrupted and continuous inner conductor are presented and compared. Their limits of applicability, advantages, and drawbacks are discussed with respect to frequency, sample characteristics, and test procedure. The analysis is performed by the use of simple equivalent circuit models, experimentally validated measuring thin copper films of different thicknesses which are deposited on kapton substrates by magnetron sputtering. It is demonstrated that the use of the three methods, properly combined, provides reliable SE results in the overall considered frequency range. It is also shown that the measurement of conducting films between two dielectric layers is critical at frequencies lower than some tens of MHz.