Evaluation of Conductivity of Sandwich-type Dielectric Substrates in Millimeter Wave Region and Comparison with Copper-clad Dielectric Substrates

This paper presents a measurement method for conductivity of sandwich-type dielectric substrates in millimeter wave region. In this method, the conductivity is evaluated from the measured resonant characteristics of Whispering Gallery modes in a circular disk sample. The method has an advantage for millimeter wave measurement in terms of the size of resonators with the diameter is about 20~40 times of wavelength. The WG mode resonances can be arisen periodically in order of the mode number in a wide frequency band. We present the measured conductivity of the sandwich-type dielectric substrates with three types of copper foils and compare with the results of copper-clad dielectric substrates in 50- 65 GHz band. The result of conductivity of electrodeposited copper foil is 53% to 60% of conductivity of pure copper and the results of rolled copper foils and both side shinning copper foils is 63% to 71%. These results are almost equal to the results obtained by the use of copper-clad resonator.