This paper presents a measurement method for determining effective conductivity of copper-clad dielectric laminate substrates in the millimeter-wave region. The conductivity is indirectly evaluated from measured resonant frequencies and unloaded Q values of a number of Whispering Gallery modes excited in a circular disk sample, which consists of a copper-clad dielectric substrate with a large diameter of 20-30 wavelengths. We can, therefore, obtain easily the frequency dependence of the effective conductivity of the sample under test in a wide range of frequency at once. Almost identical conductivity is predicted for two kinds of WG resonators (the copper-clad type and the sandwich type) with different field distribution; it is self-consistent and provides the important foundation for the method if not for the alternative method at this moment. We measure three kinds of copper foils in 55-65 GHz band, where the conductivity of electrodeposited copper foil is smaller than that of rolled copper foil and shiny-both-sides copper foil. The measured conductivity for the electrodeposited copper foil decreases with an increase in the frequency. The transmission losses measured for microstrip lines which are fabricated from these substrates are accurately predicted with the conductivity evaluated by this method.

Microwave measurement methods necessary to characterize copper-clad dielectric laminate substrates are reviewed to realize more precise design of planar circuits: that is, the balanced-type circular disk resonator method for the relative complex permittivity in the normal direction εrn and tan δn, the cavity resonator method and the cut-off waveguide method for one in the tangential direction εrt and tan δt, and the dielectric resonator method for the surface and interface conductivity of copper foil σs and σi. The measured results of the frequency and temperature dependences of these parameters are presented for a PTFE substrate and a copper-clad glass cloth PTFE laminate substrate.