Conductivity measurements in the 10–40 GHz band using Fabry–Pérot open resonator

Abstract

In this paper we introduce a novel approach to measurement of electric conductivity of bulk, metallic samples. The method is based on a Fabry–Pérot Open Resonator operating in the plano-concave configuration. Because the resonator supports multiple TEM0,0,q modes, the proposed method provides properties of the sample-under-test at multiple frequencies in the band of 10 to 40 GHz with separation of ca. 1.5 GHz between consecutive frequency points. Contrary to other methods, it does not require that a sample is cut to any particular shape, as long as it is significantly larger than the TEM-mode beam waist and thicker that the penetration depth. It also provides high measurement accuracy that is comparable to alternative methods based on single-mode resonant setups and superior to the broadband transmission methods known in the literature. We demonstrate that the open resonator due to its relatively high Q-factor reaching 135000 can be successfully used in measurements of different samples of conductivity ranging from ca. 103 S/m (e.g. laminates based on carbon-fiber) up to the order of 107 S/m (e.g. aluminum alloys, copper). Using the measurement set-up we also demonstrate an interesting effect of strong frequency dependence of the effective conductivity of stainless steel. It is expected in all magnetic materials and results from large complex permeability of such samples. It is important to emphasize that no similar characterization campaigns are to be found in the literature.