Planar sensors for dielectric and magnetic materials measurement: A quantitative sensitivity comparison

Planar transmission lines are frequently used to characterize the RF properties of materials. However, the question arises which geometry should be chosen for optimal measurement sensitivity to the material under test. Thus far, this question appears to go unanswered. In this paper, the suitability of the three most popular planar geometries is compared for material characterization. To this end, the impact of a material under test on the apparent properties (i.e. the equivalent homogeneous cross-sections) is examined. This is done for the complex permittivity and the complex permeability, using conformal mapping methods, full-wave simulations and measurements. It is shown that the coplanar waveguide (without conductor backing) is the most suitable structure of the three, since it is the most sensitive to changes in the properties of the material under test.