Open planar resonators like single and stacked microstrip resonators were used in the past for the measurement of dielectric constants and thicknesses of lossy and lossless dielectrics at microwave frequencies [1–3]. With a large width, the microstrip resonator effectively acts as a planar antenna in which case the fringing field is significant for the two slots at the two ends of the resonator and the resonator Q-factor is low. One of the limitations of the microstrip resonator is its spatial resolution which is determined by the size of the resonator. It is envisaged that this problem can be overcome by the use of planar slot resonators, Fig.1. Furthermore, compared to the microstrip-fed microstrip resonator, a microstrip-fed planar slot resonator would provide a better isolation between the feed and the material under test.