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The evanescent perturbation technique utilizing open-ended coaxial resonator probes was investigated as a nondestructive method for measuring the dielectric properties of insulating substrates in the microwave frequency region. As an investigative result, we have proposed a new formulaic method of data analysis by which the complex permittivity of samples, from changes in resonant frequency (f) and the quality factor of the resonance (Q), may be extracted in a concise and highly reproducible manner. The proposed formula has been developed based upon experimentation and detailed numerical studies of full-wave Maxwell equations coupled with physical observation and interpretation of experimental data. The new formula is applicable to both bulk and film samples with zero and finite tip-sample distances. The geometric factors derived were analyzed for variable parameters such as tip curvature, sample thickness, and tip-sample distance. Additionally, the calibration procedures necessary for experimental determination of these geometric factors were established.