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A direct and efficient method for determining the complex permittivity of materials at microwave frequencies using a Split Ring Resonator (SRR) metamaterial structure is presented. A single SRR unit fabricated on a substrate arranged between transmitting and receiving probes acts as a test probe. Dielectric samples having at least one flat surface of area greater than or equal to the area of SRR structure are used as test samples. The relative permittivity and the loss tangent of the dielectric are evaluated from the resonant frequency shift and the bandwidth of the SRR resonator, by placing the sample over it. The LC resonance of the SRR test probe is theoretically related to the permittivity by considering its equivalent circuit in terms of the dielectric filled capacitances formed on the upper and lower surfaces of the SRR. The permittivity measurements are performed using test probes of different resonant frequencies, and the results are compared with the values obtained by the cavity perturbation method. [ABSTRACT FROM AUTHOR]