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The frequency response of dielectric properties of coolant liquids used in the gyrotron RF window is measured. FC-40, FC-75, and CCl4 are considered for the measurements due to their reported application in gyrotron and excellent dielectric properties required for high-power RF transmission. A performance probe kit based on the reflection method is used for measurements in wide frequency range from 200 MHz to 50 GHz. Vector network analyzer (VNA) is used as the RF source as well as receiver. The dielectric constant and loss tangent are calculated directly by using the scattering matrix data obtained from the VNA. Further, ac conductivity and relaxation time are also computed by using measured data of loss tangent and dielectric constant.