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This paper presents a method for measuring the complex permittivity (relative dielectric constant ε'r and dielectric loss tangent tan δ) of sheet materials in the 1-15 MHz range, as part of efforts to achieve low dielectric loss. We were able to measure tan δ of 2×10-4 at low frequency by employing the following procedure. (1) The coil is immersed in liquid nitrogen, LN2. (2) Coupling of the resonating coil to the input-output coil is decreased to less than -30 dB. (3) A network analyzer is used to measure frequency and Q value. (4) Low-loss material is used as the standard to calibrate the capacitance of the measurement circuit. (5) A Litz wire coil is used to measure permittivity.