This paper proposes a jig structure for a large-diameter coaxial-feed type stepped cut-off circular waveguide to enable accurate measurement of permittivity for liquids with good repeatability in the MHz band. The jig combines an N connector and a metal sample holder with an inner diameter slightly larger than that of the connector flange’s outer conductor. The accuracy of S₁₁ evaluation under short termination conditions was improved by this construction. To verify the validity of the proposed structure, several jig sets and short metal rods for S₁₁ calibration were made. S₁₁ was measured multiple times using the VNA when the jig tip was short, open, and with a reference material inserted (here, pure water), with methanol and ethanol as unknown liquids. The permittivity of each liquid was estimated by substituting the measured S₁₁ value above into a formula involving comparison with SOM termination (short/open conditions and a known material). The liquid estimation results obtained were compared with those evaluated based on an inverse problem via MMT (mode-matching technique), with results showing good agreement, simple and stable evaluation with little variation over multiple dielectric evaluations at low frequency.