This report describes procedures for automated measurements of Q-factor that are based on a Vector Network Analyser (VNA). An efficient algorithm (NLQFIT) for fitting Q-factor and resonant frequency to swept-frequency complex S-parameter data is outlined. This can be applied to measurements by transmission and by reflection, and can fit the length of uncalibrated line between calibration reference planes and couplers as an option. A weighting scheme to improve measurement precision when the VNA

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... uses a linear frequency sweep is also described. Listings of computer code for Matlab and Python implementations of NLQFIT are given in an appendix. Experimental studies of Q-factor measurements made using both transmission and reflection methods are presented. Aspects that are reported on include measurement repeatability, VNA calibration, measurements using an uncalibrated VNA, simulations of the effect of uncalibrated line sections, traceability, and the effect of varying the VNA power-level setting. The evaluation of the uncertainty associated with Qfactor measurements is studied. An overview of methods for determining dielectric loss angle and the associated uncertainty from measurements of the Q-factor of resonators is presented.