Measurement uncertainty has a direct impact on the reliability of test instruments. To determine if there is a quantifiable difference in measurement uncertainty between the TDR and VNA, W. L. Gore & Associates performed a series of experiments, initially testing six cable assemblies in controlled conditions on each instrument. The instruments’ measurement uncertainty under best-case scenario was evaulated using the highest-performing assembly. To ensure TDR/VNA test parity, the VNA’s performance was evaluated using a s11 one-port reflection method as well as the more traditional s21 two-port transmission method.