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An accurate radio-frequency and microwave attenuation-measurement technique with a small mismatch uncertainty for continuous-frequency application has been developed by shifting the phase characteristics of the multiple reflected signals. A loss-measurement system in four different phase networks is set for this purpose by introducing two low-reflective lines (airlines) where the phase-shift values are already known. From substitution-loss measurements carried out for variable attenuators as devices under test, the attenuation is simply obtained, which is unrelated to the source and load reflection coefficients of ΓG and ΓL, respectively. Experimental measurements on a 10-dB step attenuator performed with a 40-mm length of airlines at almost continuous frequencies from 12.3 to 14.1 GHz are demonstrated and show a good agreement with measurements where the test ports are impedance-matched.