Accurate measurement of the electrical length of long coaxial radio-frequency cables

The electrical length of long coaxial cables can be difficult to measure accurately, because of the weakness of the reflected signal and the presence of stray reflections from discontinuities in the cable. A method has been devised which makes use of a modulated reflector, a tuned hybrid network and a phase-sensitive detector to determine the phase of the reflected signal to within 0.5°, even when this is as much as 60 dB weaker than the transmitted signal.