The theory of a single directional-coupler reflectometer having two auxiliary tuners is reviewed, giving emphasis to precision-measurement possibilities.Problems in setting up this system for measurements in coaxial line are discussed, and a system for measurements in the frequency range 3.95–5.85 Gc/s is described.The development of λ/4 short-circuit reflection standards and sliding terminations is presented.The results of measurements of the reflection from connectors and various loads at 4Gc/s are given.