The paper describes a type of directional coupler called a reflectometer used for connecting test equipment to a radio-frequency transmission line. A single loop of wire with a resistor in one arm is mounted at a fixed point and is loosely coupled to both the electric and the magnetic fields of waves in the line. The relative phase of the two fields for a travelling wave depends upon the direction of propagation, and the couplings with these fields are arranged to cancel for one direction of propagation and add for the other. Thus the coupling will detect, or launch, waves travelling in one direction only.This type of coupler was used originally in America to measure the reflected waves in a 400-Mc/s coaxial feeder: hence the name “reflectometer.”1, 2In the present paper the general theory is given, and a design suitable for waveguides at a wavelength of 10 cm is described. Applications to the monitoring of standing waves and transmitted power in a radar transmitter system and to the testing of radar receiver sensitivities are considered, and the methods which make use of a reflectometer are shown to have considerable advantages over others.