Abstract: Phase modulation apparatus particularly suitable for coherent radar applications wherein an interrupted carrier-wave is phase modulated according to a predetermined coded sequence. Apparatus comprises a carrier wave generator having an output for connection to an aerial, a delay line suitably coupled at one end to the output of the generator, and a modulation signal generator for controlling the reflecting actions of a reflector at the other end of the delay line so that signals travelling in the delay line are reflected with a phase-shift dependent on the signal produced by the modulation signal generator. The carrier-wave generator is switched on and off for transmit and receive periods respectively and when switched on its output signal will build up from noise levels injection-phase-locked to a delayed version of the previous transmitted signal, thus preserving phase coherence between successive transmitted signals.