Abstract: A monitor for detecting faults in the output signals of a transmitter in which a selected modulation component of the signal is used to activate fault-detecting means. In the case of a DVOR beacon system, the sub-carrier frequency is filtered from the detected and amplified signal obtained by a monitor antenna and applied to a full-wave rectifier, and the fault detecting means responds to the absence of one or more of the unidirectional pulses thus produced.