Abstract: A jammer comprises a frequency modulated transmitter for jamming, a receiver and a mixer for the receiver. One input of the mixer is connected to a receiver antenna circuit and a record input is connected to the output of the transmitter, the carrier of which is frequency modulated according to an arbitrary function of time. The receiver is tuned to receive a frequency including side bands which lie within one range of beat frequencies produced by the mixer, varying in dependence on the transmitter modulation wave. An indicating device indicates the presence of output signals of the receiver unit in dependence on the carrier frequencies received which lie within approximately the range of frequencies transmitted by the transmitter.