Abstract: A method for the detection of a source generating electro-magnetic oscillations wherein for the purpose of eliminating disturbances caused by foreign transmitters there is measured the field intensity at at least two spatially separate locations, the determined measurement values are compared with one another, and when the difference of both measurement values exceeds a value or amount dependent upon the spacing between the measurement locations there is triggered an alarm signal.The installation for the performance of the aforesaid method comprises a receiver and a first antenna and a second antenna. An antenna reversing switch may be provided for connecting the receiver into circuit with the one or the other antenna. An alternate embodiment provides a second receiver, wherein each antenna is connected with one of the receivers. A comparator serves to compare the signals generated in the receiver or receivers and which signals are dependent upon the fields acting upon the antennae.