Abstract: Measuring apparatus for providing a reading representing a magnetic field component or the electric field component of incident radio frequency electromagnetic radiation comprises a set of magnetic field sensors and a set of electric field sensors interchangeably connectable to a measuring device which has an indicator instrument and an optical output to which can be connected a repeater device by means of an optical fibre cable which thus does not exert a perturbing influence on the field being measured. The sensors are omnidirectional balanced devices in which the detector elements are constituted by suitably shaped metallic film deposits on dielectric substrates, and can be interchanged by a simple plug-in connection which also holds the sensor physically in position on the measuring device.