there is disclosed an apparatus for measuring the strength of an electromagnetic field . the apparatus comprises a plurality of antennas arranged such that the field is received from all directions , each antenna supplying an rf signal as output . the apparatus also comprises a plurality of transforming modules , each of said modules being fed with the rf signal supplied by one of the antennas , each of said modules transforming the impedance of the rf signal it receives as input , so as to supply another rf signal as output . the apparatus also comprises a plurality of converting modules , each of said modules being fed with the rf signal supplied by one of the transforming modules , each of said modules converting the rf signal it receives as input into a dc current it supplies as output , said dc current varying substantially as a logarithmic function of the said rf signal .