The aim of this Tech. Mem. is to produce an expression for the field strength of an interfering signal which provides a 3 dB loss of signal to noise ratio in an FM channel delivering 46 dB4W in the absence of interference. The FM channel is taken as having 75 kHz peak programme deviation and 15 kHz audio bandwidth. The receiver aerial is assumed to be a half wavelength dipole for the purpose of the calculation.