A series of measurements in urban, suburban and city-centre areas has been conducted at frequencies in the range 40 MHz to 900 MHz. Analysis of the detected envelope of man-made noise in this frequency range has shown that notwithstanding the different experimental conditions, there is a general decrease in the level of noise as the monitoring frequency increases. At lower frequencies external noise is dominant but this is not the case at u.h.f. There is no evidence of a change in the fundamental character of manmade noise as frequency changes.