Distribution and Frequency Dependence of Unintentionally Generated Man-Made VHF/UHF Noise in Metropolitan Areas. Part II-Theory

The frequency and range dependence of unintentionally generated man-made VHF/UHF noise in metropolitan areas may be interpreted by assuming that it is composed of random impulses which have been attenuated by propagation over irregular terrain. Man-made noise power per cycle of bandwidth measured for the frequency interval 150 to 500 Mc/s is shown to possess decrements of -10.4 dB/decade change in frequency and -17.9 dB/decade change in range. Theoretical computations of the frequency and range decrements based on a random impulse, irregular terrain model yield frequency and range decrements of -11.1 to -11.3 dB/frequency decade and -17.3 to -18.3 dB/range decade which are in good agreement with the measured results. Comparison of the measured VHF/UHF decrements for unintentionally generated man-made noise in urban and suburban areas with those derived from data which were predominantly of automotive ignition origin supports the conclusion that automotive ignition interference is the major source of man-made noise at these frequencies.