ABSTRACT
This chapter is concerned with natural extremely low frequency (ELF)/very low frequency (VLF) noise; however, man-made noise, in particular power line harmonic radiation, is of great significance. It concentrates on providing typical examples of the major radio noise statistics for the ELF/VLF range: average amplitudes, voltage deviation, antenna noise factor, and some amplitude probability distributions. The envelope amplitude probability distribution (APD) is widely accepted as a useful tool for the analysis of radio noise, and it is particularly useful for estimating the interference to communication systems caused by this noise. E. C. Field and M. Lewinstein developed an analytical model for ELF and VLF APDs that goes a long way toward providing a theoretical basis for the form of the APD plots, including the knee observed in many of the APDs. The model explicitly illustrates the effect of earth-ionosphere waveguide propagation on the APDs, and it appears to be in good agreement with most of the currently available APDs.
