Page "government" Paragraph 614

The 20-to-1 ratio for cochannel interference embodies one of the fundamental limiting principles which we must always take into account in AM assignments and allocations -- that signals from a particular station are potential sources of objectionable interference over an area much greater than that within which they provide useful service.

A second fundamental principle is that involved particularly in the present proceeding -- the difference between nighttime and daytime propagation conditions with respect to the standard broadcast frequencies.

This is a phenomenon familiar to all radio listeners, resulting from reflection of skywave signals at night from the ionized layer in the upper atmosphere known as the ionosphere.

All AM stations radiate both skywave and groundwave signals, at all hours ; ;

but during the middle daytime hours these skywave radiations are not reflected in any substantial quantity, and during this portion of the day both skywave service and skywave interference are, in general, negligible.

But during nighttime hours the skywave radiations are reflected from the ionosphere, thereby creating the possibility of one station's rendering service, via skywave, at a much greater distance than it can through its groundwave signal, and at the same time vastly complicating the interference problem because of the still greater distance over which these skywave signals may cause interference to the signals of stations on the same and closely adjacent frequencies.

Because of the difference between daytime and nighttime propagation conditions, it has been necessary to evolve different allocation structures for daytime and nighttime broadcasting in the AM band, with many more stations operating during the day than at night.

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