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Author: Edmund A. Laport
World Noise Zones and Required Minimum Field Strength for Commercial Telephone Communication
Natural atmospheric noise frequently is the limiting factor in radio communication. Until recent years the existing magnitudes of atmospheric noise were known only in a qualitative way. Recent systematic measurements made throughout the world have given some statistical values for such noise which is of basic engineering importance. More data on this subject are needed, and the available information will increase as the result of work that is now in progress.
Atmospheric noise varies throughout the day and throughout the yearly cycle. It also varies with frequency, being of greatest magnitude statistically at the lowest radio frequencies and decreasing as the frequency increases. An arbitrary system of designation for average noise levels of different values has been devised through international cooperation on the problems of noise cataloguing, and are indicated as various noise grades from 1 to 5. The world's noise zones, under this system of grading, are shown for the four seasons in Appendix VI-A, B, C, and D. In the absence of more specific data on particular locations, these figures may be used as a general guide to probable noise grades in any locality. Because of the fact that reliable radio communication depends upon the signal-to-noise ratio at the receiver, antenna engineers must be as interested in the subject of noise as they are in the power of their transmitters, because a 10-decibel rise in noise level is equivalent to reducing the transmitter power to one-tenth its original value. It is therefore of prime importance to make atmospheric noise measurements at locations where important receiving facilities are to be installed. Such measurements should be made in such a way as to reveal the diurnal and yearly range of noise field strengths in the parts of the radio spectrum that would be used for radio communication. The choice of a site for a receiving station must take noise into account always, although both atmospheric and man-made noise are involved in site selection.
Appendix VI-E to J is included here to show the average values of field strength required for standard double-sideband amplitude-modulated radio-telephony to secure 90 percent reliability with different frequencies for different noise grades at different hours of the day during different seasons. It is useful to replot these data curves for specific operating frequencies for any particular receiving location to obtain the necessary information in simpler form. It must be emphasized that these data apply only to atmospheric noise. If other sources of electrical noise are present near a receiving location, and such noise equals or exceeds the atmospheric noise, the required minimum received field strengths
for the signal, shown in these figures, must be increased in the same ratio as the extraneous noise exceeds the atmospheric noise. If locally received noise exceeds atmospheric noise at certain times by 10 decibels, the minimum required received field strength of the signal must be 10 decibels higher than shown by these figures. It is for this reason that extraneous noise picked up at a receiving site should preferably always be less than the minimum atmospheric noise in order that radio communication will not be limited more than nature permits.
It is evident from these data curves that atmospheric noise is a severe limitation on the lower radio frequencies, because relatively high field strengths are needed to give the same degree of communication reliability.
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