Carrier power requirements for long-distance communication by microwaves

Formulae are derived for the path attenuation between two aerials of given effective surface areas, or two parabolic aerials, for conditions of free-space propagation. Abacs for the case of identical transmitting and receiving aerials enable the attenuation to be found for distances up to 100 miles and wave-lengths from 1 to 300 cm. Output s./n. ratios for multiplex telephony and for television and receiver noise factor, are considered, with particular reference to the Federal Telecommunication Laboratories 23-channel pulse-time-multiplex equipment Type 10B, and to f.m. and p.c.m. systems. Comparison of corresponding f.m. and p.c.m. systems shows the latter to require 17 db less power (0.1 W instead of 5 W in a particular case), but f.m. is the better under conditions of deep fading. Taking all factors into account, however, the comparison becomes more and more in favour of p.c.m. as the number of repeaters in the transmission chain is increased.