Abstract :
The application of frequency modulation to multi-channel v.h.f. radio links for mainland-to-island communication and for use across broad estuaries is discussed. As the radio links form part of the trunk telephone network of the country, a high standard of performance and reliability is essential, and the equipment has to be capable of continuous unattended operation. To facilitate connection with the trunk network the system is designed to accommodate up to twelve channels, each of 4 kc/s bandwidth, in the frequency range 60¿108 kc/s. The radio links at present in use operate in the frequency range 45¿80 Mc/s, the frequency deviation being ± 300 kc/s. The factors determining the signal/noise ratio are discussed, and it is shown that most applications permitting optical paths between transmitter and receiver aerials require radiated powers of about 10 watts when using frequency modulation, whereas, with amplitude modulation, 100 watts are needed. An additional advantage of frequency modulation is that the inter-channel crosstalk is not a function of valve-characteristic curvature; it is pointed out, however, that when using frequency modulation it is essential that the phase/frequency characteristic of the system should be substantially linear if the interchannel crosstalk is to be reduced to an adequately low level. Typical transmitting and receiving equipments are described, and their individual performance characteristics given. Finally, the performance of such equipment on optical and long non-optical links is discussed.
