Multi-station v.h.f. communication systems using frequency modulation

The reasons for the use of multi-station V.H.F. systems of communication are discussed, and an historical survey of previous technical experiments on F.M. systems is given. The standard equipment normally used for a single-station scheme is described and its technical performance stated. From the basis of this standard equipment the required performance of multistation equipment is derived. It is shown that one of the most important factors for such schemes is that the main carrier frequencies should be nearly identical and the permissible difference of frequency determined. Linking frequencies available for a control link are discussed and, in conjunction with the permissible frequency difference, the basic design factors of the equipment are evaluated. Details of a particular type of equipment are described, this equipment being that used for the initial experiments and for a completed installation. The factors affecting distortion caused by multipath transmission are discussed and a quantitative analysis made of the distortion occurring under the worst conditions. From this analysis the maximum permissible path difference for reasonable speech quality is determined, this being ±30 miles and this is shown to agree with practical tests. The practical tests carried out in the London area and in Scotland are described. Possible future developments of such systems are indicated.