A method of increasing the range of v.h.f. communication systems by multi-carrier amplitude modulation

The initial development of a method of extending the range or improving the coverage of v.h.f. communication systems of the type used for police services is described. The method is based on the simultaneous amplitude modulation of a number of carriers closely spaced in frequency. The frequency spacing between the carriers is so chosen that they lie within the band-width of the v.h.f. receiver, without producing audible interaction components of importance. Two-carrier schemes employing separate transmitters at the same site have been found to give improved coverage, while two- and three-carrier schemes using separate sites have been found to give greatly increased range. An unsuccessful attempt to achieve the same object with frequency-modulated (f.m.) transmitters using the same nominal carrier frequency is described. The difficulties of employing frequency modulation with common modulation are discussed and considered to be fundamental. Single-station v.h.f. schemes upon which the development of the multi-carrier scheme is based are briefly described.