| Summary: | The thesis describes a technology which can make good use of the U.K. private mobile radio allocation of the VHF Band 47-68 MHz, known as Band I, despite the fact that the allocation is as a secondary user and propagation disturbances must be expected. A detailed description of the variable nature of the propagation of radio signals in Band I is given as well as results of continuous observation of signals in the band over several years. In particular it is the phenomenon known as Sporadic-E which gives strong over-the-horizon propagation, lasting several minutes, or on occasions hours. The likely origin of Sporadic-E is to cause the powerful European primary TV signal transmissions to appear in the mobile radio allocation, coming from distances particularly between 800-2600 km. Likewise a radio system using the band could interfere with TV receivers far removed in distance. A simulation of the central station and of the mobile unit with two TV interfering stations shows that channels with least interference or causing least interference are situated between the colour carrier and the sound carrier of either TV signal. This thesis proposes and then goes on to describe an adaptive system aiming to use the least interfered part of the spectrum. A central radio scans the band for interfering TV (television carriers) and then decides on the optimum channels from a set of channels. A mobile station will ask for service through the control channel(s) by which the information of the optimum channel set is received from the central station. The mobile then switches to a recommended optimum channel and operates as a normal private mobile radio, for example, using 12 1/2 kHz FM channelling. The central radio continues to monitor the band status, because Sporadic-E can arise quite unannounced. The circuit technology is described in full with in particular details of a rapidly settling synthesizer under control of a microcontroller. Both a central station and mobile units are described.
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