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1986 Edition, January 1, 1986

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The RF spectrum of frequency-modulation sound-broadcasting transmitters

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The RF spectrum of frequency-modulation sound-broadcasting transmitters is infinite in theory. In practice, the need may arise to have a precise knowledge of either the necessary (No. 146 of the Radio Regulations) or the occupied bandwidth (No. 147 of the Radio Regulations). For example, only when the occupied bandwidth is known, is it possible to plan for acceptable service in adjacent bands.

Rules for the calculation of the necessary bandwidth of emission for numerous services are laid down in the Radio Regulations and examples are given in Appendix 6 to these Regulations. According to these rules the necessary bandwidth of frequency-modulation emissions

Measurement results particularly when obtained for stereophonic emissions, differ greatly from the calculated values. [n the light of Report 418 it is rather doubtful whether Carson's rule may be used in bandwidth calculations for stereophonic emissions, since the pre-supposition that D» fmax is not met, and since consequently. a value K ≠1 may have to be used.

Difficulties are also experienced when calculating the occupied bandwidth and also the out-of-band emissions. For example no reference to out-of-band emissions in FM sound broadcasting is contained in Recommendation 328 "Spectra and bandwidths of emissions", although out-of-band emissions are defined there. Even Reports 275 and 324, which are quoted in the Recommendation and which deal with measurements of bandwidths of emissions. do not include any useful information with respect to this question. Measurements of the frequency spectrum of transmitters which are frequency-modulated with white noise are described in Report 977. However, the results of these measurements do not permit conclusions to be made with regard to the radio-frequency spectrum of frequency modulation sound-broadcasting transmitters. [t was with this in mind that the measurements described below were performed [CCI R, 1982-86a].