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This paper provides formulas for evaluating RF spectral shape and interfering carrier distortion in connection with FM trunk radio systems having low modulation ratios. Numerical results are given for 1800-speech-channel systems. The curves showing interfering carrier distortion are of particular interest in connection with systems using RF multiplex (i.e., systems consisting of a number of carriers each modulated with a signal built up from a large number of speech channels). Preferred carrier spacings, originally determined for 600-channel systems, have been recommended by the CCIR 9th Plenary Assembly, 1959, for use with up to 1800 channel systems. It appears from the analysis that with these larger numbers of channels, a considerable amount of attenuation by filtering or by isolators will be required to give adequate protection against intermodulation distortion.