Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

THE SERVICE RANGE OF FREQUENCYMODULATION*!ByMurray G. CrosbyEngineering Department, RCA Communications, Inc.Summary— An empirical ultra-high-frequency propagation formula iscorrelated with experimentally confirmed frequency modulation improvementfactors to develop a formula for the determination of the relationbetween signal-noise ratio and distance in a frequency-modulation system.Formulas for the distance between the transmitter and the point of occurrenceof the threshold of frequency-modulation improvement, as well asformulas for the signal-noise ratio occurring at this distance, are alsodeveloped.Results of calculations and listening tests are described which evaluatethe signal-noise ratio gain obtained by applying pre-emphasis to the highermodulation frequencies of both amplitude and frequency modulation systems.Examples using a typical set of conditions are given and discussed.ITHAS BEEN shown that thepropagation characteristics of frequencymodulation are such as toconfine the use of this type ofmodulation insofar as telephony is concerned to the ultra-high frequencieswhich do not use the ionosphere as a transmission medium. 1Consequently, for most purposes, calculations of service range of afrequency modulation transmitter may be based totally on the propagationcharacteristics of ultra-high-frequency waves and in this paperthat base will be used. Formulas and empirical data now availablemake it possible to calculate the field strength at the receiver if transmitterpower, antenna heights, and distance are known.Consequently,the signal-noise ratio for amplitude modulation may be determined ifthe field strength of the noise is known. From this known amplitudemodulationsignal-noise ratio,the signal-noise ratio obtainable with agiven frequency-modulation system may be determined by multiplyingthe corresponding amplitude-modulation signal-noise ratios by the frequency-modulationimprovement factors as given inthe author's previouslypublished paper 2on frequency-modulation noise characteristics.It is the purpose of this paper to perform this correlation of thesepropagation and frequency-modulation improvement formulas anddevelop formulas so that from the known constants of the frequencymodulationsystem, the signal-noise ratio at a given distance may bedirectly calculated.* Decimal Classification: R630.ll.f Reprinted from RCA REVIEW, January, 1940.207