Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

PROPAGATION CHARACTERISTICS 99Fd/Fm since sin a/2 enters in the quantity Z in the same manner asdoes the quantity F d/F,„. However it can also be seen that the distortionis zero when a is equal to 0, 2w, -ir, etc. Thus the most severe distortionoccurs when a = ir radians or 180 degrees. Since a = 2irDFm ,where D = timc delay between the two paths, for a given time delaysin a/2 will go through maximums and minimums as the modulationfrequency is varied. Hence bands of maximum and minimum distortionwould be expected throughout the modulation frequency band.By an application of the same theoretical method, the distortiondue to three or more paths could be determined. The addition of a wavearriving over the third path to the resultant given by (4) would addanother arc-tangent term to the phase of the resultant in which Rwould be equal to the ratio between the resultant amplitude of thefirst two paths and the amplitude of the third. This would add distortionwhich would result in a wave form undoubtedly more distortedthan the two-path case. However, the degree of distortion would beproportional to the same parameters as the two-path case since the distortionterm added by each path issimilar to that produced by thefirst two paths.The distortion encountered by amplitude modulation in multipathtransmission has been theoretically considered in previous literature. 13The general results of this theory are that the distortion consists of achange of the amplitude of the fundamental modulation frequency togetherwith the introduction of second harmonic distortion. However,the higher order harmonic distortion so prone to appear in frequencymodulation, and depending upon the ratio F d /F m , is not present. Itis this extreme high order harmonic distortion which makes the receptionof frequency modulation over a multipath medium far moredistorted than the reception of amplitude modulation over the samemedium.ConclusionsThe general conclusion derived from the tests and theory is that oncircuits where multipath transmission is encountered, frequency modulationis impracticable. This, of course, precluded the use of frequencymodulation on frequencies which are transmitted by refraction fromthe Kennelly-Heaviside layer. For the frequencies in the immediatevicinity of eighteen megacycles, where transmission is largely by a13Charles B. Aiken, "Theory of the detection of two modulated waves by alinear rectifier," Proc. I.R.E., vol. 21, pp. 601-629; April, (1933). The "Case ofIdentical Modulating Frequencies" considered on page 616 covers exactly thesame situation as the two-path multipath transmission case.