Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

MULTIPATH DISTORTION 277the distortion is a constant phenomenon over periods of weeks in thosespots where it occurs. DuMont and Goldsmith 2 found that multipathconditions are common in which two signals of approximately equalstrength arrive at a receiving antenna, and therefore this type of distortionwith frequency-modulation sound transmission can be expectedto occur frequently.A similar type of distortion had previously been encountered inlong-distance transmission. In 1930, Eckersley 3 was working withamplitude-modulation transmitters which had incidental frequencymodulation. Because of this frequency shift he found "most appallingdistortion" resulting from delayed echoes caused by deflections fromthe Heaviside layer. He found that it was necessary to use specialprecautions to keep any frequency shift from getting into the amplitude-modulationtransmitters. The distortion when frequency modulationand amplitude modulation were both present was worse than thatfor amplitude modulation only.A study of frequency-modulation propagation over long distanceswas made by Crosby. A 4 transmitter was set up in Bolinas, California,and the transmissions were observed on receivers at the Eiverhead,New York, station. He found that there was considerable distortion,and that in some cases a signal which gave fair intelligibility on amplitudemodulation was practically unintelligible on frequency modulation.He stated, "The general conclusion derived from the tests andtheory is that, on circuits where multipath transmission is encountered,frequency modulation is impracticable." In a later paper, 5 he againreported similar distortion. Other studies 67 have shown that reflectionsfrom the ground and nearby buildings cause multipath transmission,for both horizontal and vertical polarization, which can introducedistortion.Reflections from airplanes flying overhead sometimes causeinterference for short intervals.The same difficulty is encountered in television reception. Thehigher frequencies used tend to increase the difficulty,changes encountered are greater.since the phaseThis causes light and dark bands in2Allen B. DuMont and Thomas T. Goldsmith, Jr., "Television broadcastcoverage," Proc. I.R.E., vol 32, pp. 192-205; April, 1944.3 T. L. Eckersley, "Frequency modulation and distortion," Exp. Wirelessand the Wireless Eng., vol. 7, pp. 482-484; September, 1930.4Murray G. Crosby, "Frequency-modulation propagation characteristics,"Proc. I.R.E., vol. 24, pp. 898-913 ; June, 1936.5Murray G. Crosby, "Observations of frequency-modulation propagationon 26 megacycles," Proc. I.R.E., vol. 29, pp. 398-403 ; July, 1941.6P. S. Carter and G. S. Wickizer, "Ultra-high-frequency transmissionbetween the RCA building and the Empire State Building in New YorkCity," Proc. I.R.E., vol. 24, pp. 1082-1094; August, 1936.7R. W. George, "A study of ultra-high-frequency wide-band propagationcharacteristics," Proc. I.R.E., vol. 27, pp. 28-35; January, 1939.