Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

;276 FREQUENCY MODULATION, Volume 1tered which were of sufficient magnitude to cause partial or completecancellation of the waves at one or more frequencies in a 150-kilocyclewidechannel as used by frequency-modulation broadcast stations.large number of field tests have been made since that time, and theyshow that this trouble can occur in nearly any location in Manhattanas well as in many of the surrounding towns in northern New Jersey.In midtown Manhattan there are many steel buildings which causemultipath transmission. If the receiver is inside such a structure, anoutside antenna is desirable. If such an antenna is not practicable, itprobably will be necessary to use more than one inside receivingantenna in order to obtain full performance on the high-fidelity programsfrom all the New York frequency-modulation stations. This willbe true even if the receiver has the best performance characteristicsnow commercially available. For a single stationary antenna, distortionwill probably be noticed in many locations on at least one of the stations.Cases have been observed where the movement of a person aroundin the room near the receiving antenna has changed the relative fieldstrengths enough so that, when the person was in some parts of theroom, the reception was satisfactory, and when he was in other partsit was unsatisfactory. A parasitic element such as a metal rod ordipole can sometimes be moved around in a room to produce the sameeffect. Maximum distortion occurs at the time of cancellation of thetwo waves.This causes a great reduction in the voltage in the intermediate-frequencychannel and it may fall below the threshold of limitingin the receiver.If the cancellation occurs near one edge of the band, the signal maysound fairly good during the soft passages but it becomes distorted onthe loud passages.It will then sound like an overloaded audio-amplifierstage. If the cancellation occurs near the middle of the channel, theentire output may be so distorted that it becomes almost unintelligibleit then sounds somewhat like severe selective fading in amplitudemodulation. Since high-order harmonics are produced, the distortionmay appear as buzzes, rattles, or swishes, and if it occurs during theplaying of a phonograph record, one may think the pickup is not trackingproperly. The high frequencies often tend to become irritatingdue to intermodulation.Two amateurs 1 have reported distortion of the programs fromWMIT when the receiver is in the mountainous region near Asheville,N. C. At times they found that the audio signal was badly distorted tothe point of becoming unintelligible, even for strong signals,Aand that1A. D. Mayo and Charles W. Sumner, "P. M. distortion in mountainousterrain," QST, vol. 28, pp. 34-36; March, 1944.