Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FREQUENCY-MODULATION DISTORTION CAUSEDBY MULTIPATH TRANSMISSIONSBymurlan s. corringtonHome Instrument Department, RCA Victor Division,Camden, N. J.Summary— When a frequency-modulated wave is received by more thanone path, so that two or more of the voltages which are induced in theantenna have nearly the same amplitude, considerable distortion can result.Large objects, such as hills or high buildings, can reflect and absorb thewaves and thus cause interference. Two interfering waves will be in phasepart of the time and out of phase part of the time during modulation. Thiscauses amplitude modulation on the resultant carrier, and a sharp irregularityin the instantaneous frequency, corresponding to each hole in thecarrier caused by the interference.The distortion can often be greatly reduced by detuning the receiveror discriminator until the hole in the carrier coincides with the zero-balancepoint of the discriminator. Directional antennas are helpful when thesignals from the desired station are not coming from the same direction.Formulas are derived for the modulated envelope and for the distortion.A Fourier-series analysis of the distorted audio output makes it possibleto calculate the effects of de-emphasis networks and other audio selectivity.DURINGIntroductiona demonstration of a frequency-modulation receiver inNew York in the spring of 1942, difficulty was experienced inlocating an antenna in the RCA Building which would producea signal of quality comparable with that known to be transmitted bystation W2XWG. The antenna was located in a room in this steel buildingon the side away from the transmitter.The distortion was presenteven on relatively small deviations.The tests were repeated in an apartment house on 93rd Street, andsimilar distortion was encountered. Further tests were made laterwith a second frequency-modulation receiver to determine whether thedistortion was caused by a defective receiver or was introduced duringthe transmission of the signal.of distorted output.Both receivers produced the same typeSince intermediate-frequency amplifier selectivitywould not cause this effect, it was suggested that it might be caused bymultipath reception. During July and August of 1942, further observationson a qualitative basis were made on the eighty-fifth floor of theEmpire State Building. Path differences due to reflections were encoun-* Decimal classification: R113.110XR148.2.t Reprinted from Proc. I.B.E. December, 1945.275