Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

,322 FREQUENCY MODULATION, Volume 1Some of the state police frequency-modulation systems have reportedserious skip interference on numerous occasions. In Missouri, on thetalk-back frequency of 39.78 megacycles, the interfering signals are usuallythose of the New Jersey State Police and the North Carolina HighwayPatrol Cars, although cars of the Ohio State Patrol and those of RhodeIsland occasionally cause interference. The signal strengths of the undesiredstations are greatest during May, June, and July and rangefrom weak to strong. The strong signals are of sufficient intensity toswamp out the local cars and may be received for an hour or two or forthe whole day, from about two hours after sunrise to an hour or so aftersunset.The Florida State Patrol have reported considerable interference onfrequency modulation from stations in California , New Jersey , Connecticutand Massachusetts, and they have made car-to-car contacts with Pittsfield,Massachusetts. The Michigan State Police reported that signalsfrom the Alabama State Patrol stations were received by their patrol carswith signal levels at the input to the receiver as high as 300 microvolts,and these stations in Alabama have taken control of their receiversthroughout Michigan for hours at a time.The Indiana State Police have had their cars blocked out by stationsin Virginia and Oklahoma for all cars more than three miles from the transmitter.During the hunt for escaped German war prisoners near Carlisle,Indiana, on June 10th, the interference was so bad they had considerabledifficulty maintaining contact with their cars.On June 22nd, during aman-hunt and road blockade following a bank holdup at San Pierce,Indiana,cars were completely blocked out at various times by cars inVirginia and Massachusetts. Further disruption of service was causedmany afternoons by the second harmonic of short-wave broadcast stationsin Massachusetts and New York.Recent observations by the Federal Communications Commissionshow that such bursts or sudden increases in strength of signals receivedbeyond the line of sight occur regularly. 14-15 The long-distance transmissionthat occurs during such bursts can be interpreted as reflections frommedia of height comparable to the E layer, but lying at each side of thegreat-circle plane. It is assumed that when meteors pass through theupper atmosphere, the air is ionized and this causes the bursts.If a local station is on the same channel as a distant one which is beingreceived in bursts, interference may be expected to occur for intervals as14"Measurement of V-H-F Bursts," Electronics, Vol. 18, No. 1, p. 105;January, 1945.15K. A. Norton and E. W. Allen, Jr., "Very-High-Frequency and Ultra-High-Frequancy Signal Ranges as Limited by Noise and Co-Channel Interference,"Pn:. T.R.E., Vol. 33, No. 1, p. 58; January, 1945.