Radio broadcast .. (1922-30)

When Broadcast Stations Interfere An Explanation of "Heterodyne" Interference Produced By Broadcast Stations — What the Department of Commerce Is Doing to Minimize the Difficulty — How the Listener Can Aid BY C. B. JOLLIFFE Physicist, Bureau of Standards more ihan a year, RADIO BROADCAST has been printing informative articles about bow various kinds of interference troublesome to the broadcast listener may be traced, reduced, or altogether conquered. Among the first of these was a series on "Man-Made Static" by A. F. Van Dyck, the first of which appeared in this magazine for April, 1924. In the July RADIO BROADCAST, there were two articles, one by John V . L. Hogan and the other by Dr. Alfred N. Goldsmith, which told how to use single-circuit receivers without annoyance to one's neighbors. This article, which is published by permission of the Director of the Bureau of Standards of the Department of Commerce, tells bow the steady squeals produced by any two broadcast stations which are heterodyning each other occur, and the efforts being made by the Department of Commerce to lessen this rather unusual form of interference. — THE EDITOR A TIMES, when tuning-in a broadcasting station, there is heard in the receiving set a whistling sound whose pitch (frequency) cannot be changed no matter what is done to the controls of the set. As the tuning adjustments are changed, the whistle reaches greatest intensity at one point on the dials and dies away gradually as they are turned from this tuning point. The fact that the note remains the same pitch distinguishes it from the whistle of varying pitch ("birdies") produced by your own or some other person's generating (oscillating) receiving set. If the tuning con'trols are turned slowly while one listens carefully it will usually be found that there are two stations which can be heard very close together when the whistle is at its maxi HOW BEAT NOTES ARE PRODUCED In No. i the curve A-A1 covers a range of from 795 to 805 kilocycles with its peak at 800 kc. Curve B-B1 with its peak at 80 1 kc. covers a band of 796 to 806 kc. The beat is equal to the difference of the two — in this case 1,000 cycles. In No. 2 we have a less pronounced example. Here the peak of C-C1 is at 820 kc. while that of D-D1 is at 828. Since the transmitted wave is assumed to cover a band 10 kc. wide it is obvious that there will be an overlapping. The difference here is 8 kc. or 8000 cycles. Example No. 3 shows no overlapping and no beat is produced. It is possible in some instances where there is overlapping that the difference is so great as to produce a beat above the frequency range which the human ear can hear mum loudness. These two transmitting stations are "beating" and producing the whistle. Let us take, for example, two stations that are on frequencies of 800 and 80 1, kilocycles per second (wavelengths 375, and 374.5 meters). Signals from both of these stations enter the receiving set and in addition to giving up to the set the messages (music, etc.) which they carry, the radio-frequency currents produced by the carrier waves combine and produce a note which has a frequency equal to the difference between the frequencies of the two received waves, in this case i ooo cycles per second. This is a highpitched whistle. Any two stations that are closer together than 3000 cycles will give a whistle which can be heard and which is very annoying. The frequency of the whistle