Radio Broadcast (May-Oct 1922)

6 RADIO BROADCAST interference troubles is the use by diflferent stations of different frequencies of wave vibration. Radio signals, whether telephonic and in the form of music and spoken words, or telegraphic and in the form of Morse dots and dashes, are carried through space across the earth's surface by waves in the ether. Except for their frequencies or rates of vibration, these radio waves are identical with light waves; they pass through the same ether of space which conveys light waves from the sun to the earth and from a candle to our eyes. Like the flash from an exploding bomb, these radio waves rush outward from a wireless sending station at the almost inconceivable speed of 186,000 miles per second; some of them reach listeners at receiving stations (just as some of the bomb-flash strikes the eyes of onlookers) but the greatest part is lost in space. Unlike light waves, however, the radio waves are completely invisible to us; they are passing through us and around us constantly, but our bodies contain no sensory organs which can detect them. Their frequencies of vibration are so low that the waves cannot produce any physiological effect such as would cause the sensation of light or heat, although the character of the radio waves is exactly that of light waves or heat waves except in the one respect, namely, the number of vibrations per second. What is the range of frequency of these radio signal waves which we can neither see nor feel? What number of vibrations in one second is too small to set up the sensation of illumination or that of warmth? Curiously enough, these radio frequencies (while far lower in the scale than light or heat) are away beyond the rates of which we usually think. An airplane motor rotates 3,500 revolutions per minute; the middle-C string of a piano vibrates 256 times in one second; everyday radio waves oscillate at the tremendous rates of from 20,000 to 3,000,000 times in one second. The fact that radio waves may have any frequency from above 3,000,000 to below 20,000 per second opens the way to a great increase in the number of wireless stations which may operate at the same time in the same geo Receiver Accepts Receiver Excludes | Receiver Excludes 400 375 360 345 METERS WAVELENGTH 750 760 770 780 790 800 810 820 830 840 850 860 870 880 890 900 ^ FREQUENCY IN THOUSANDS OF CYCLES PER SECOND A chart giving an approximate idea of the way in which four transmitting stations of high character will distribute their power in space, if they are of equal power and if they are all equally distant from a receiving station. Each of these stations has its total energy confined to a band of 10,000 cycles, their basic wave frequencies being respectively 750,000, 800,000, 833,000 and 870,000 cycles. These frequencies correspond closely to wavelengths of 400, 373, 360 and 345 nieters. The accepting and excluding action of a tuned receiver is indicated by the heavy line above the chart. If this receiver accepts radio waves only within a range of 5,000 cycles, or 2,500 cycles above and below its central frequency, no difficulty would be experienced in receiving exclusively from anyone of the four transmitters indicated, with no interference from any of the others. In fact, when adjusted to frequencies intermediate to those of the sending stations (for example 770,000 or 81 5,000 or 830,000 cycles) nothing would be heard