KILOCYCLES AND METRES

Radio Record, Volume I, Issue 26, 13 January 1928, Page 2

KILOCYCLES AND METRES

A SIMPLIG WXPLANATION. In rearranging the licenses of stations lately, the United States I'ederal Radio Comumission, like all technical men, has laid special emphasis on kilocycles, rather than on wave-lengths (or metres). Yet the public has previously failed to respond to efforts to impose upon ib this more scientific method of reckoning. ‘There seems to be a popular feeling that a wave-length is something ‘tangible, while a kilocycle is an abstract idea. Yet the matter should be simple enough. A cycle is a reversal from positive to negative, and back again from negative to positive, in the electricity in an alternating-current circuit, or in the impulses creating the field of a radio waye. "Cycles," used as a measure of the rapidity of these changes, implies always per second; and ‘kilocycle" is simply a short expression for thousands of cycles (per second). A thousand kilocycles, therefore, means a million double changes . per second in the polarity of the wave, as measured at any point in its progress and, as the wave advances 300,000,000 metres (more accurately, 299,820,000, more or less) per second, thie "peaks" will be highest at points 300 metres apart, along the path of a 1000ke. wave. We have here the idea of a simple wave, corresponding to a wave in water, with approximately equal spaces between its highest crests. However, the water does not move steadily forward-it rises and falls-and the radio wave is not a flow of current, it is a rise and fall of voltage. ACCURATE MEASUREMENT. The frequetcy of currents, alternating from thousands to millions of times a second, has been very accurately measured, the wavelength, with a lesser degree of accuracy, hy other complicated devices. From the standpoint of classifying stations in a_ broadcast list, we might use either kilocycles or metres readily enough. We may also describe a distance as 66 feet, or as

1-80 of a mile, with equal accuracy ; it is merely a question of convenience in reckoning. But, in the technical problem of arranging stations so that they will not interfere with each other, it is necessary ‘to calculate in cycles; because what is impressed on a radio carrierwave is not a wavelength. It is a fre~ quency. VIBRATIONS AND CYCLES, A musical note is a vibration at the rate, for instance, of 800 cycles a sec~ ond, causing air waves about four feet long. In an elettric speech amplifier this would correspond to electric waves abont six hundred miles long, because of the great speed of electricity. But we do not add a six-hundred-mile wave to a thousand-foot wave; we impress a frequency of 3800 cycles (per second) upon one of a million (per second), The result is a ‘modulated wave.’ Tlie function of a radio detector is to iron out, so to speak, the million-per-second wave and leave the 300-per-second wave, which enters the loudspeaker and reproduces a 3800-cycle note, of sound in air. Now, at the upper end of the broadcast band, a 800-cycle change affects the wavelength (measured in metres) about ten times as much as it does at the lower end of the band. At 5200 metres, the wavelength used for transAtlantic radiopllone work, one kilocycle added to the frequency makes a difference of about 100 metres in the wavelength. At 5.2 metres, down near the very short wavelengths at which amateurs are now working, a kilocycle makes a difference of only about one ten-thousandth part of a metre in the wavelength. As a mathematician might put it, the difference in the length of a wave represented by a kilocycle varies inversely as the square of the frequency (approximately), CONVERSION TABLE. The following table, to the nearest tenths cf metres, is therefore published for the information of our readers, who may desire to convert kilocycles into metres, or vice versa, Metres K.C. Metres K.C. Metres K.C,

344.6 340.7 336.9 333.1 829.5 825.9 322.4 319.0 815.6 312.3 309.1 305.9 302.8 299.8 296.9 "293.9 291.1 285. 5 282.8 280.2 277.6 275.1 272.6 270.1 267.7 265.3 263,0 260.7 258.5 256.3 254.1 870 880 890 900 910 920 930 940 950 960 970 990 1000 1010 1020 1030 1040 1050 1060 1070 1080 1090 1100 1110 1120 1130 1140 1150 1160 1170 1180 252.0 249.9 247.8 245.8 243.8 241.8 239.9 238.0 236.1 234.2 232.4 230.6 228.9 227.1 225.4 223.7 222.1 220.4 218.8 217.3 215.7 214.2 212.6 QUL.1. 209.7 208.2 206.8 205.4 204.0 202.6 201.2 199.9 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300: 1310 1320 1330 1340 1350 1360 1870, 1380" 1390 1400 1410 1420 1430 1440 1450 1460 1470 1480 1490 1500