MEGACYCLES OR METRES?

New Zealand Listener, Volume 1, Issue 21, 17 November 1939, Page 55

MEGACYCLES OR METRES?

Explanation by BBC Engineers

BC announcements, while always mentioning the frequencies (expressed as megacycles per second) used to serve the respective areas, do not give exact details of the corresponding wavelengths in metres. The reasons for this practice are described in this article by the Engineering Division of the BBC. The term "wavelength" has been associated with wireless communication since the first practical applications of the science were instituted. In the early

days, relatively "long" waves were used, and subsequently, when the broadcasting of general-interest programmes began, the practice of describing such transmissions in terms of wavelengths in metres was adopted by many organisations. Manufacturers of receiving sets, consequently, calibrated the tuning dials so that these indicated wavelengths, and this method of description, therefore, had become well established by common usage before short waves were used to any extent for broadcasting. (A notable exception to this practice occurs in the United States of America, where frequency in kilocycles per second has been used almost exclusively.)

It is, however, possible to specify a wave not only by its wavelengtth in metres, but by its "frequency" in kilocycles per second (abbreviated "kc/s’"’) or megacycles per second (abbreviated "me/s"). There is a definite relationship between the two units, and one can easily be calculated if the other is known-viz., if the wavelength in metres is known, this figure divided into three hundred gives the frequency in megacycles per second, and conversely, one megacycle

per second equals 1,000 kilocycles per second, It has been found that, in order to ensure undistorted, interference-free reception of high-quality transmissions, wireless stations may not operate on adjacent waves spaced from each other by less than 10 kilocycles per second. Consider for example, two stations in the medium-wave broadcasting band that are spaced 10 ke/s from each other -e.g., a station working on 285.7 metres, and a neighbour working on 288.5 metres. There is here a wavelength difference of 2.8 metres. Consider next, in the short waveband, the British station | GSJ, 21.53 mc/s, and the American

station WCAB, Philadelphia, 21.52 mc/s. These two stations are also spaced 10 kc/s apart, but the corresponding difference in their wavelengths is only 0065 of a metre. In other words, the lower the number of metres, the higher the number of kilocycles, so that between 200 and 300 metres-a band of 100 metres-there is space for fifty channels 10 kc/s apart, but between 20 and 30 metres-a band of only 10 metres-is space for 500 such channels. The fact that stations generally work on exact multiples of 10 kc/s further emphasises the expediency of announcing short wavelengths in terms of frequency.