Radio Broadcast (May 1923-Oct 1923)

242 Radio Broadcast frequency assignments to broadcasting stations. The waves used by the broadcasting stations are spaced 10 kilocycles apart (3 meters at a wavelength of 300 meters). Thus one station is on 990 kilocycles, another on 1000, and another on 1,010 kilocycles. If one of these is inaccurately adjusted by as much as 0.1 per cent, this would mean a variation of 1 kilocycle from the established value. The next station having a frequency only 10 kilocycles different, this variation of 1 kilocycle is decidedly appreciable in comparison with the 10 kilocycle separation. A variation of the frequency of 1 per cent, for example, would be a variation of 10 kilocycles and could cause one station to be using exactly the wave that had been assigned to another. The whole success of American broadcasting is thus tied up with the placing of broadcasting stations on the correct frequencies to an accuracy approaching 99.9 per cent. Since receiving sets are now available by which an individual can hear the stations from all over the United States on the same night, the importance of this accuracy is apparent. The basis of frequency measurement of the Bureau of Standards includes absolute frequency measurements by four entirely independent methods. These are: A. Measurement or calculation of capacity fig. 2 Lissajous figures giving accurate frequency ratios and inductance of carefully constructed standard condensers and inductors. B. Measurement of frequency of the sound from a tuning fork, together with measurement of ratio of such frequency to radio frequencies by use of harmonics from an electron tube generator. C. Measurement of frequency of the sound of a tuning fork, together with measurement of the ratio of such frequency to radio frequencies by the use of Lissajous figures produced by the direct application of the two frequencies to a cathode-ray oscillograph. This method is free from the assumption of integral ratios involved in the harmonic method (B). D. Measurement of the actual length of very short waves and calculation of the frequency from this and the known velocity by the relation, f = v I \. (X = wavelength in meters). Ratios of frequencies thus measured, to lower radio frequencies, are obtained by the same general methods for ratio measurements as in B and C. These four methods are not all that are theoretically possible. Another one that could be used is the accurate measurement of the speed of a radio-frequency alternator. Every method goes back ultimately to the measurement of a time interval. Method A was the first method to be used by the Bureau of Standards. Where this system is employed, the capacity of specially designed condensers are measured by their charge and discharge at about 1 kilocycle. The inductance of the standard inductors is measured at the same frequency. The design of both capacity and inductance standards is such that there is no appreciable change with frequency. The capacity of coil and leads is measured by the use of harmonics as described on page 100 of the Bureau of Standards Circular 74, entitled "Radio Instruments and Measurements." This standard wavemeter, with its instruments for indicating resonance, is shown in Fig. I.1 Method B employs the frequency of a tuning fork as the starting point, measuring the ratio of such frequency to radio frequencies by means of harmonics of the current in an electron tube generator. The use of harmonics for establish 1 Further information on the standard wavemeter is given in an article now in preparation entitled, "The Standards of Radio Frequency of the Bureau of Standards," by Mr. E. L. Hall, who is in charge of radio standards and testing. The time and place of publication of that article, and others referred to below, will be announced in the Radio Service Bulletin.