Radio broadcast .. (1922-30)

398 Radio Broadcast Frequertcv FIG. I Pure, unmodulated continuous wave figure may be taken as a safe upper limit. Hence a 3OO-meter broadcasting station will radiate waves having frequencies confined between 995,000 and 1,005,000 cycles per second, these extreme values having little importance. Fig. 33 represents such a voicemodulated current; and Fig. 3b represents the relative intensities of the component waves, which consist of the "carrier" and the two "side bands." Broadcast stations whose carrier frequencies differ by 10,000 cycles per second evidently have no overlap in their frequency bands and can be distinguished by sufficiently selective receivers, provided that their signal intensities are not too different. HOW A "SPARK STATION" RADIATES ITS WAVE A SPARK telegraph station, on the other hand, produces a radiating current in a succession of groups, each of which is of short duration, compared with the interval between groups, as represented in Fig. 43. This is essentially equivalent to modulating a continuous wave by a variation in intensity which rises very rapidly to a maximum, then falls rapidly, and is sensibly zero for a large portion of the group cycle, as represented by the dotted envelope in Fig. 43. Such a modulation curve is very rich in high harmonics. If the rate of building up of the oscillating current is very high and the decrement is at the legal limit of o. 2, a wave which nominally has the frequency of one million cycles per second will actually consist of waves of almost uniform intensity ranging from about 970,000 to 1,030,000 cycles per second, and of waves of rather slowly decreasing intensity extending down to very low frequencies and up to a few million cycles per second, as represented in Fig. 4b. Such a wide band of frequencies will over aide. Carrier 1 Freou.e rxcy ooo qycles per sec FIG. 2 Continuous wave modulated by pure tone lap a great many broadcasting bands and is the cause of the great amount of interference from spark stations. SO MUCH for the transmitted waves. Now let us see how the receiver responds. Selectivity is accomplished by tuning the receiver to a certain frequency. But the receiver will respond not only to that frequency but also to neighboring frequencies. The relation between response and frequency with a fixed tuning adjustment is represented by a "resonance curve," of which examples are given in Figs. 5 and 6. (As with the preceding figures, it has not been possible to draw these to scale.) For broadcast reception without sensible distortion of the music or speech, it is necessary that the resonance curve embrace a band of frequencies corresponding to that usefully radiated, as represented in Fig. 3b. On the other hand, the wider the band embraced, the greater will be the tendency to pick up interfering signals and atmospheric disturbances ("static"). For pure continuous-wave telegraph reception a very narrow band is best. WHAT MAKES RECEIVERS SELECTIVE THE shape of a resonance curve and the effective width of the frequency band are controlled in two ways: first, by the ratio of the resistance to the reactance in each tuned circuit; and secondly, by the number of successive tuned circuits. The effect of changing the resistance of a tuned circuit is illustrated in Fig. 5. The middle curve represents conditions when the resistance of the coil and the condenser are kept low by proper design and construction, jcocooocyclesperaec FIG. 3 Continuous wave modulated by voice "Spark" or damped wave