Radio broadcast .. (1922-30)

332 Radio Broadcast super-heterodyne receiver, in action. It may be considered in one way unfortunate that before receivers of this type could be manufactured and put into service on the field, the armistice had been signed. But, as though the super-heterodyne were destined to assist in the making of history, it quickly found its way into the hands of a few fortunate, delighted, appreciative amateurs — created a worshipful enthusiasm — and was packed away to the dreary shores of Scotland. Here, during a short ten days' vigil, it showed its historymaking calibre when it initiated international amateur radio communication. As this is being written, considerable tinkering is being done with super-heterodyne receivers. The occasion for this tinkering is the transatlantic amateur tests of December, 1922. Before this reaches the reader's hands, there is little doubt but that the super-heterodyne will have shown again its great dependability as a receiver for sorting out, classifying, and recording extremely weak signals from great distances. HOW IT WORKS A THOROUGH understanding of the operJ\ ation of the super-heterodyne is no difficult task. There are three actions which take place in the receiver in the following order: (i) a changing of the frequency of the incoming oscillation; (2) amplification of the oscillation at its new frequency; and (3) rectification (commonly considered as detection). To the novice, the most mystifying function of the receiver combination is the frequencychanging action. This is simplicity itself. A detector tube has fed into it the incoming signal oscillations. This same detector has fed into it simultaneously oscillations which are generated at the receiving station by an oscillating vacuum-tube circuit. The circuits of the local vacuum-tube oscillator are so adjusted as to be either slightly lower or slightly higher in frequency than the incoming signal oscillations. The mixing of the two currents of different frequency in the detector tube gives rise to a new current of entirely different frequency in the output (plate) circuit of the detector tube which is known as the "beatfrequency current." By way of example in explaining the above, let us suppose that the detector circuit was tuned to receive signals on a wavelength of 200 meters. The oscillatory currents at this wavelength would be recurring at a frequency of 1,500,000 times per second. Let us now adjust the local oscillator circuits so as to produce a frequency of i ,400,000 cycles per second. Under these circumstances the two currents differing in frequency by 100,000 cycles per second would re-act one upon the other in the circuits of the detector, so as to produce the "beat frequency" above mentioned, which would be equal to the difference of the two initial frequencies. This frequencychanging method was devised by Professor R. A. Fessenden, an American, and is known as the heterodyne method. Now, a frequency of 100,000 cycles per second corresponds to a wavelength of 3,000 meters. As mentioned above, there is no great difficulty encountered when alternating currents having frequencies as low as 100,000 cycles per second are to be amplified. Three, five, seven or even nine stages of radio-frequency amplification may be used with complete success if a few necessary precautions are taken. Subsequent to this amplification, a rectifier tube (detector) receives the amplified energy, rectifies it, and, if desired, passes it on to a second amplifier in order that the volume of the now audible signals may be increased. A review or the foregoing explanation and reference to Figure i will quickly show that in its usual form, the super-heterodyne receiver is exceedingly easy to tune. There are only two prime adjustments. One controls the FIG. 2 wavelength of the "collector" or antenna circuit, while the other controls the wavelength (or frequency of oscillation) of the vacuum-tube oscillator circuit. Having set the one at the desired wavelength, it is only necessary to adjust the other until the dif