Prologue

THIS book, although designed to be read independently, is in one sense a continuation of an earlier work, Syntony and Spark: The Origins of Radio, published in 1976. That book dealt with the very earliest phase of radio technology, when the scientific work of James Clerk Maxwell and Heinrich Hertz was being transformed into a technology of communication by men like Oliver Lodge and Guglielmo Marconi and the first attempts were being made to base commercial enterprises on that technology. The present volume picks up the story in the closing decades of the nineteenth century and carries it through the 1920s, when the advent of popular broadcasting transformed radio from a means of point-to-point communication, competing with the wired telegraph, into the agency of mass communication it is today. I discuss the origins of broadcasting only briefly in this book. My interest is in the origins of the technology that made broadcasting possible. This was the technology of the continuous wave.

In the earliest days of "signalling without wires" the only known method of generating radio waves was by means of sparks. An induction coil, or sometimes a bank of capacitors, was used to place a high voltage across a spark gap; when a spark jumped the gap it created an electromagnetic disturbance that could be detected at a distance. A series of sparks following each other in rapid succession gave rise to a chain of such disturbances — a radio wave, in short — that could be interrupted to form the dots and dashes of the Morse code and thereby convey information. Such waves travelled at a constant velocity: the speed of light. Each wave had a specific wavelength — the distance between succeeding peaks or troughs — usually measured in meters; and therefore, given the constant velocity, it had a specific frequency (the number of cycles per second). Each wave, that is to say, had a particular "place" on the electromagnetic spectrum, defined by its wavelength or frequency. If it was to be detected, the apparatus used for receiving had to be capable of responding to waves of that frequency — that is, it had to find and react to a signal at that "place" and, if possible, reject all others. Today we do this by a process we call tuning. In the earliest days of radio it was more common to speak of "syntony." Receiving and transmitting circuits were said to be in syntony when they resonated at the same frequency.

Syntony and spark were the characteristics that gave unity to that first phase of radio history. Technological development consisted of devising more effective spark transmitters, receivers that could detect and respond to spark-generated waves, and syntonic circuits that made it possible for transmitters and receivers to "find" each other in the radio spectrum. Important elements in this process were the development of antennas that could radiate and pick up signals efficiently, and the trial-and-error discovery of which wavelengths were most suitable for transmission over long distances.

The radio wave generated by a spark transmitter was a wave of a particular type. Each spark discharge generated a series of oscillations that diminished rapidly in amplitude as its energy was radiated into space and absorbed by the internal resistance of the components. A common simile, and an appropriate one, was to compare the antenna to a bell struck by a clapper. The bell, when struck, would sound a note, radiating energy in the form of sound waves. But the strength of the note would diminish more or less rapidly, as the vibrations of the bell diminished in amplitude. If the vibrations were "damped," as for instance if one placed a hand on the bell's surface, the sound would die away very quickly. So it was with a spark discharge: it had a degree of damping, depending on the internal resistance of the circuit and the rate at which it radiated energy into space. The radio wave generated by a succession of spark discharges consisted of a series of these damped oscillations. In that sense, a spark transmitter, although it might radiate continuously, did not generate a true continuous wave. (See Fig. 1.1)

It can be shown mathematically, by a technique known as Fourier analysis, that a damped oscillation such as that generated by a spark transmitter (or indeed any other complex waveform) can be decomposed into a large number of other oscillations, each with a frequency and wavelength of its own. These constituent oscillations are sine waves, in which the signal changes in an exactly prescribed way through a full cycle, going first positive, then negative, following the sine function in trigonometry. (See Fig. 1.2) This is no mere mathematical transformation: if such a train of damped oscillations were radiated from an antenna, its constituent sine waves would appear on the electromagnetic spectrum and (unless filtered out by tuned circuits) affect any receiver with enough sensitivity to detect them. From this fact certain important practical implications followed. A spark radio transmitter generated not one radio wave, but a very large number of them. Its signal was not at a single "place" on the electromagnetic spectrum but at a very large number of places. A true unmodulated continuous sine wave, in contrast, if one could have been generated, would have had one frequency only; it would have appeared at one place in the spectrum and only at that place.

This is the reason why, today, spark transmitters are universally outlawed. The radiofrequency spectrum is, to be sure, a unique resource, in that it can never be used up. But it can be overused; it can be overcrowded. Congestion of the radio spectrum creates a form of pollution in which transmitters interfere with each other and receivers are unable to select the signal that conveys the desired information from among interfering signals that are essentially noise. The danger always exists of a new "tragedy of the commons," in which overuse of a resource freely available to all creates a situation in which it is available to no one. To prevent such a situation, governments and international supervisory agencies resort to frequency allocation — essentially, the rationing of scarce space on the spectrum.

A spark transmitter is inevitably a dirty transmitter. It pollutes the spectrum by contaminating frequencies far removed from those nominally being used to carry the message. Its undesired effects can be minimized by reducing the degree of damping, which is to say by approximating more and more closely to a continuous wave. But there is a point, with spark, beyond which amelioration cannot go. Furthermore, spark transmissions make selective tuning much harder to...