Fundamentals of Radio

In the usual sense, radio refers to equipment used to send or receive electromagnetic waves in the range of frequencies lying, more or less, between one hertz and a few gigahertz.

Electromagnetic radiation occurs in wave form, that is, in a train of regularly rising and falling strengths. Distance from one crest to the next makes up one wavelength. In an ordinary AM broadcast signal, say 1000 kHz on your AM dial, wave crests are spaced at about 969 feet apart. The number of crests going by in one second is called the frequency of the wave. Thus, if the speed of a wave is known (it's light speed, of course, for radio waves) then wavelength and frequency may be calculated one from the other according to the formula: v = fw, where v stands for velocity,f for frequency, and w for wavelength.

The height of a wave is its amplitude—usually that's expressed in volts for a radio wave. Common sense suggests that for waves of the same amplitude, those of higher frequency, more of them arriving in each second, are more intense, more powerful. Indeed, this is the case. A single gamma ray (extremely high in frequency) packs a concentrated wallop, while a wave of energy spread out in time, across longer wavelengths, doesn't knock things apart.

Early Radio Concepts and Equipment

Radio communication requires, at the very least, a transmitter that produces, amplifies, and radiates power at a useful radio frequency, while at the same time incorporating some kind of information into its signal; and a receiver that can detect the selected frequency, separate the signal's information content, and present it to a recipient.

Once physics had advanced far enough to understand and describe electromagnetic waves, the biggest hurdle for practical use lay in achieving sufficiently high frequencies and voltages for radio transmission. Tesla obtained both with his versatile resonant and "magnifier" coils (Tesla coils). Getting information into and out of a radio wave remained, however, a rather clumsy process until the development of electronic vacuum tubes, most notably Lee De Forest's triode in 1906.

For a decade or two after radio had become an accomplished fact, signals worked more or less like Morse dots and dashes, either on or off. A train of pulses separated by intervals made up the message. To know whether a signal was present or not, early receivers often relied on devices called coherers—in effect, just switches that turned on when a pulse excited an antenna, and had to reset themselves before the next pulse arrived. No one ever invented a coherer that was really satisfactory, Tesla included, but in his visionary way he solved another problem in communications whose implications reach right through modern computing and encryption techniques.

Tesla's Individualization

Tesla understood immediately, from the construction of the first radio transmitter, that a confusing welter of signals would soon cover the world. With this in mind he invented circuits that would respond only when a preselected set of frequencies were detected at the same time or in a specific sequence. A sender, thereby, could feel assured that messages would be received only at their intended destinations and would remain identifiable against a noisy background of unrelated radio traffic.

His designs for "individualization" (Tesla's term) operate in the same way—indeed they introduced the principle—as logic gates in computer circuitry. And the idea of breaking up signals, moving them around in frequency or time, lies at the heart of present-day communications security.