Go on any long trip with several other people, as I did over the weekend, and a major source of conflict is sure to arise: what to listen to on the radio. But amid the debate on the relative merits of country, jazz, Top-40 and oldies (not to mention loud and soft), it struck me how we take for granted what would have seemed magical just 100 years ago: sending voices and music through empty air.
Radio’s principles were first demonstrated by early-19th-century scientists such as Michael Faraday and Joseph Henry, who theorized that a current flowing in one wire could produce a current in another wire it wasn’t connected to. In 1866 German physicist Heinrich Hertz set up two pairs of electrodes, and passed a high-energy spark between the first two. A similar spark was generated between the second, isolated pair.
Hertz had produced “electromagnetic waves,” best described as electric and magnetic fields at the same frequency vibrating at right angles to each other. When these vibrating fields, travelling outward from a source at the speed of light, strike another conductor, they set up new currents at the same frequency.
Hertz couldn’t see a practical application for his work, but Guglielmo Marconi did. The Italian read of Hertz’s experiments and reproduced them, with improvements. He patented his “wireless telegraphy” system in London in 1896. That same year he managed to send messages over a few kilometres using a “spark gap transmitter,” which produced its radio signal by generating a spark between electrodes, as in Hertz’s experiment. The signal was received as a series of clicks in a telephone earpiece.
On December 12, 1901, Marconi transmitted the Morse-code signal for the letter “s” (dot-dot-dot) across the Atlantic, from Cornwall, England, to Saint John’s, Nfld. It required a 25,000-watt generator connected to 50 aerials on masts 70 feet high, but it was more strong evidence for the value wireless communication, and silenced many scientists who were convinced trans-Atlantic radio communication was impossible because the curvature of the Earth would block any signal after about 300 kilometres. Marconi was able to achieve the “impossible” because, as was later determined, his radio waves bounced off the ionosphere, a layer of charged atoms high in the atmosphere. (Short wavelengths reflect at a higher altitude than long wavelengths, which is why shortwave radio is used for long-distance communication.)
Clicks and dots and dashes were one thing, but transmitting voices quite another. The frequency of radio waves is measured in hertz, named after old Heinrich. One hertz is one vibration per second. A signal normally uses several frequencies at once; the number of frequencies it uses, its bandwidth, is measured in the difference in hertz between the lowest frequency and the highest. The wider the bandwidth, the more information the signal can carry.
Marconi’s wireless only required a bandwidth of a few hertz. Speech, on the other hand, needs at least 4,000 hertz, while high-fidelity music requires about 20,000 hertz. (Television signals require more than five million hertz!)
Transmitting speech and music only became practical with the invention of the diode by Sir Ambrose Fleming in 1905, which permitted the detection of high-frequency radio waves, and the triode in 1907 by Lee De Forest, which could amplify both radio and sound waves. The diode and triode led to tunable radios–and a big headache for governments, which had to quickly step in and regulate frequencies to keep everything from interfering with everything else.
When someone talks into a microphone, his or her voice sets up vibrations that are turned into electrical currents that vary in strength. To reproduce that voice, you have to amplify those varying electrical currents and feed them into a speaker, which then recreates the original vibrations. A transmitter turns the varying currents from the microphone into varying radio waves, which generate matching electrical currents in the radio receiver.
There are two main ways to vary, or “modulate,” the radio waves: amplitude modulation (AM) and frequency modulation (FM). Amplitude modulation varies the height of the radio waves, while frequency modulation varies the speed at which the waves vibrate. AM is simpler and came first, but FM, developed in 1933, is less affected by interference like lightning. (On the other hand, AM, thanks to the ionosphere, can carry for thousands of kilometres, while FM is limited to line-of-sight transmission, giving it a maximum range of about 110 kilometres.)
After the invention of the triode and diode, radio made its next big leap with the accidental invention in 1948 of the transistor, which functioned like the triode vacuum tube but took up far less space, used less power, produced less heat, and lasted longer. That began the revolution in electronics that continues today.
When you consider that radio began as literally nothing more than a few sparks, it’s astonishing what has been done with it. In fact, new applications are being found all the time, from digital radio-controlled garage-door openers to tracking polar bears.
All of which does nothing to solve the problem of what kind of music to listen to in a van with seven other people on a long, cold trip to Manitoba. But I guess that’s a pretty small price to pay for the “magic” of radio.