Radio

• 1. Introduction

  ›

  On the 11th of November 1886, a German physicist named Heinrich Hertz watched a tiny spark jump across a gap in his laboratory, and in doing so confirmed that an invisible wave was traveling through the room. Radio is the technology of communicating using radio waves, electromagnetic waves with frequencies between 3 hertz and 300 gigahertz. They are made by a transmitter connected to an antenna, and caught by another antenna wired to a receiver. That simple loop, transmitter to receiver, is the entire foundation of the thing. So how did a spark in a German lab become the system that carries cell phone calls, locates aircraft, and tracks spacecraft across the solar system? Why did people argue for decades about what to even call it? And how does a single resource, the air itself, get divided fairly among millions of competing voices? The answers run from a Latin word for the spoke of a wheel to a wartime scramble for control of the airwaves.

• 2. The Word Itself

  ›

  Radius, the Latin word meaning spoke of a wheel, beam of light, or ray, is where the modern term begins. It was first applied to communications in 1881, when Alexander Graham Bell adopted radiophone, meaning radiated sound, as an alternate name for his photophone optical transmission system. That choice came at the suggestion of a French scientist, and it had nothing to do with the wireless devices we now picture. After Hertz proved the waves existed, people called the radiation Hertzian waves for years. The earliest practical systems carried telegraph signals, so the whole field went by the name wireless telegraphy. That label was a problem, because until about 1910 it also covered experiments in electrostatic induction, electromagnetic induction, and even conduction through water and earth. A more precise word was needed, one that meant electromagnetic radiation and nothing else. The radio- prefix supplied it. The French physicist Edouard Branly, who in 1890 built the wave-detecting coherer, named his device a radio-conducteur. From there the prefix spread, especially across Europe, and in early 1898 the British publication The Practical Engineer printed the words radiotelegraph and radiotelegraphy. The standalone word would soon follow.

• 3. Wireless Versus Radio

  ›

  On the 30th of December 1904, the British Post Office issued telegram instructions stating that the word Radio is sent in the Service Instructions, one of the earliest uses of radio as a word on its own. The 1906 Berlin Radiotelegraphic Convention then carried it worldwide, with a Service Regulation declaring that radiotelegrams must show in the preamble that the service is Radio. Lee de Forest pushed the term hard in the United States. In early 1907 he founded the DeForest Radio Telephone Company, and in a letter published in the 22nd of June 1907 issue of Electrical World, he warned that Radio chaos will certainly be the result until such stringent regulation is enforced. The change from wireless to radio crept through the English-speaking world unevenly. The United States Navy translated the 1906 Berlin Convention using the older terms wireless telegraph and wireless telegram, then by 1912 began promoting radio instead. The general public only settled on the word in the 1920s, when broadcasting arrived and put receivers in ordinary homes.

• 4. From Maxwell To Marconi

  ›

  James Clerk Maxwell predicted the waves before anyone had seen them. In his 1873 theory of electromagnetism, now called Maxwell's equations, he argued that a coupled oscillating electric field and magnetic field could travel through space as a wave, and that light itself was such a wave at short wavelength. Hertz set out to confirm that theory, generating his first radio waves with a primitive spark-gap transmitter. He was not working alone for long. Experiments by Jagadish Chandra Bose, Oliver Lodge, Lord Rayleigh, and Augusto Righi showed these waves reflected, refracted, diffracted, polarized, formed standing waves, and moved at the speed of light. Light and radio waves were the same thing, separated only by frequency. Guglielmo Marconi turned the physics into a working tool. In 1895 he built the first radio communication system, using a spark-gap transmitter to send Morse code over long distances, and by December 1901 he had reached across the Atlantic Ocean. In 1909 Marconi and Karl Ferdinand Braun shared the Nobel Prize in Physics for their contributions to the development of wireless telegraphy.

• 5. From Dots To Voices

  ›

  For radio's first two decades, the radiotelegraphy era, the transmitters could only send pulses, not the continuous waves needed to carry audio. So radio stayed a tool for text: person-to-person commercial, diplomatic, and military messaging in Morse code. Around 1908 the industrial nations strung worldwide networks of powerful transoceanic transmitters, swapping telegram traffic between continents and reaching their colonies and naval fleets. The breakthrough to sound came during World War I, with continuous wave transmitters and new detectors, the rectifying electrolytic and the crystal radio receiver. These let Reginald Fessenden and others achieve amplitude modulation radiotelephony, putting actual audio onto the air. The era of voice opened to the public on the 2nd of November 1920, when Westinghouse Electric and Manufacturing Company in Pittsburgh aired the first commercial radio broadcast under the call sign KDKA. The program carried live coverage of the 1920 United States presidential election, letting listeners hear the returns as they came in.

• 6. How A Signal Travels

  ›

  A time-varying electrical signal, called the modulation signal, is where every transmission begins. A transducer creates it: a microphone turning sound into an audio signal, a video camera producing a video signal, or a computer supplying a digital signal of bits. Inside the transmitter, an electronic oscillator generates an alternating current at radio frequency, the carrier wave, and the modulation signal varies some aspect of that carrier to impress the information onto it. The methods differ by system. Amplitude modulation, or AM, varies the strength of the carrier. Frequency modulation, or FM, varies its frequency. Frequency-shift keying shifts the carrier between frequencies to send digital signals. Orthogonal frequency-division multiplexing, or OFDM, spreads many closely spaced carriers across a channel, sending multiple bits in parallel; it powers Wi-Fi networks, cellphones, digital television, and digital audio broadcasting, with higher spectral efficiency and more resistance to fading than AM or FM. At the far end, the radio wave induces a tiny oscillating voltage in the receiving antenna, a weaker copy of the transmitting current. The receiver amplifies that whisper, demodulates it, and a transducer returns it to a usable form, sound from a loudspeaker or images on a display. Because each transmitter oscillates at its own frequency, countless signals share the air without colliding, and a tuned circuit, behaving like a tuning fork, picks out the one station the listener wants.

• 7. The Crowded Spectrum

  ›

  A modulated radio wave occupies a range of frequencies, and the width of that range in hertz is its bandwidth, a direct measure of how much information it can carry. A television signal demands a greater data rate than an audio signal, so it needs more room. The trouble is that the radio spectrum is a limited resource. Each transmission claims a slice of it, and that slice has real monetary value; in some parts of the spectrum, the right to use a frequency band or even a single channel sells for millions of dollars. That scarcity has driven invention. A slow shift from analog to digital began in the late 1990s, partly because digital modulation packs more information into a given bandwidth, resists noise better, and lets one method carry many kinds of data. As the spectrum grows more congested, engineers have answered with trunked radio systems, spread spectrum transmission, frequency reuse, dynamic spectrum management, frequency pooling, and cognitive radio. The International Telecommunication Union imposes order on the whole map, dividing the spectrum into twelve named bands, each starting at a wavelength that is a power of ten meters. They run from extremely low frequency at 3 to 30 hertz up to tremendously high frequency at 300 to 3,000 gigahertz, and each band holds ten times the bandwidth of the one before it.

• 8. Sharing The Airwaves

  ›

  Two transmitters in the same area on the same frequency will garble each other's reception, and the cost is not only economic. Interference with emergency communications or air traffic control can be life-threatening, which is why the emission of radio waves is regulated by national laws and coordinated by the International Telecommunication Union. Transmitters must be licensed, restricted to set frequencies and power levels, and broadcasting stations carry a unique call sign of letters and numbers in every transmission. Anyone adjusting or repairing a transmitter needs a government license, such as the general radiotelephone operator license in the United States, earned by passing a test on safe radio operation. The public still gets a great deal of unlicensed access. Cell phones, cordless phones, walkie-talkies, citizens band radios, wireless microphones, garage door openers, and baby monitors operate as low-power short-range devices, falling under Part 15 of the Federal Communications Commission rules and often using the ISM bands set aside for unlicensed use. Against this ordered system stands its opposite: radio jamming, the deliberate radiation of signals designed to drown out others. Militaries jam enemy tactical communication in wartime, and some totalitarian countries jam foreign broadcasts to keep their citizens from listening. United States federal law bans the nonmilitary operation or sale of any jamming device, including those that interfere with GPS, cellular, Wi-Fi, and police radars.