Going Wireless
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GOING WIRELESS An Old Radio and turned it on. We saw the ancient tubes We had a Philco radio that was about six light up with a dim glow and smelled the feet tall. It had 287 knobs on it, of which characteristic odor produced by a warm cir- only two worked: off-on-volume and the sta- cuit and carbonizing dust particles. We tion selector. -Bill Cosby, The Chicken Heart Routine checked the tubes to make sure that they were not loose and even replaced tubes that I grew up in an extended family in Tul- seemed to be going bad. Static was all that sa, Oklahoma. Often, I found myself in the we got for our reward. After a time we gave breakfast room of my grandparent’s house at up and let it sit there as an enigmatic muse- dinner or breakfast. I loved to eat there. It um piece that must have worked at least dur- was a fairly open room, dominated by a ing “the war”. large table in the center. Besides the tele- Although I was very unsuccessful with phone table, there was almost no other furni- that radio, I was fascinated by the idea of ture except a large Philco radio. It was a information arriving through the air. I or- floor model with beautiful veneer work and dered circuit boards and made circuit boards several knobs and a station selector panel. in many electronic projects that I attempted, That radio was witness to some of the best most of which were failures. However, I did meals and family stories, both of which not give up and did manage to make a sev- seem to grow better as memory dims. I also eral crystal radios, which included making remember that the radio never worked, at my own speakers. Mostly, I heard static, but least within my memory. Several times, my occasionally I heard words and music. cousins and I moved it away from the wall Figure 1. A diagram of the electromagnetic (EM) spectrum which depicts the relative lengths of its component members. Note that visible light occupies a very small band and the range in wavelength goes from less than 10-13 cm to more than a kilometer. From nasa.gov. TABLE 1. Descriptions of the Electromagnetic spectrum. Note that radio waves have the longest wavelengths, the lowest frequencies, and the lowest energies of the compo- nents of the EM spectrum. http://imagine.gsfc.nasa.gov/docs/science/know_l1/spectrum_chart.html Wavelength (m) Frequency (Hz) Energy (J) Radio > 1 x 10-1 < 3 x 109 < 2 x 10-24 Microwave 1 x 10-3 - 1 x 10-1 3 x 109 - 3 x 1011 2 x 10-24- 2 x 10-22 Infrared 7 x 10-7 - 1 x 10-3 3 x 1011 - 4 x 1014 2 x 10-22 - 3 x 10-19 Visible 4 x 10-7 - 7 x 10-7 4 x 1014 - 7.5 x 1014 3 x 10-19 - 5 x 10-19 UV 1 x 10-8 - 4 x 10-7 7.5 x 1014 - 3 x 1016 5 x 10-19 - 2 x 10-17 X-ray 1 x 10-11 - 1 x 10-8 3 x 1016 - 3 x 1019 2 x 10-17 - 2 x 10-14 Gamma-ray < 1 x 10-11 > 3 x 1019 > 2 x 10-14 The EM Spectrum to measure the relative temperatures of a Rattlesnakes May Inject Venum Under light beam that was separated into its com- Xtreme aGitation. ponent colors by a prism. The highest tem- –a mnemonic for remembering peratures were in the red end of the visible the Electromagnetic Spectrum. light spectrum. However, when he placed Radio waves and microwaves range the thermometer beyond the red band, the from about 3 cm to more than 300 m long temperature was even higher. He called the (Table 1). The relative energy of the waves invisible light, calorific ray. Today, we call of the EM spectrum is the inverse of the it infrared light. The real importance was wavelength through the whole spectrum that he had demonstrated the existence of from gamma-rays to very long-wave radio. rays of “light” that exist, but are invisible to Only a very narrow band occupies the visi- the eye. ble range1; thus, most of the EM spectrum is Johann Ritter (Germany; 1776-1810), invisible and discovered only indirectly. during his short life, piled up a number of Frederick William Herschel2 (Germany inventions, including the dry cell battery and and England; 1738-1822) in 1800 attempted electroplating. Also, in the year following the discovery of infrared radiation, he began to search for an invisible light at the violet 1 The mnemonic for visible light is usually given as end of the visible light spectrum. He used the name, Roy G Biv (for red, orange, yellow, green, blue, indigo, and violet). Red has the longest wave- silver chloride and exposed it to different length and violet has the shortest wavelength. colors of light using the same method as Herschel. He noted that the red end of the 2 Herschel is most well-known for having discovered Uranus. For more about Herschel read The Planets, spectrum was relatively ineffective at de- an essay in Paths of Science by Jack Holt. composing silver chloride compared to the was the earlier development and used the violet end. When he exposed silver chloride activation of electromagnets to send a non- to “light” beyond violet, the breakdown oc- verbal code along a wire. The telephone curred even more effectively. Thus, he built upon the telegraph and utilized a mi- demonstrated that the visible spectrum was crophone to convert the human voice to straddled both above and below by invisible electric impulses through a wire, and the rays with different properties. At first, he impulses then created sound by activating a called this new light chemical ray, which we device similar to a microphone that func- know as ultraviolet radiation. tioned as a speaker. The greatest breakthrough came when Radio was a disputed technology like James Clerk Maxwell (United Kingdom, those of telephone and telegraph. The in- Scotland; 1831-1879) united electricity and vention of radio has been claimed by the magnetism into a set of equations that we Americans, Italians, Russians, and British. know as the Electromagnetic Theory3. His In truth, all participated in the invention. work implied that light was an electromag- Many of the participants were inventors in netic wave and that electromagnetic waves the old style with little mathematical or sci- of shorter and longer wavelengths must ex- entific training. David Edward Hughes (UK ist. Heinrich Rudolph Hertz (Germany; and USA; 1831-1900) and Thomas Alva Ed- 1857-1894) confirmed that electromagnetic ison (USA; 1847-1931) were such inventors. waves with wavelength of 0.66 meters could Indeed, Edison’s achievements were legend- be produced by one electrical circuit and ary, mainly due to his persistence and pa- received by another. The theoretical foun- tience. Both men had created radio trans- dation of wireless communication had been mitters and receivers long before Hertz suc- achieved. ceeded in doing so. However, they were discouraged by a mistaken understanding RADIO, EARLY DEVELOPMENTS that they were just dealing with electromag- I do not think that the wireless waves I have netic induction, thus, so they thought, the discovered will have any practical applica- phenomenon was nothing new. tion. Heinrich Rudolph Hertz Even the casual users of the early tele- phones were aware that if a spark jumped Radio did not emerge as a technology by across a loose wire in another circuit, they itself. It grew upon the technologies of the could hear a moment of static. The spark 4 5 telephone and telegraph . The telegraph had generated electromagnetic waves that were picked up by the telephone. This is the phenomenon that generally was mistaken for 3 Find more about James Clerk Maxwell and the Elec- electromagnetic induction until Hertz con- tromagnetic Theory in Electricity? What Good is It?, an essay in Paths of Science by Jack R. Holt. 5 The telegraph evolved together with successive 4 Although the true inventor of the telephone was developments in the technology of the electromag- disputed, the patent for the Telephone was awarded net. Samuel Finley Breese Morse (USA; 1791-1872) to Alexander Graham Bell (UK –Scotland and USA; was the first to patent a generally useful system to- 1847-1922) in March 1876. gether with a code made of dots and dashes. firmed that it was something different. free electrons in the wire to oscillate, thus Hertz died on January 1, 1894, after having making an alternating current. Because the developed a radio transmitter and receiver. radio waves provide all of the power to the crystal radio, the antenna wires have to be long. In fact, it should be at least one-fourth of the wavelength, which in the case of radio waves can be very long, indeed (see Table 1 and Figure 1). The antenna is not selective in which waves it receives, so if there is more than one radio source, the listener would just hear a cacophony of sounds. The tuner, which is a coil (see Figure 3), uses resonance6 to en- hance particular wavelengths, thus tuning in particular transmitting stations. For exam- ple, The carrying frequency for the local public radio station is 89.9 (89,900 Hertz or vibrations per second). Simple tuners are made of a simple coil with which the chan- nel is selected by running a contact over the wire, thus changing the length of the coil.