|MadSci Network: Physics|
Greetings, Amy: The primary waves associated with television are known as "electromagnetic waves". These waves have many names: radio waves, short-waves, microwaves, infrared radiation, visible light, ultraviolet light, X-rays, gamma rays, and so on. All of them travel at the speed of light, since all of them are merely "varieties" of ordinary light. Here are two crude sketches of two waves: . . . . , note shorter wavelength . . . . . . , note longer wavelength . . Whenever two electromagnetic waves can be called different from each other, a simple way to recognize that difference is to note their wavelengths. It is possible for one wavelength to be many kilometers long! --And it is possible for another wavelength to be smaller than the diameter of an atomic nucleus. A second and more common way to distinguish two different electromagnetic waves involves measuring them in terms of time. Recall that all these waves move at the speed of light: that is about 300,000 kilometers per second. If we had an electromagnetic wave that was 1000 kilometers long, and if we measured the time it took to pass by, that would turn out to be 1/300 of a second! In other words, one second is enough time for 300 waves, each 1000 kilometers long, to pass by, one after another. Or we can say that one 1000-kilometer electromagnetic wave could oscillate 300 times in one second. Normally, each full oscillation of a wave is called a 'cycle'; we can call a 1000-kilometer wave a 300-cycle-per-second wave. And of course a different wavelength would be associated with a different number of cycles-per-second, or 'frequency'. Take another look at the two waves sketched above: they are each only a few centimeters long. BILLIONS of these waves could pass you by, one after another at the speed of light, in a single second. Their frequencies can therefore be expressed in terms of "gigacycles per second". Electromagnetic waves are generally associated with electrically charged particles. These particles are found wherever you find static electricty, for example. If you scuff your feet on a carpet in a cold dry place, you can often build up a shocking amount of static-electric charge. The tiny spark that occurs when you surprise a friend is harmless, startling, and ALWAYS generates electromagnetic waves. Some of those waves are the visible light of the spark; others are radio waves, microwaves, and infrared. When you turn on an ordinary A.M. radio, the crackling you hear is caused by electromagnetic waves generated by lightning bolts -- Nature's giant and far-from-harmless static-electric sparks. The ordinary and understandable sounds coming from a radio, and of course the stuff you encounter on television, get there via radio waves and microwaves, respectively. Again, electrically charged particles generate these waves, but here there is dynamic electricity at work, not static. Please examine this sketch: | | magnetic pole |____N____| wire is part of wire ----------------------------- a large _________ loop (not shown) | S | | | magnetic pole A section of a loop of wire is shown; lets pretend that the rest of the loop comes out of the screen and connects behind your back. Also, pretend that you are holding the portion of wire shown, near yourself. Next, pretend that you can move the wire, still holding it as shown, all the way through the gap between the magnetic poles. Due to the laws of electromagnetism, this will cause some electrons to start moving (they ACCELERATE for a moment) inside the wire loop, in a particular direction. If you now imagine yourself pulling the section of wire back towards yourself, through the gap, then this would cause electrons to move/ accelerate the other way around the loop. Since electrons are electrically charged particles, each time you move the wire through the gap, BECAUSE you are causing electrons to accelerate, you would be generating some electromagnetic waves. If you could do this steadily, through-and-back once per second, then you would naturally be generating waves at a frequency of one cycle per second. They would have wavelengths of 300,000 kilometers! We generally use special electronic circuits to accelerate electrons back-and-forth, thereby producing electromagnetic waves. Depending on the circuit, we may generate waves of a few cycles per second (the Navy does this, because very low-frequency waves can travel through water to reach submarines) -- or we may generate waves having frequencies of many gigacycles per second. Almost every frequency in that range is used by somebody, somewhere, for some sort of communications. Even higher frequencies are also in use: infrared waves have frequencies of hundreds of gigacycles per second, and are used in lots of remote-control units (communicating with devices like TVs, VCRs, etcetera). Visible light waves oscillate many trillions of times per second, and a major new way to communicate is to send laser beams through many miles of optical fiber. It should be obvious that to generate electromagnetic waves is to do only about one-fourth of the process of using them to communicate. Some sort of data must be associated with the waves; then they must be detected and, finally, the data must be extracted. The simplest kind of electromagnetic communication is to merely generate and then not-generate waves. Long and short bursts of waves were used for years to represent information in the classic form of Morse Code. Gathering the information happens automatically, as the waves are detected.... In the middle 1800's a physicist named James Clerk Maxwell discovered the basic mathematical equations describing electricity, magnetism, and electromagnetic waves. For one thing, he computed that all these waves move at the speed of light -- and therefore it seemed reasonable that light itself was an electromagnetic wave. But he had no way to prove this. He wasn't even sure how to go about proving that any sort of electromagnetic waves actually existed. Later that century, another physicist named Heinrich Hertz figured out a way. He set up a coil of wire with a small gap in it, and forced electricity to spark across the gap. Nearby he placed another coil, with another gap in it. When a strong spark jumped the gap in the first coil, Hertz expected that invisible electromagnetic waves would cross the distance to the second coil, and some of them would be absorbed. The absorption process is simply backwards to the generation process: If causing electrons to accelerate produces electromagnetic waves, then when electrons absorb electromagnetic waves, they must become accelerated! And since accelerated elctrons are MOVING electrons, they must constitute a flowing electric current! Hertz expected to detect this current by watching it jump the gap in the second coil. It worked! Today we honor this physicist by having formally named "one cycle per second" "one Hertz". It is certainly easier to say "five hundred megahertz" than "five hundred megacycles per second", and considering how valuable we find the consequences of his initial detection of electromagnetic waves, the honor has been well earned. An antenna is a device that is specially designed to either emit or absorb electromagnetic waves efficiently. The more efficiently we can produce or detect these waves, the less power we need, to either send a signal from the trasmitter, or to extract useful information at the receiver. The coils that Hertz used were rather inefficient. In theory, the most efficient antenna is a simple straight wire that is exactly as long as the wavelength of the electromagnetic wave that you want to deal with. This would obviously be impractical for transmitting/receiving 100-kilometer waves! Also, we often want a single antenna (especially for a receiver unit) to handle a whole range of wavelengths as efficiently as possible. So even today, after more than a century of advances, an improved antenna design still makes its debut every so often. (The latest improved antenna designs use special geometric shapes known as fractals.) Any primary electromagnetic wave that carries information is known as a "carrier wave". A great many carrier waves of different frequencies are being generated by radio and TV stations; electronic circuits known as "filters" allow a receiver of electromagnetic waves to select the particular frequency that the user wants. If you tune your A.M radio to 800 kilohertz, then you are setting it to filter out all except a carrier wave of that frequency. "A.M." stands for "Amplitude Modulation", and this describes the way data is added to the carrier wave. Amplitude modulation is really just a more refined version of turning the carrier wave on and off, as described earlier for Morse Code. Because the receiver has an easier time detecting the carrier wave if that wave is transmitted continually, the information is transmitted in the form of a sequence of higher-power and lower-power waves: . . . . , note lower amplitude (power level) . . note same wavelengths (carrier wave) . . . . . . , note higher amplitude (power level) . . . . The rate at which the power level of the carrier wave changes is very rapid, thousands of times per second. The overall shape of each wave in the carrier wave is very smoothly wavelike, but the wave preceding or following it may have rather greater or lesser amplitude. Each change in the power of the carrier corresponds to a small piece of the overall message being transmitted. The receiver just stays synchronized with the carrier, and uses variations in the detected power level to control a speaker. When you switch your radio to receive F.M., or "Frequency Modulation" signals, you might notice that the markings on the tuner are numbers of Megahertz. Suppose a particular radio station transmits its carrier wave at the frequency of 88.80 Megahertz. Frequency Modulation means that this carrier wave frequency is deliberatly modified. Sometimes the radio station may transmit waves of 88.81 Megahertz, and sometimes it may transmit waves of 88.79 Megahertz. Of course, a whole range of frequencies surrounding the main carrier of 88.80 Megahertz would be used. An F.M. radio is able to constantly convert the changes in the freqency of the carrier wave into appropriate sound waves. The major reason that we widely use F.M. technology is that lightning doesn't interfere very much. While lightning generates electromagnetic waves all across the spectrum, those waves are not immediately followed by more waves of slightly different frequencies. And since F.M. radios are designed to pay attention to changes in the frequency, the emissions of lightning are mostly ignored. (Well, ok, there are multiple-flash lightning strikes, and a small amount of static does get through -- but it is nowhere near as bothersome as what an A.M. radio picks up.) One of the physical phenomena of electromagnetic waves is that they can don't have to be absorbed; they can bounce. Some things they bounce off easier than others; for light, when we have an especially good light-bouncer, we call it a "mirror", and the bouncing is called "reflectance". In general, the ability of anything to act as a reflector depends on how many unpaired electrically charged particles are concentrated in a given amount of surface area, and it depends on what frequency of electromagnetic wave happens to encounter that surface. The higher the frequency, the greater the concentration of electric charges are needed to make a good mirror. Or simply compare the wavelength to the size of the average gap between electric charges: the smaller the wave, the smaller a gap is needed to keep the wave from getting through. At the top of the Earth's atmosphere is a layer of charged particles collectively known as "the ionosphere". This is a very low-density environment, a near-vacuum, and while the ions move about a lot, there is considerable amount of space between them, on the average. Nevertheless, they are close enough together to reflect A.M. radio waves quite well. Once upon a time I used to think that a neat way to detect alien civilizations would be to listen for the electromagnetic waves produced by alternating-current power -- that's billions of watts and thousands of miles of powerlines acting as antenna -- but then I realized that the ionospheres of those alien planets would be reflecting those waves, and keeping them from getting to us. Probably those planetary ionospheres are doing all electric-powered civilizations a favor, preventing vast energy losses from power grids to outer space.... The ionosphere is not a good enough mirror to reflect the radio frequencies used in F.M. transmissions. To receive an F.M. radio signal, you must be in "line of sight" with the transmitting antenna. (To receive an A.M. radio signal you can be over the horizon from the antenna, because the ionosphere reflects the signal from the antenna to you.) And even if you can't actually see the antenna without a telescope (or even with a telescope, through smoke or fog or brick buildings), you can receive an F.M. signal because most ordinary objects do not reflect those radio waves very well. Steel does, so a building with a lot of steel in it can seriously interfere with radio reception. If you recall what you saw, the last time you looked at a microwave oven, you might remember that the door to the oven was glass, with a piece of metal full of small holes. The microwaves in the oven are too big to get through those holes, so for them the metal is a great mirror. Meanwhile, ordinary light consists of quite-small electromagnetic waves that pass through the holes easily, letting you see what is happening to the food cooking in the oven. And if you recall seeing any windows with a special heat- reflective coating, you might have noticed that the coating reflects some light, but a reasonable amount gets through. In such a coating (known as "partially silvered") the charged particles are close enough together to reflect the frequencies of electromagnetic waves which are infrared, but are still far enough apart to let most light through. As you might expect, a fully silvered coating works as a full mirror for ordinary light (but it would have to be even-more-fully-silvered to reflect ultraviolet light...). The next relevant topic concerns the amount of data that can be loaded onto a carrier wave. There is a limit, after all, and this limit exists because any data represents some kind of change in the nature of the carrier wave. The more data you try to load onto the wave, the less the carrier wave resembles its unloaded state. That can make it rather difficult to detect the carrier wave! Fortunately, the amount of data that can be effectively carried is directly related to the frequency of the carrier wave: The higher the frequency of the carrier, the more data it can carry. In general, there is a 10-to-1 relationship between the frequency of the carrier wave, and the maximum rate at which it can carry data. That is, if a carrier wave has a frequency of 100 Megahertz, then the most it can carry is 10 million bits per second of data. Any more than that, and the carrier wave is distorted into unrecognizability. In our world of backwards-compatible, many-decades-old radio technology, we usually arrange a 20-to-1 or more relationship between the carrier frequency and the rate of data transmission. You can note that while our ears can detect a maximum frequency of about 20 Kilohertz, the LOWEST frequency on the A.M. radio band is 530 Kilohertz -- a 26.5-to-1 ratio. A.M. radio frequencies are quite well able to carry a good-quality audio signal (that is, if lightning didn't interfere with it so much). For F.M. radio, the intent is to broadcast in stereo, thus requiring enough carrier-wave capacity (or "bandwidth") to handle two complete audio signals. When you note that the lowest frequency on the F.M. radio band is 87.5 Megahertz, you can see that there is plenty! And now...just take a quick look at your television screen, and imagine how much data per second that represents! Television waves are Frequency Modulated. Even more than for F.M. radio, you must be in the line of sight of a TV transmitter to receive a good signal. The frequencies of TV-channel carrier waves tend to range from roughly 200 Megahertz to more than one Gigahertz. The digital revolution of recent years will be utterly changing television technology. All old TVs will need to be either replaced or upgraded. The process is starting slowly, but will gain speed as more and more of the newest units are sold. Digital technology lets us easily transmit six times the data, in the same bandwidth, as the older technology. Furthermore, the data being transmitted will actually be compressed data. Uncompressed, the results on your TV screen will be as clear as looking through a simple window, letting you see the world as never before.
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