Riding the Electromagnetic Wave
by Laurie Peach Toupin
You're picking up your new puppy, Belle, and you can't wait to show your friends! So you take Belle's picture with your cell phone and send it out from the Humane Society. One friend picks up the picture on her computer at home. Another friend sees it on her laptop in McDonald's, where she's doing her homework as she waits for a ride. Both friends are waiting in your driveway, eager to see Belle, when you get home. How did this one picture travel so far so fast? It rode the electromagnetic waves!
A cell phone is nothing more than a sophisticated two-way radio—a device that transmits, receives, and decodes electromagnetic waves.
Electromagnetic waves come from the same kind of electric force that flows through your wall plug, and from the same kind of magnetic force that keeps your younger brother's painting stuck to the refrigerator door. When electric and magnetic forces act together, they vibrate at right angles to each other and make electromagnetic radiation, or waves. The waves are shaped like sinusoidal curves, which resemble smooth, up-and-down hills. The distance between the tops or the bottoms of these hills is known as the wavelength. These electromagnetic waves move across space at a certain number of repetitions, or cycles, per second; this speed is known as their frequency. It is measured in hertz—named after Heinrich Hertz, the German scientist who first measured them. Frequency and wavelength are indirectly proportional: a wave with a high frequency has a short wavelength, and one with a low frequency has a long wavelength.
There are seven groups of electromagnetic frequencies, each moving through space at its own wavelength (all electromagnetic waves travel at the speed of light). Radio, microwave, infrared, visible light, ultraviolet, X–ray, and gamma ray waves make up what is known as the electromagnetic spectrum.
A Simple Radio
To create a radio wave, an electric signal is fed to the antenna of a transmitter. The signal makes the electrons in the metal of the antenna change energy levels and emit an electromagnetic wave or a radio-frequency carrier wave. To send information, a modulator in the transmitter encodes the sound signal onto the carrier wave. (See below, “AM vs. FM.”)
This encoded wave is broadcast from a radio tower, which transmits several carrier waves at once, each at a different frequency.
These frequencies give us our channels. A station at 99.5 FM, for example, is broadcasting at a frequency of 99.5 megahertz or 99,500,000 hertz.
A radio receiver—the radio in your bedroom—has an antenna that is tuned to capture your favorite station or frequency. When it picks up the radio waves, a demodulator separates or decodes the sound signal from the carrier wave. The sound signal is amplified and sent to a loudspeaker, where the sound is reproduced as your radio plays.
Analog vs. Digital
Your cell phone contains both a radio transmitter and a receiver.
The digital picture that you took of your new dog was sent as an electromagnetic wave that contains numerical information instead of sound. And instead of using radio towers, it was transmitted and received over a cellular network. (See “What's in a Name?” below) Because a cell phone's transmitter, embedded in the phone, is so much smaller than a radio tower, it can transmit maybe only 8 to 30 miles, as opposed to what can be done by a tall radio tower, which can transmit for a hundred miles or more. This creates a need for many cellular base towers, which contain a large antenna and radio equipment. These base towers “hand off” your call to one another, relaying the clarity of your message from your phone to whomever you are calling.
But before the picture could be sent, it had to be converted to a digital signal, meaning that the picture became a numerical formula—a series of 0s and 1s.
Radios and the very first cell phones carried only analog signals—a continuous flowing wave. Unfortunately, analog signals cannot carry huge amounts of information quickly. So engineers developed a way to convert data—voice, pictures, movies—into bits, or a series of binary digits (0s or 1s) that a computer recognizes. By combining these bits in various ways, a computer or cell phone reads the information and converts it back into a format that we can understand.
Sending the Picture
After your cell phone changes the picture to a set of numbers, it sends this information, along with the e-mail or instant messaging address of your friends, across the cell phone frequency to a cell base tower.
At the tower, the radio wave is converted to another kind of signal, such as optical or electrical, and sent to the head office of the cell phone company. The receiving system at the company sees the e-mail address and sends it out to your friend's Internet provider.
On the Receiving End
The Internet provider doesn't know where your friends are, so the picture of Belle waits for them. When the friend at home checks her e-mail, she is using a wired connection to the Internet. Two of the most popular kinds of Internet connections are telephone lines or cable TV lines. In this case, your friend is using a Digital Subscriber Line (DSL). DSL is a high-speed connection that uses the same wires as the telephone. Voice travels through the copper wires on frequencies of up to 3,400 hertz. But the wires have the potential to handle frequencies of up to several million hertz. DSL takes advantage of this extra frequency space. By transmitting digital data signals at higher frequencies, the voice and data transmissions can travel along the same line simultaneously without interfering with one another. Filters are placed along the wire to direct the signal to the phone, the fax, or the computer.
The friend at McDonald's isn't connected directly to the Internet via a wire. Instead, she is using Wi-Fi—short for Wireless Fidelity. Wi-Fi may be used over a Wireless Local Area Network, or WLAN, which is used to network computers together or to the Internet, within a small area, without a physical connection. To do this, the McDonald's friend has a special Wi-Fi modem in her laptop that acts as another kind of radio transmitter and receiver. The modem sends and receives electromagnetic signals back and forth to a computer network station commonly called a Wi-Fi hot spot. The computer station serves the same function as the cell phone towers. It decodes radio signals and sends the information back and forth. The radio in the laptop has an even smaller range than the one in the cell phone. It is only good for 100 feet, and sometimes less. But it can send 100 times more information back and forth, making it fast enough to check e-mail and play computer games. When your friends have received the message with Belle's picture, it's up to them to figure out how to get a ride to your house to meet her. But it was the electromagnetic wave that got your message to them!
- The name is short for modulator and demodulator, a device or program that enables a computer to transmit data. Because computer information is stored digitally, and information transmitted over telephone lines, for example, is typically analog, a modem changes—or modulates—between the two forms.
- What kind of a comparison could you make between an analog signal and a kind of electric motor? Why?
[anno: An analog signal is like a DC motor because both the analog signal and the electric charge move in one direction only.]
- What advantage does a digital signal have over an analog signal?
[anno: A digital signal allows much more information to be transmitted more quickly than an analog signal.]
- Draw a diagram that shows the path that a picture photographed by a digital cell phone takes from the cell phone to a person's computer via e-mail. The computer is connected to the Internet using a DSL connection. Label each part of your diagram. If the signal is converted from one kind of signal to another, include this information.
[anno: Each diagram should begin with a cell phone. From the cell phone, digital information is transmitted via digital signal to a cell base tower. This cell base tower transfers the digital signal to another cell base tower. This process may be repeated a number of times. A cell base tower eventually converts the signal into either an optical or an electrical signal and sends this information to a receiver at an Internet provider. The signal is probably converted from digital to analog and then sent from the receiver at the Internet provider to a telephone line. The signal passes through a switching station on the phone line and is then directed to the phone line running into the correct home. There the signal is received by a modem. The modem translates the analog signal into digital information, which is then decoded by the person's e-mail program.]