The radio receives electromagnetic waves from the air that are sent by a radio transmitter. Electromagnetic waves are a combination of electrical and magnetic fields that overlap. The radio converts these electromagnetic waves, called a signal, into sounds that humans can hear.
Radios are a part of everyday life. Not only are they used to play music or as alarms in the morning, they are also used in cordless phones, cell phones, baby monitors, garage door openers, toys, satellites, and radar. Radios also play an important role in communications for police, fire, industry, and the military. Although there are many types of radios—clock, car, amateur (ham), stereo—all contain the same basic components.
Radios come in all shapes and sizes, from a little AM/FM "Walkman" to a highly sophisticated, multi-mode transceiver where both the transmitter and receiver are combined in one unit. The most common modes for a broadcast radio are AM (amplitude modulation) and FM (frequency modulation). Other modes used by ham radio operators, industry, and the military are CW (continuous wave using Morse code), SSB (single sideband), digital modes such as telemetry, radio teletype, and PSK (phase shift keying).
Guglielmo Marconi successfully sent the first radio message across the Atlantic Ocean in December 1901 from England to Newfoundland. Marconi's radio did not receive voice or music. Rather, it received buzzing sounds created by a spark gap transmitter sending a signal using Morse code.
The radio got its voice on Christmas Eve 1906. As dozens of ship and amateur radio operators listened for the evening's traffic messages, they were amazed to hear a man's voice calling "CQ, CQ" (which means calling all stations, I have messages) instead of the customary dits and dahs of Morse code. The message was transmitted by Professor Reginald Aubrey Fessenden from a small radio station in Brant Rock, Massachusetts.
In the years from 1904 to 1914, the radio went through many refinements with the invention of the diode and triode vacuum tubes. These devices enabled better transmission and reception of voice and music. Also during this time period, the radio became standard equipment on ships crossing the oceans.
The radio came of age during World War I. Military leaders recognized its value for communicating with the infantry and ships at sea. During the WWI, many advancements were made to the radio making it more powerful and compact. In 1923, Edwin Armstrong invented the superhetrodyne radio. It was a major advancement in how a radio worked. The basic principles used in the superhetrodyne radio are still in use today.
On November 2, 1920 the first commercial radio station went on the air in Pittsburgh, Pennsylvania. It was an instant success, and began the radio revolution called the "Golden Age of Radio." The Golden Age of Radio lasted from the early 1920s through the late 1940s when television brought in a whole new era. During this Golden Age, the radio evolved from a simple device in a bulky box to a complex piece of equipment housed in beautiful wooden cabinets. People would gather around the radio and listen to the latest news and radio plays. The radio occupied a similar position as today's television set.
On June 30, 1948 the transistor was successfully demonstrated at Bell Laboratories. The transistor allowed radios to become compact, with the smallest ones able to fit in a shirt pocket. In 1959, Jack Kilby and Robert Noyce received the first patent for the integrated circuit. The space program of the 1960s would bring more advances to the integrated circuit. Now, a radio could fit in the frame of eyeglasses or inside a pair of small stereo earphones. Today, the frequency dial printed on the cabinet has been replaced with light emitting diodes or liquid crystal displays.
Today's radio consists of an antenna, printed circuit board, resistors, capacitors, coils and transformers, transistors, integrated circuits, and a speaker. All of these parts are housed in a plastic case.
An internal antenna consists of small-diameter insulated copper wire wound around a ferrite core. An external antenna consists of several aluminum tubes that slide within one another.
The printed circuit board consists of a copper-clad pattern cemented to a phenolic board. The copper pattern is the wiring from component to component. It replaces most of the wiring used in earlier radios.
Resistors limit the flow of electricity. They consist of a carbon film deposited on a cylindrical substrate, encased in a plastic (alkyd polyester) housing, with wire leads made of copper.
Capacitors store an electrical charge and allow alternating current to flow through an electrical circuit but prevent direct current from flowing in the same circuit. Fixed capacitors consist of two extended aluminum foil electrodes insulated by polypropylene film, housed in a plastic or ceramic housing with copper wire leads. Variable capacitors have a set of fixed aluminum plates and a set of rotating aluminum plates with an air insulator.
Coils and transformers perform similar functions. Their purpose is to insulate a circuit while transferring energy from one circuit to another. They consist of two or more sets of copper wire coils either wound on an insulator or mounted side-by-side with air as the insulator.
Transistors consist of germanium or silicon encased in a metal housing with copper wire leads. The transistor controls the flow of electricity in a circuit. Transistors replaced vacuum tubes used in earlier radios.
The integrated circuit houses thousands of resistors, capacitors, and transistors into a small and compact package called a chip. This chip is about the size of the nail on the little finger. The chip is mounted in a plastic case with aluminum tabs that allow it to be mounted to a printed circuit board.
Radios consist of many specialized electronic circuits designed to perform specific tasks—radio frequency amplifier, mixer, variable frequency oscillator, intermediate frequency amplifier, detector, and audio amplifier.
The radio frequency amplifier is designed to amplify the signal from a radio broadcast transmitter. The mixer takes the radio signal and combines it with another signal produced by the radio's variable frequency oscillator to produce an intermediate frequency. The variable frequency oscillator is the tuning knob on the radio. The produced intermediate frequency is amplified by the intermediate frequency amplifier. This intermediate signal is sent to the detector which converts the radio signal to an audio signal. The audio amplifier amplifies the audio signal and sends it to the speaker or earphones.
The simplest AM/FM radio will have all of these circuits mounted on a single circuit board. Most of these circuits can be contained in a single integrated circuit. The volume control (a variable resistor), tuning knob (a variable capacitor), speaker, antenna, and batteries can be mounted either on the printed circuit board or in the radio's case.
There is no single process for manufacturing a radio. The manufacturing process depends upon the design and complexity of the radio.
Manufacturers purchase the basic components such as resistors, capacitors, transistors, integrated circuits, etc., from vendors and suppliers. The printed circuit boards, usually proprietary, may be manufactured in house. Many times, manufacturers will purchase complete radio modules from an vendor. Most of the manufacturing operations are performed by robots. These include the printed circuit boards and mounting of the components on the printed circuit board. Mounting of the printed circuit board and controls into the case and some soldering operations are usually done by hand.
- The blank printed circuit board consists of a glass epoxy resin with a thin copper film cemented to one or both sides. A light sensitive photoresist film is placed over the copper film. A mask containing the electrical circuitry is placed over the photoresist film. The photoresist film is exposed to ultraviolet light. The photoresist image is developed, transferring the image to the copper film. The unexposed areas dissolve during etching and produce a printed circuit on the board.
- Holes are drilled in designated locations on the printed circuit board to accept the components. Then, the board is pre-soldered by dipping it in a bath of hot solder.
- Smaller electronic components such as resistors, capacitors, transistors, integrated circuits, and coils are installed in their designated holes on the printed circuit board and soldered to the board. These operations can be performed by hand or by robots.
- Larger components such as power transformer, speaker, and antenna are mounted either on the PCB or cabinet with screws or metal spring tabs.
- The case that houses the radio can be made either of plastic or aluminum. Plastic cases are made from pellets that are melted and injected into a mold. Aluminum cases are stamped into shape from sheet aluminum by a metal press.
- External components not mounted on the printed circuit board can be the antenna, speaker, power transformer, volume, and frequency controls are mounted in the case with either screws, rivets, or plastic snaps. The printed circuit board is then mounted in the case with screws or snaps. The external components are connected and soldered to the printed circuit board with insulated wires made of copper and plastic insulation.
Since most of the components or a radio are manufactured by specialized vendors, the radio manufacturer must rely on those venders to produce quality parts. However, the radio manufacturer will take random samples of each component received and inspect/test them to ensure they meet the required specifications.
Random samples of the final radio assembly are also inspected to ensure quality. The overall unit is inspected for flaws—both physical and electrical. The radio is played to ensure it can select radio frequencies it's design to receive, and that the audio output is within specifications.
Today's environmental awareness dictates that all waste be disposed of properly. Most byproducts from the construction of a radio can be reclaimed. The etching solutions used in the printed circuit board manufacture are sent to chemical reclamation centers. Scraps from the leads of electronic components are sent to metal waste recovery centers where they are melted to create new products.
Radios are being combined with computers to connect the computer to the Internet via satellites. Eventually radios will convert from analog to digital broadcasting. Analog signals are subject to fade and interference, digital signals are not. They can produce high quality sound like that found on a CD.
Digital radios can be programmed for specific stations, types of music, news, etc. Eventually, radios will have mini-computers built in to process sounds in numerical patterns "digits" rather than an analog waveform. This will allow listeners to program their radios for favorite radio stations, music type, stock quotes, traffic information, and much more.
Where to Learn More
Carter, Alden R. Radio From Marconi To The Space Age. New York: Franklin Watts, 1987.
Floyd, Thomas L. Electric Circuit Fundamentals. Columbus: Merrill Publishing Company, 1987.
The American Radio Relay League. The ARRL Handbook for Radio Amateurs. Newington, CT: ARRL, 1996.
Canadian Broadcasting Company Web Page. "The Future of Digital Radio.: December 2001. < http://radioworks.cbc.ca/radio/digital-radio/drri.html >.
UC Berkley Web Page. December 2001. < http://www.cs.berkeley.edu/~gribble/cs39c/Comm/radio/radio.html >.
Ernst S. Sibberson