March 12, 1995
Radio is a form of communication in which intelligence is transmitted without wires from one point to another by means of electromagnetic waves. Early forms of communication over great distances were the telephone and the telegraph. They required wires between the sender and receiver. Radio, on the other hand, requires no such physical connection. It relies on the radiation of energy from a transmitting antenna in the form of radio waves. These radio waves, traveling at the speed of light (300,000 km/sec; 186,000 mi/sec), carry the information. When the waves arrive at a receiving antenna, a small electrical voltage is produced. After this voltage has been suitably amplified, the original information contained in the radio waves is retrieved and presented in an understandable form. This form may be sound from a loudspeaker, a picture on a television, or a printed page from a teletype machine.
The principles of radio had been demonstrated in the early 1800s by such scientists as Michael Faraday and Joseph Henry. They had individually developed the theory that a current flowing in one wire could induce (produce) a current in another wire that was not physically connected to the first.
Hans Christian Oersted had shown in 1820 that a current flowing in a wire sets up a magnetic field around the wire. If the current is made to change and, in particular, made to alternate (flow back and forth), the building up and collapsing of the associated magnetic field induces a current in another conductor placed in this changing magnetic field. This principle of electromagnetic induction is well known in the application of transformers, where an iron core is used to link the magnetic field of the first wire or coil with a secondary coil. By this means voltages can be stepped up or down in value. This process is usually carried out at low frequencies of 50 or 60 Hz (Hertz, or cycles per second). Radio waves, on the other hand, consist of frequencies between 30 kHz and 300 GHz.
In 1864, James Clerk Maxwell published his first paper that showed by theoretical reasoning that an electrical disturbance that results from a change in an electrical quantity such as voltage or current should propagate (travel) through space at the speed of light. He postulated that light waves were electromagnetic waves consisting of electric and magnetic fields. In fact, scientists now know that visible light is just a small portion of what is called the electromagnetic spectrum, which includes radio waves, X rays, and gamma rays (see electromagnetic radiation).
Heinrich Hertz, in the late 1880s, actually produced electromagnetic waves. He used oscillating circuits (combinations of capacitors and inductors) to transmit and receive radio waves. By measuring the wavelength of the waves and knowing the frequency of oscillation, he was able to calculate the velocity of the waves. He thus verified Maxwell's theoretical prediction that electromagnetic waves travel at the speed of light.
It apparently did not occur to Hertz, however, to use electromagnetic waves for long-distance communication. This application was pursued by Guglielmo Marconi; in 1895, he produced the first practical wireless telegraph system. In 1896 he received from the British government the first wireless patent. In part, it was based on the theory that the communication range increases substantially as the height of the aerial (antenna) is increased.
The first wireless telegraph message across the English Channel was sent by Marconi in March 1899. The use of radio for emergencies at sea was demonstrated soon after by Marconi's wireless company. (Wireless sets had been installed in lighthouses along the English coast, permitting communication with radios aboard nearby ships.) The...