The radio waves are a form of electromagnetic radiation. This theory was first emitted by the British James Clark Maxwell in 1864 and later confirmed by the German Heinrich Hertz in 1880. In 1887, Hertz showed in public the transmission and reception of radio waves. His emitter produced an electric current discharged as sparkles, alternated rapidly and changed flow direction. The rapidly changing current determined two metal plates emit radio waves, which Hertz received initially at a distance of about 3 m (10 ft), using the simplest form of receptor, a wire loop. In an obscure chamber, it could be clearly seen a sparkle jumping over the loop's space, each time the emitter was turned on.
In 1896, the Italian Guglielmo Marconi patented the first functional system of telegraphy through radio. Like the electric telegraph, it could send message on a certain distance, using short and long impulses, the points and lines of the Morse signals. The radio transmission had the advantage that it did not require wires between the emitter and receptor (they are wireless). Marconi's receptor had a device called corer, made of a glass tube containing metal filings. The filings decreased resistance, so that a bell could ring or an eco-meter could click as a response to the signals. Now the action range of the radio system could be extended.
Marconi established, in 1896, a telegraphic radio connection between several buildings in London and, in 1897, he realized a 13 km (8 mi) connection over the Bristol Channel, and a 29 km (18 mi) connection between Poole (Dorset) and Wight Island.
In 1897, the English Oliver Lodge created the tuning. A circuit made of an electric capacitor and a bobbin were used for controlling the speed of the current alternation through the emitter, thus the speed of the radio waves. This determined the frequency of the emitted waves. In the receptor, another circuit selected the desired waves. This system showed that various telegraphy systems could be used at the same time without interferences.
In 1901, Marconi managed the first radio transmission over the Atlantic. In St. John (Newfoundland) he received a signal transmitted from Poldhu (Cornwall), on a distance of over 3,000 km (1,900 mi). Many ships were endowed with radio gears for keeping contact with ports and call for help in cases of emergency.
In 1900-1906, the Canadian Reginald Fessenden developed radiotelephony, through which not signals, but real sounds were transmitted. The base principle was the use of a microphone that modulated the radio waves. The receptors had radio earphones. The sparkle emitter gave an impure sound, with noisy background clicks. Fessenden developed an alternator (generator of alternative power), especially conceived to produce the rapid alternation of the electric current required by the emitter. The newly developed electric lamps induced new techniques of emission-reception.
The triode (3 electrodes) lamp generated a pure radiofrequency signal which could be used by an emitter to modulate a sonic signal and amplify the modulated frequency, before being released by the emission antenna. Electric lamps could be used in receptor to amplify the signal received by its antenna to separate the sonic signal from the carrying wave and to amplify it before being reproduced in earphones or speakers.
When public radiodiffusion emerged in 1920, people listened to earphones connected to simple radios. Inside the radio earphones, an electromagnet acted on a thin metal plate, called diaphragm. Electric signals passed through earphones, exciting the electromagnet and making the metal vibrate, emitting proper sounds. Most of these radios detected the sonic composition of the received signal using a galena (lead sulfide) crystal and a wire with sharp point, called "mustache". When the wire made contact to a sensitive point on the crystal, it acted like a redresser (diode), allowing the passing of the current just in one direction.
These radios did not required batteries or a power source, as they used the energy provided by a wire antenna. But this required long wires for receiving signals from remote stations and had weak selectivity, not being able to select similar frequencies, a severe issue with the increase in number of commercial radios. The introduction of receptors with amplifying electric lamps solved the issue. This also allowed a proper volume so that radio could be heard in a room.
Medium to long band radios emit using amplitude modulation (AM). Sonic signals modulate or vary the amplitude or intensity of the wave frequencies. These signals can be jammed by storm and various house electric devices, because their impulses overlap the radio signals. Amplitude changes are detected together with sonic signals, causing background undesired clicks and hisses.
In frequency modulation (FM), the issue is solved because sonic signals are used for varying the frequency of the wave. The FM receptor is conceived to detect frequency changes, not amplitude changes, so that any background interference is not detected. A FM commercial radio occupies a wide frequency band, so that this type of emission must be adapted in VHF (very high frequency) or even larger wave frequency bands.
Along the years, receptors turned smaller and more compact, as the electric lamps dwindled gradually and later were replaced by small transistors. In most current portable radios, the majority of the electronic parts are contained in a sole chip, about the size of the thumb's nail.
Today, radio transmission is more than commercial radios. Truck drivers use CB (citizen's band) radios to keep in touch. Troops use emission-reception devices in combat situations.
With so many commercial radios today, selecting the right one can be difficult. Intelligent systems have been developed for this. RDS systems transmit a supplementary not audible signal, containing information about each station. A chip from the receptor sort this information, displaying on a signaling screen. If more emitter transmit on the selected program, the receptor connects to the strongest signal, providing a good reception to the listener/driver.
Satellite communication too uses radio waves, of ultrahigh, very high and superhigh frequencies (4, 6 or 8 gigahertz), as these waves are not reflected by ionosphere (the atmosphere layer located at heights of 330-560 km).