Radio is the wireless transmission of signals through free
space by electromagnetic radiation of a frequency
significantly below that of visible light, in the radio frequency
range, from about 3 kHz to 300 GHz. These waves are called
radio waves. Electromagnetic radiation travels by means of
oscillating electromagnetic fields that pass through the air
and the vacuum of space.
An antenna (or aerial) is an electrical device which converts
electric currents into radio waves, and vice versa. It is usually
used with a radio transmitter or radio receiver. In reception, an
antenna intercepts some of the power of an electromagnetic
wave in order to produce a tiny voltage at its terminals, that is
applied to a receiver to be amplified. An antenna can be used
for both transmitting and receiving.
In 1894 the young Italian inventor Guglielmo Marconi began
working on the idea of building a commercial wireless
telegraphy system based on the use of Hertzian waves (radio
waves), a line of inquiry that he noted other inventors did not
seem to be pursuing. In 1895 he built a radio wave system
capable of transmitting signals at long distances (1.5 mi./
2.4 km).Marconi found from his experiments the phenomenon
that transmission range is proportional to the square of
antenna height, known as "Marconi's law."