Radio antennas take their name from the humble centre pole of a tent
In summer 1895 a young Guglielmo Marconi was testing wireless signals on his father's estate outside Bologna, hanging long wires from a pole. Italians call a tent's central pole l'antenna centrale, so his wired pole became simply l'antenna. Thanks to his fame, the word spread worldwide, replacing the drab earlier label, terminals.
Marconi did not build the first ones, though. In 1886 the German physicist Heinrich Hertz constructed antennas to prove that electromagnetic waves, predicted by James Clerk Maxwell's theory of 1867, really existed. Hertz set dipoles at the focus of curved reflectors for both sending and receiving. Marconi's contribution, from 1895, was making antennas that worked over long distances for wireless telegraphy, and in 1898 he opened a factory in Chelmsford, England, to produce his equipment.
An antenna works as a translator between two worlds: currents flowing in metal and waves travelling through space at light speed. The same shape behaves identically whether sending or receiving, a property called reciprocity. A simple horizontal dipole broadcasts in most directions but stays silent along the line of its own wire, a blind spot engineers call its null.
One surprise is what antenna gain means. An amplifier genuinely adds power. An antenna cannot; it is a passive device that only redistributes the fixed power a transmitter feeds it, beaming more one way by sending less elsewhere. Focusing tightly requires an antenna large relative to the wavelength, which is why dishes and horns suit the high frequencies from UHF upward, while AM stations need arrays of tall towers spread across big plots of land.
Rooftop television aerials show two clever designs. The log-periodic type strings together roughly 10 to 20 connected dipoles of steadily changing length, so it works across a wide band. The Yagi–Uda looks similar but wires up only one element; the rest are unconnected parasitic rods that shape the field into a sharp but narrow-band beam.
Source: Antenna (radio)
