An antenna is one of the fundamental parts of a radio; it is not only needed to receive radio waves, but to transmit them as well.

In 1887 Heinrich Hertz (1857-1894) discovered that electromagnetic radio waves were produced by the oscillations of alternating current in a wire. Using a simple dipole antenna (two metal plates connected to a rod), he was able to detect what he called " Hertzian waves." His Hertzian dipole antenna allowed him to measure both the shape and intensity of the invisible waves. The rate of the oscillation of the current that produces radio waves, known as the frequency, is now measured in kilohertz or megahertz in honor of Hertz's discovery.

A transmitting antenna converts the oscillating electric current into radio waves, whose frequency is identical to that of the original electrical oscillations. The waves radiate outward from the antenna in all directions like ripples on...