An antenna is defined by Oxford Dictionary as ? a usually metallic device for radiating or receiving radio waves ?. The IEEE Standard Definitions of terms for Antennas ( IEEE Std 145-1973) defines the antenna or aerial as ? a means for radiating or receiving radio waves ?. In other words the antenna is the transitional structure between free-space and a guiding device. The guiding device or transmission line may take the form of a coaxial line or a hollow pipe (wave guide) and it is used to transport electromagnetic energy from the transmitting source to the antenna or from the antenna to the receiver. In the former case we have a transmitting antenna and in the latter a receiving antenna.
History of radio antennas began as early as 1887 with the first design taking the form of a loop. It was not until 1901 that Marconi used an array of fifty copper wires to perform the first transatlantic transmission. The greatest advancement was due to the introduction of microwave antennas taking the form of reflectors, apertures and arrays. With the introduction and advancement of computer techniques, antenna theory was again revised. The introduction of new elements and design of effective approaches have enhanced the efficiency and effectiveness of modern antennas.