History and Applications

Since 1901, the time of Marconi's first experiments with transmitting electromagnetic waves, antennas have found several important applications over the entire frequency range, and numerous designs of antennas now exist. Antennas are an integral part of our everyday lives and are used for a multitude of purposes. All antennas operate on the same basic principles of electromagnetic theory formulated by James Clark Maxwell. An antenna is used to either transmit or receive electromagnetic waves, and it serves as a transducer that converts guided waves into free-space waves in the transmitting mode, or vice-versa in the receiving mode. Maxwell put forth his unified theory of electricity and magnetism in 1873 [1] in his famous book A Treatise on Electricity and Magnetism, incorporating all previously known results on electricity and magnetism and expressing these mathematically through what we refer to as Maxwell's equations, which hold over the entire electromagnetic spectrum. His theory was met with much skepticism, and it was not until 1886 that Heinrich Hertz [2], considered the Father of Radio, was able to validate this theory with his experiments. The first radio system, at a wavelength of 4 m, consisted of a I»/2 dipole (transmitting antenna) and a resonant loop (receiving antenna) [3]. By turning on the induction coil, sparks were induced across the gap and detected at the receiving antenna.