ABSTRACT
Antennas are used to radiate efficiently electromagnetic energy in desired directions. Antennas match radio frequency systems, sources of electromagnetic energy, to space. All antennas may be used to receive or radiate energy. Antennas transmit or receive electromagnetic waves at radio frequencies. They convert electromagnetic radiation into electric current, or vice versa. Antennas are a necessary part of all communication links and radio equipment. They are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless local area network (LAN), cell phones, radar, medical systems, and spacecraft communication. Antennas are most commonly employed in air or outer space, but can also be operated under water, on and inside the human body, or even through soil and rock at low frequencies for short distances. Physically, an antenna is an arrangement of one or more conductors. In transmitting mode, an alternating current is created in the elements by applying a voltage at the antenna terminals, causing the elements to radiate an electromagnetic field. In receiving mode, an electromagnetic field from another source induces an alternating current in the elements and a corresponding voltage at the antenna’s terminals. Some receiving antennas (such as parabolic and horn) incorporate shaped reflective surfaces to receive the radio waves striking them and direct or focus them onto the actual conductive elements [1-11].
