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ANTENNA

ANTENNA is a device which, used in conjunction with a transmitter or receiver, is used for radiating or receiving radio waves. An antenna is a specialized transducer that converts radio-frequency (RF) fields into alternating current (AC) or vice-versa. Antennas take many forms and sizes dependent upon the job they are to perform. There are different kinds of antennas. The two basic types are the receiving antenna, which intercepts RF energy and delivers AC to electronic equipment, and the transmitting antenna, which is fed with AC from electronic equipment and generates an RF field. _____Magnetic antennas are designed for temporary mounting on vehicles. _____Flexible whip antennas are intended for permanent mounting on vehicles and may also be used on specialized aircraft such as helicopters. _____Weatherproof antennas are intended for permanent installation on boats and for use on fixed towers or masts for monitoring, homing and triangulation. _____Beam antennas are designed for extended weak signal reception and reflection rejection for hand held use. _____In computer and Internet wireless applications, the most common type of antenna is the dish antenna, used for satellite communications. Dish antennas are generally practical only at microwave frequencies (above approximately 3 GHz). The dish consists of a paraboloidal or spherical reflector with an active element at its focus. When used for receiving, the dish collects RF from a distant source and focuses it at the active element. When used for transmitting, the active element radiates RF. _____At frequencies below 3 GHz, many different types of antennas are used. The simplest is a length of wire, connected at one end to a transmitter or receiver. More often, the radiating/receiving element is placed at a distance from the transmitter or receiver, and AC is delivered to or from the antenna by means of an RF transmission line, also called a feed line or feeder ______There are several critical parameters affecting an antenna's performance that can be adjusted during the design process. They are resonant frequency, impedance, gain, radiation pattern, polarization, efficiency and bandwidth.