Antennas (or aerials) are devices that radiate (or couple) electromagnetic energy into space. Their effectiveness depends on their ability to concentrate (or direct) the energy into the desired direction. An efficient antenna is one in which very little or no energy is lost in the radiation process. The transmitting and receiving antennas have similar characteristics and in many applications, a single antenna is used to perform both the functions. As most of the radars and communication systems operate in microwave range, the antennas used in these applications are referred to as microwave antennas. Since the basic principles of operation of microwave antennas are similar to those used at lower frequencies, their basic characteristic parameters are also similar. The design of antennas relating to their sizes, shapes and types is mainly governed by the operating frequency, space occupied, weight and some other practical considerations. The electrical characteristics of antennas derived from the antenna theory are summarised in Section 9.2.