ABSTRACT
In the radio-frequency-microwave-frequency range, antennas are often used to launch and focus the radiation to a limited beam width so that the signal to noise ratio at the receiver is maximum and the interference to other wireless links in the same frequency band is minimized. An antenna is therefore a device that converts confined radiation from a transmission line or waveguide into an unguided but directed electromagnetic wave in the ambient medium. The receiving characteristics of an antenna are identical to its transmitting characteristics; thus, descriptions of the properties of an antenna are equally valid in terms of the reception characteristics and transmission characteristics. There are many different requirements for antenna systems. In broadcast applications, it is desirable that the transmitted radiation can be detected over a large area. The directivity of an antenna is the ratio of the power density in the main beam to the average power density.
