11.1 INTRODUCTION

The antenna is the most important device in any transmitting station. Antennas are very commonly used for radio and television broadcasting. Also, antennas are used for dedicated point-to-point radio communication, radar, wireless LAN, space exploration, etc. An antenna is a logical arrangement of several conductors also called "elements". An antenna also called an aerial is basically a transducer. The antenna is the device that sends a radio frequency (RF) signal through the space to the receiver. This antenna is also called the transmitting antenna. An alternating current is produced by applying a voltage to the transmitting antenna elements, which causes the elements to radiate the electromagnetic field. The receiving antenna picks up the RF signal (basically a travelling electromagnetic field or wave through the space) that is being sent by the transmitting antenna. When an RF signal is received by the receiving antenna, it induces a voltage into it. Thus, it can be easily said that an antenna converts electromagnetic waves into electrical currents and vice versa. In the receiving antenna, this electrical current produced due to the induced voltage is converted back into the information that was transmitted in the form of the RF signal. Thus, a receiving antenna does exactly the reverse of a transmitting antenna. Due to reciprocity, all antenna parameters applicable to a transmission antenna are also applicable to a receiving antenna.

The size (dimensions) and design of an antenna should be such that it is efficiently able to radiate the generated RF signal. In no case should the power supplied by the transmitter to it go waste. The transmitting antenna size should be exact, which is determined by the transmitting frequencies. The receiving antenna size is not an important issue especially in relatively low RFs. The receiving antenna size, design and installation become an important issue as the frequency of the signal being received increases. It happens in the case of a household television-receiving antenna. When we raise it from the ground and/or give it a turn (change in direction), then a snowy or blurred picture can be changed into a vivid picture. Some receiving antennas like the parabolic and the horn incorporate shaped reflective surfaces for collecting electromagnetic waves from free space. It is then directed or focused onto the actual conductive elements. Some special-purpose short-distance antennas can even be placed in the water or soil or rock. Antenna tuning is basically altering an electrical resonance antenna. An antenna's electrical resonance is changed by adjusting its length. Antenna tuning is done by adjusting the values of inductance or capacitance, which are externally combined to the active antenna. The inductive and capacitive reactances combine with the active antenna's internal reactance and establish a resonance in the circuit, which also includes an active antenna. Such resonance frequency is different from the active antenna's electrical resonant frequency. Various antennas are the Yagi-Uda beam antenna, rooftop TV antenna, shortwave antenna, broadband antenna, multi-band rotary directional antenna, terrestrial microwave radio antenna, rotatable log-periodic array antenna, etc. Antenna shields are used for noise rejection.