Antenna & Wave Propagation

                                INTRODUCTION TO ANTENNA

The antenna is a device, which radiates or receives electromagnetic waves.

The antenna is the transition structure between a guiding device (transmission line, waveguide) and free space. 

CONCEPT OF RADIATION:-

The radiation from the antenna takes place when the electromagnetic field generated by the source is transmitted to the antenna system through the transmission line and separated from the Antenna into free space.

The four methods of Excitation of Antenna are:

1. Centre-fed

2. Shunt- fed

3. End-fed

4. Tapped-fed.

 

The Antenna system can be shown by a Transmission line Thevenin equivalent circuit in Fig. 1.1 (a) and (b)

In ideal condition is, the source should transfer emitted radiation totally into radiation resistance Rd, whereas in practical condition RL represent conduction and dielectric loss at the interface of transmission line and antenna. Now the maximum power transfer could be possible when the source matches the load. So, the antenna must have the ability to match the transmission line and the load impedance. The matching is specified in terms of VSWR. Th5 standing waves are to be avoided because they may cause arching or discharging the transmission line. The standing waves can be reduced by impedance matching” the transmission line, and the antenna.

 

The time deviation of a current source is proportioned to the amount of charge q in the volume of current element and to its acceleration. Putting Al on both side of equation 1.2.

In summary:

• If charge is, not moving, current is zero, no radiation.

• If charge is moving with a uniform velocity, no radiation.

i.e, Radiation is produced by accelerated or decelerated. charge (time varying current element)

• l charge accelerated due to electromotive force or due to discontinuities, TadiáETs.