Excitation of antennas and impedance matching

There exist four methods of excitation of antennas. They are centre-fed, shunt-fed, end-fed and tapped-fed. In a centre-fed excitation type, a dipole is fed at the centre by using an open-wire transmission line. In shunt-fed mechanism, feeding is done by using solid dielectric low impedance lines. This method is also known as delta-match excitation which provides good impedance match. Additional advantage here is that it is possible to eliminate standing waves. In end-fed type, antenna is fed up with a single vertical wire. Although this is a simple method, unwanted radiation is a serious cause of concern. In tapped-fed type, horizontal antenna is tapped by a vertical wire. Here impedance and radiation are very low. The main techniques used for impedance matching are stubs, folded dipoles and baluns. A stub is a piece of transmission line. The transmission line whose impedance is to be matched is connected to one end of the stub. Folded dipoles are used to increase the impedance. Balun is a balanced to unbalanced transformer.