Definition of antenna An antenna can be defined in the following different ways: 1. An antenna may be a piece of conducting material in the form of a wire, rod or any other shape with excitation. 2. An antenna is a source or radiator of electromagnetic waves. 3. An antenna is a sensor of electromagnetic waves. 4. An antenna is a transducer. 5. An antenna is an impedance matching device. 6. An antenna is a coupler between a generator and space or vice-versa.
Radiation Mechanism 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. Radiation from a Single Wire Assume the existence of a piece of very thin wire where electric currents can be excited. The current I flowing through the wire cross-section charge passing through i Where S is defined as the amount of S in 1 second: S l1 S 1.1 v, A c / m 3 is the electric charge volume density, vm / s is the velocity of the charges normal to the cross-section. l1 m / s is the distance traveled by a charge in 1 second. Equation (1.1) can be also written as v , A / m2 J 1.2 Where j is the electric current density. The product is the charge per unit 1 S length (charge line density) along the wire. Thus, from (1.1) it follows that 1.3 i v 1 A. It is then obvious that di dt 1 dv 1 dt A / s 1.4 a, Where am / s 2 is the acceleration of the charge. The time-derivative of a current source would then by proportional to the amount of charge q enclosed in the volume of the current element and to its acceleration : di A l 1 m / s dt a q a, 1.5
Radiation is produced by accelerated or decelerated charge (time-varying current element) Figure 1 It is not immediately obvious from Maxwell’s equations that the time varying current is the source of radiated EM field. However, the system of the two first-order Maxwell’s equations in isotropic medium, h t e 1.6 e j t h Can be easily reduced to a single second-order equation for the E vector, or for the H vector. By taking the curl of both sides of the first equation in (1.6) and by making use of the second equation in (1.6), we obtain 2 e h j 1.7 t2 t From the vector wave equation (1.7), it is obvious that the time derivative of the electric currents is the source for the wave-like propagation of the vector e in homogeneous and isotropic medium. In an analogous way, one can obtain the wave equation for the magnetic field H and its sources:
To accelerate/decelerate charges, one needs sources of electromotive force and/or discontinuities of the medium in which the charges move. Such discontinutities can be bends or open ends of wires, change in the electrical properties of the region, etc. . In summary: It is a fundamental single wire antenna. From the principle of radiation there must be some time varying current. For a single wire antenna, 1. If a charge is not moving, current is not created and there is no radiation. 2. If charge is moving with a uniform velocity: a. There is no radiation if the wire is straight, and infinite in extent. b. There is radiation if the wire is curved, bent, discontinuous, terminated, or truncated, as shown in Figure. 3. If charge is oscillating in a time-motion, it radiates even if the wire is straight.