SHRI VISHNU ENGINEERING COLLEGE FOR WOMEN :: BHIMAVARAM DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING ANTENNAS & WAVE PROPAGATION Antenna fundamentals: Unit - 1 Introduction An antenna is defined by Webster’s Dictionary as “a usually metallic device (as a rod or wire) for radiating or receiving radio waves.” The IEEE Standard Definitions of Terms for Antennas (IEEE Std 145–1983) defines the antenna or aerial as “a means for radiating or receiving radio waves.” In other words the antenna is the transitional structure between free-space and a guiding device, as shown in Figure. The guiding device or transmission line may take the form of a coaxial line or a hollow pipe (waveguide), and it is used to transport electromagnetic energy from the transmitting source to the antenna or from the antenna to the receiver. In the former case, we have a transmitting antenna and in the latter a receiving antenna. An antenna is basically a transducer. It converts radio frequency (RF) signal into an electromagnetic (EM) wave of the same frequency. It forms a part of transmitter as well as the receiver circuits. Its equivalent circuit is characterized by the presence of resistance, inductance, and capacitance. The current produces a magnetic field and a charge produces an electrostatic field. These two in turn create an induction field.
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: