Antenna Fundamentals PDF

Lecture - 1- Antenna Fundamentals What is an Antenna? An Antenna is a transducer, which converts electrical power into electromagnetic waves and vice versa. An Antenna can be used either as a transmitting antenna or a receiving antenna.  Their name is borrowed from zoology, in whi...

Lecture - 1- Antenna Fundamentals What is an Antenna? An Antenna is a transducer, which converts electrical power into electromagnetic waves and vice versa. An Antenna can be used either as a transmitting antenna or a receiving antenna.  Their name is borrowed from zoology, in which the Latin word antennae is used to describe the long, thin feelers possessed by many insects. A metallic conductor's structure is called an "antenna" while the wire form is called an “aerial”.  Just as in humans the ears are the transducers that convert acoustic waves into electrochemical impulses.  The antenna is the transition between a guiding device (transmission line, waveguide) and free space (or another usually unbounded medium). Its main purpose is to convert the energy of a guided wave into the energy of a free space wave (or vice versa) as efficiently as possible, while at the same time the radiated power has a certain desired pattern of distribution in space.  An antenna intercepts some of the power of an electromagnetic wave to produce a tiny voltage at its terminals, which is applied to a receiver to be amplified.  Typically an antenna consists of an arrangement of metallic conductors ("elements"), electrically connected (often through a transmission line) to the receiver or transmitter. Radiation Mechanism  To create radiation, there must be a time-varying current or an acceleration (or deceleration) of charge.  For a transmission line, to become a waveguide or to radiate power, has to be processed as such: 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: -There is no radiation if the wire is straight, and infinite in extent. - There is radiation if the wire is curved, bent, discontinuous, terminated, or truncated, as shown in Figure 1.1 3. If charge is oscillating in a time-motion, it radiates even if the wire is straight  The radiation from an antenna can be explained with the help of Figure 1.2 which shows a voltage source connected to a two conductor transmission line. When a sinusoidal voltage is applied across the transmission line, an electric field is created and these results in the creation of electric lines of force which are tangential to the electric field.  The free electrons on the conductors are forcibly displaced by the electric lines of force and the movement of these charges causes the flow of current which in turn leads to the creation of a magnetic field.  Due to the time varying electric and magnetic fields, electromagnetic waves are created and these travel between the conductors.  As these waves approach open space, free space waves are formed by connecting the open ends of the electric lines.  Inside the transmission line and the antenna, the electromagnetic waves are sustained due to the charges, but as soon as they enter the free space, they form closed loops and are radiated.  1.1  Maxwell's equations describe how electric charges and electric currents create electric and magnetic fields. Further, they describe how an electric field can generate a magnetic field, and vice versa E: electric field [volt/m] H: magnetic field intensities [ampere/m] D: electric flux densities also called the electric displacement [coulomb/m2] B: magnetic flux densities or magnetic induction [weber/m2] or [tesla]. ρ: free electric charge density [coulomb/m3] J: electric current density (charge flux) of any external charges (that is, not including any induced polarization charges or magnetization currents in a material [ampere/m2] ∇× is the curl operator ∇· is the divergence operator The charge and current densities ρ, J may be thought of as the sources of the electromagnetic fields. For wave propagation problems, these densities are localized in space; for example, they are restricted to flow on an antenna. The generated electric and magnetic fields are radiated away from these sources and can propagate to large distances to the receiving antennas. ,