Electromagnetic radiation When electric power is applied to a circuit, a system of voltages and currents is set up In it, with certain relations governed by the properties of the circuit. Thus the voltage may be high if the impedance of the circuit is high. Similarly , any power escaping into the free space is governed by the characteristics of free space. Radiation describes any process in which energy emitted by one body travels through a medium or through space. Free space is space that does not interfere with the normal radiation and propagation of radio waves. Thus it has no magnetic or gravitational fields, no solid bodies and no ionized particles. The theory of electromagnetic radiation was proposed by the British physicist James Clerk Maxwell in 1857 and finalized in 1873. EMW are energy propagated through free space at the velocity of light (3 * 10 power 8 meter/sec). Consider a object dropped in a pond. After this there would be path of bubbles generated in the vertical direction also there would be circular wave pattern radiating from the point of contact and spreading horizontally across the body of water. The energy created by the displacement of the liquid is converted into both a vertical and a horizontal component. The energy level of these components varies inversely to the distance i.e horizontal wavefront covers a larger area and spreads the total energy generated over this expanding wavefront. Energy reduces with the increasing distance from point of contact. This can be related to power density. PD is radiated power per unit area. It is seen that the PD is reduced to one-quarter of its value when the distance from the source is doubled. The vertical and horizontal component for the example above can be directly related to the electric and magnetic component in EMW. The fig in 8-1 shows the electromagnetic wave, where it has two components horizontal (magnetic field denoted by H) and vertical (electric field denoted by V). The direction of the electric field, the magnetic field and the propagation are mutually perpendicular in EMW. Waves in free space EMW spread uniformly in free space in all directions from a point source. The wavefront produced is spherical. Rays are imagined which radiate from the point source in all directions. They are everywhere perpendicular to a tangential plane of the wavefront just like spokes of a wheel. Wavefront is the plane joining all points of identical phase. It is spherical here. Length of Q = 2 length of P, area of new sphere = 4 area of inner sphere Power density is defined as radiated power per unit area. Here power density is reduced to one- quarter of its value when the distance from the source has doubled. Power density is inversely proportional to the square of the distance from the source. This is inverse square law which applies universally to all forms of radiation in free space. Stated mathematically, P = Pt / 4 ∏ r2……………………………………….8-1 Where P = power density at a distance r from an isotropic source Pt= transmitted power Isotropic source is one that radiates uniformly in all directions in space. The electric and magnetic field intensities of electromagnetic waves are direct counter parts of voltage and current in circuits. They are measured in voltage per meter and amperes per meter respectively. As we have V=IZ…..Z is the impedance (related to inductor in the circuit) Similary, Permittivity relates to a material's ability to transmit (or "permit") an electric field. Permittivity is measured in farads per meter. (F/m) In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter µ. Permeability is measured in the henry per metre (H/m) Permeability is the equivalent of inductance and permittivity is the equivalent of capacitance in electric circuits. Now to calculate a value for the characteristic impedance of free space. It is seen that field intensity is inversely proportional to the distance from the source, since it is proportional to the square root of the power density. Radiation and Reception Antennas radiate EMW. Radiation will result from electron flow in a suitable conductor. This is predicted mathematically by Maxwell equations, which show that current flowing in a wire is accompanied by a magnetic field around it. Polarization EMW are transverse and the electric and magnetic fields are at right angles. As Magnetic field surrounds the wire and is perpendicular to it, it follows that the electric field is parallel to the wire. Polarization refers to the physical orientation of the radiated waves in space. Waves are said to be polarized if they all have the same alignment in space. It is the characteristic of most antennas that the radiation they emit is linearly polarized. A vertical antenna will radiate waves whose electric vectors will be vertical and will remain so in free space. Light emitted by incoherent sources such as the sun has a haphazard arrangement of field vectors and is said to be randomly polarized. The wave in fig 8-1, linearly polarized also said to be vertically polarized since all the electric intensity vectors are vertical. Thus vertical antennas radiate vertically polarized waves and similarly horizontal antennas produce horizontal polarization.
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