Wireless signal propagation (antenna patterns) Thanasis Korakis University of Thessaly [email protected] Dipole, monopole and isotropic antenna ● The simplest antennas, monopole and dipole antennas, consist of one or two straight metal rods along a common axis ● Isotropic antenna is a hypothetical antenna that radiates equally in all directions and is used as a reference to calculate antenna gain Dipole or monopole antennas Dipole radiation pattern ● The radiation pattern of a vertical 1/2λ (half-wave) dipole antenna ● The antenna is at the center of the "donut," or torus. ● Radial distance from the center represents the power radiated in that direction ● The power radiated is maximum in horizontal directions, dropping to zero directly above and below the antenna Monopole radiation pattern ● Radiation pattern of a typical monopole ● Monopole antennas up to 1/4λ long have a single "lobe", with field strength declining monotonically from a maximum in the horizontal direction ● Longer monopoles have more complicated patterns with several conical "lobes" (radiation maxima) directed at angles into the sky Dipole and monopole antennas Monopole antenna used at high frequencies. The three conductors projecting downward are the ground plane "Rabbit-ears" television dipole antenna Omnidirectional antenna ● EM wave power is radiated uniformly in all directions in one plane ● Radiated power is decreased with elevation angle above or below the plane, dropping to group of monopole antennas zero on the antenna's axis Aforming an omnidirectional antenna Directional (or beam) antenna ● Radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources ● Residential TV antennas are usually: ○ Yagi-Uda directional antennas ○ Log-periodic directional antennas ○ Corner reflector directional antennas Directional antennas Yagi-Uda array consists of a driven element (typically a dipole) and additional parasitic elements (usually a so-called reflector and one or more directors) Log-periodic array consists of a series of dipoles positioned along the antenna axis, spaced at following a logarithmic function of the frequency Lobes or beams ● The main lobe, or main beam, of an antenna radiation pattern is the lobe containing the maximum power ● The radiation pattern of most antennas shows: ○ "lobes" at various angles, directions where the radiated signal strength reaches a maximum, ○ separated by "nulls", angles at which the radiation falls to zero Main lobe, sidelobes and backlobe ● In a directional antenna one lobe is designed to be bigger than the others; this is the main lobe ● The other lobes are called "sidelobes", and usually represent unwanted radiation in undesired directions ● The sidelobe in the opposite direction from the main lobe is called the "backlobe" Main lobe, sidelobes and backlobe How do we estimate the antenna gain? Radiation pattern (another view) ● Imagine an elastic sphere that is filled with an incompressible medium ● A dot at the center represents an isotropic antenna and the sphere surface is its radiation pattern ● The radius of the sphere is proportional to the power radiated by the isotropic antenna Radiation pattern (another view) ● ...Now, let the sphere be deformed to create a new shape (or a new radiation pattern) ● The volume remains unchanged regardless of the change in shape ● The ratio of the distance from the center to any point on the surface of the new shape, to the radius of the initial sphere is the directive gain ● The maximum directive gain is its directivity Beamwidth The half-power beamwidth is the angle (in the horizontal plane) between: ● the two directions in which the directive gain of the major lobe is one half the directivity or equivalently ● the half-power (-3 dB) points of the main lobe, when referenced to the peak effective radiated power of the main lobe Beamwidth 30 o * ( 0.7072 = 0.5 ) * ( -3dB = 10log10(0.5) ) NYU Poly WiMax antenna 120o beamwidth Why we use antenna gains? ● Propagation models assume isotropic antennas ● Other antennas feature gains comparing to the isotropic antenna: ○ As a transmitting antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction. ○ As a receiving antenna, the figure describes how well the antenna converts radio waves arriving from a specified direction into electrical power Antenna gain ● When no direction is specified, gain is the peak value of the gain (under direction) ● Due to reciprocity, the gain of any antenna when receiving is equal to its gain when transmitting ● Antenna gain is usually defined as the ratio of the peak power produced by this antenna to the power produced by an isotropic antenna Antenna gain units ● This ratio is expressed in decibels and the related units are decibels-isotropic (dBi) ● Half-wave dipole antenna has gain 2.15 dBi ● Alternatively, we compare the antenna to the power received by a lossless half-wave dipole antenna, writing the units as dBd ● So, gain in dBd = gain in dBi - 2.15 How we estimate antenna gains? ● The gain is inversely proportional to beamwidth; as the beamwidth decreases, the gain increases G = 10log10(4π2 / bh * bv) (bh is the beamwidth in the horizontal plane) (bv is the beamwidth in the vertical plane) References 1. Andrea Goldsmith, Wireless Communications, Cambridge University Press, 2005. 2. T. S. Rappaport, Wireless Communications: Principle and Practice, 2nd ed. Singapore: Pearson Education, Inc., 2002.
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