Isotropic if an antenna is a point source that

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Isotropic -If an antenna is a point source that radiates power equally in all directions. Power density at any point of sphere would be total radiated power, divided by the area of this sphere: ( 29 2 4 r W r w total ISO π =
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29 An isotropic antenna would radiate equally in all directions Isotropic Antenna Power density at any point is dependent only on radius r θ P( r , θ , φ ) φ For transmission antenna, the directivity of the antenna is the ratio of max power density to isotropic power density. This ratio is usually expressed in decibels: ( 29 ( 29 ( 29 ( 29 dB r w r w D r w r w D ISO ISO y directivit = = max max log 10 : decibels The power density is always dependent on the orientation of the receiver, which is measured in polar coordinates. The direction of maximum power density is assigned to θ = 0, φ = 0. This point is also called the foresight of the antenna.
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30 Radiation diagram for horizontal plane ( φ = 0 ° ) θ Direction of maximum gain ( θ = 0, φ = 0) Foresight Radiation diagram for vertical plane ( θ = 0 ° , 180 ° ) φ Direction of maximum gain ( θ = 0 ° , φ = 0 ° ) Foresight 18 dB 18 dB 12 dB 6 dB 12 dB 6 dB ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 = = r w r w G in g r w r w G ISO ISO φ θ φ θ α φ θ φ θ , , log 10 , : decibels , , , A radiation diagram describes the ratio of the antenna’s power density at any orientation around the antenna to the isotropic power density: or gain of the antenna in this direction . Transmission antenna
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31 Receiving antenna Effective area of the antenna measures how well the antenna captures power , and is simply the ratio of the power received by the antenna to the power density at the point where the antenna is located. The effective area of the antenna can be much larger than the antenna geometric area. Because of the principle of reciprocity, the effective area of an antenna is related to the antenna gain by the following formula: ( 29 ( 29 π λ φ θ π λ φ θ φ θ 4 ) ( ) , , ( 4 , , 2 2 × = = r w r w G A ISO eff
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32 The polarization of the electromagnetic wave Polarization describes the direction of the electrical field of the electromagnetic wave There is also a perpendicular magnetic field, but we describe polarization in terms of the electric field. Electric Field E Magnetic Field H Direction of Propagation An antenna generally has a preferred polarization direction, and it is important that the polarization of the transmitting antenna matches the polarization quality of the receiving antenna.
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33 Horizontal plane ( φ = 0 ° ) θ φ 18 dB 18 dB 12 dB 6 dB 12 dB 6 dB Vertical plane ( θ = 0 ° ), 180 ° ) 3dB beamwi dth Directional antenna radiation pattern . 3 dB beamwidth , the angle between points that are 3dB below the max power output. Horizontal plane ( φ = 0 ° ) φ 18 dB 18 dB 12 dB 6 dB 12 dB 6 dB Vertical plane ( θ = 0 ° ), 180 ° ) Omnidirectional antenna radiation pattern
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34 Free space propagation Consider an isotropic point source with power P t . At distance d the radiated power is uniformly distributed over surface of a sphere.
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