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Unformatted text preview: 36 Antenna reception and links Elementary description of antenna reception and links: • Consider a pair of identical short dipole antennas in free space, one located at the origin ( x, y, z ) = (0 , , 0) and the other at a distance r away from the origin at zenith and azimuth angles of ( θ, φ ) as shown in the margin. Furthermore: x y I t R z θ φ r – ant.1 at the origin, e.g. a ˆ zpolarized shortdipole, sees ant.2 lo cated at angles ( θ, φ ) , but – ant.2’s orientation is adjusted so that it always sees ant.1 at fixed angles of, say, ( θ 2 , φ 2 ) = (90 ◦ , 0) , defined in its own coordinate system, for all possible locations ( r, θ, φ ) . Current I r that flows through the resistor R connected across the antenna terminals is induced by the tangen tial components of incident wave fields on the conducting arms of the dipole antenna; this response is a straight forward consequence of the need to maintain tangential boundary conditions on the antenna surface. • First, ant.1 is driven with an input current (phasor) I t and a time average input power P t = 1 2  I t  2 R rad , while ant.2, terminated by a resistor R , puts a current I r through R into which it delivers an average power P r = 1 2  I r  2 R ≡ S inc A ( θ 2 , φ 2 ) where: 1 1. S inc = P t 4 πr 2 G ( θ, φ ) is the incident power density (the magnitude of the timeaverage Poynting vector) of the field arriving from ant.1 expressed in terms of the antenna gain G ( θ, φ ) evaluated in the angular direction of ant.2, and 2. A ( θ 2 , φ 2 ) is called the antenna effective area and is defined to be the conversion factor between the received power P r (W) and the incident power density S inc (W/m 2 ) . Our aim is to identify the effective area function A ( θ, φ ) in terms of the antenna gain function G ( θ, φ ) and two constraints, one regarding R , and the other regarding the antenna polarization. Once that is accomplished, the receiving properties of antennas will be relatively easy to understand. • Combining the expressions above we note that P r = P t 4 πr 2 G ( θ, φ ) A ( θ 2 , φ 2 ) ....
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 Fall '08
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 Polarization, Electromagnet, Dipole antenna

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