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ECE-305-Lecture23 - Engineering Electro-Magnetics...

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Engineering Electro-Magnetics Engineering Electro-Magnetics ECE-305, Lecture 23 Antenna Properties Dennis McCaughey, Ph.D. 20 April, 2010
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04/20/2010 Dennis McCaughey, ECE305, Spring 2010 2 Polarization Polarization Choice of polarization is one of the design choices available to the RF system designer. For example, low frequency (< 1 MHz) vertically polarized radio waves propagate much more successfully near the earth than horizontally polarized radio waves, because horizontally polarized waves will be cancelled out by reflections from the earth. Mobile radio systems waves generally are vertically polarized. TV broadcasting has adopted horizontal polarization as a standard. This choice was made to maximize signal-to-noise ratios. At frequencies above 1 GHz, there is little basis for a choice of horizontal or vertical polarization. Circular polarization has also been found to be of advantage in some microwave radar applications to minimize the "clutter" echoes received from raindrops, in relation to the echoes from larger targets such as aircraft. Circular polarization can also be used to reduce multipath.
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04/20/2010 Dennis McCaughey, ECE305, Spring 2010 3 Differential Length Current Filament Differential Length Current Filament (Hertzian Dipole) (Hertzian Dipole)
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04/20/2010 Dennis McCaughey, ECE305, Spring 2010 4 Summary (Hertzian Dipole) Summary (Hertzian Dipole) 0 0 0 0 2 0 2 2 0 2 3 2 0 2 3 1 Letting 2 , , and = 2 1 sin 4 1 cos 2 2 2 1 sin 4 2 r j s r j rs r j s f f v v I d H e j r r I d E e r j r I d E e j r r j r π λ φ π λ π λ θ ϖ π λ η μ ε μ ε π θ π λ λ θ π π π λ θ π λ π - - - = = = = + = +
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