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lecture36

lecture36 - Lecture 36 Reflector Antennas In this lecture...

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1 ECE 303 – Fall 2007 – Farhan Rana – Cornell University Lecture 36 Reflector Antennas In this lecture you will learn: • Reflector antennas • Dish Antennas ECE 303 – Fall 2007 – Farhan Rana – Cornell University A Short Dipole Antenna x y φ ( ) ( ) θ φ θ 2 sin 2 3 , = G ( ) ( ) θ φ θ 2 sin , = p Pattern: Gain: ( ) φ π θ , 2 = p In the θ = π /2 plane the radiation goes everywhere – not a very efficient antenna

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2 ECE 303 – Fall 2007 – Farhan Rana – Cornell University A Plane Reflector with a Short Dipole Feed - I x y The basic idea behind reflector antennas is that metal surfaces can be used to direct radiation from antennas in desired directions (or vice versa in reception) Question: How does one get all the radiation from a Hertzian dipole to go in the + x direction? Answer: Use a reflector x y a a image dipole φ φ ( ) ( ) ( ) [ ] = 2 3 2 for 0 2 2 for cos sin sin 4 , 2 2 π φ π π φ π φ θ φ θ ka F Array Factor: { , ....... 5 , 3 , 1 2 = = n n ka π For maximum gain in the θ = π /2, φ = 0 direction, choose: feed { , ....... 5 , 3 , 1 4 = = n n a λ ECE 303 – Fall 2007 – Farhan Rana – Cornell University x y a image dipole φ ( ) ( ) ( ) 2 2 , sin 4 1 , φ θ θ φ θ F p = Pattern: { , ....... 5 , 3 , 1 2 = = n n ka π ( ) φ π θ , 2 = p for ( ) ( ) ( ) 2 2 , sin 2 3 , φ θ θ φ θ F G = Gain: A plain reflector has increased the maximum gain by a factor of 4 A Plane Reflector with a Short Dipole Feed - II
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