lecture35 - Lecture 35 Diffraction and Aperture Antennas In...

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1 ECE 303 – Fall 2005 – Farhan Rana – Cornell University Lecture 35 Diffraction and Aperture Antennas In this lecture you will learn: • Diffraction of electromagnetic radiation • Gain and radiation pattern of aperture antennas ECE 303 – Fall 2005 – Farhan Rana – Cornell University Diffraction and Aperture Antennas “Aperture antenna” usually refers to a (metallic) sheet with a hole (or an aperture) of some shape through which radiation comes out The natural spreading of electromagnetic waves in free space when emanating from a source is called “diffraction” Questions • What happens on this side? • How does the radiation coming out of the aperture looks like when the dimensions of the hole are of the order of the wavelength? • What is the radiation pattern? z x Incoming radiation
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2 ECE 303 – Fall 2005 – Farhan Rana – Cornell University Aperture Antennas in Practice: Rectangular Waveguides How does radiation coming out of a rectangular waveguide looks like? = σ = P Radiation coming out of a rectangular aperture Some fraction of the incident power is reflected from the open end and some is radiated out Metal rectangular waveguide Diffraction ECE 303 – Fall 2005 – Farhan Rana – Cornell University Aperture Antennas in Practice: Dielectric Waveguides Optical fiber Radiation coming out of a circular aperture Integrated Photonics (dielectric waveguides on a chip) Radiation coming out of a integrated dielectric waveguide (e.g. a semiconductor laser) Diffraction Diffraction
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3 ECE 303 – Fall 2005 – Farhan Rana – Cornell University z x Incident radiation Assumption and Goal Assumption: Assume that we know the field for all time right at the aperture ( ) , , 0 , t z y x E = r y This we could know for example from our knowledge of the incident (and reflected) fields behind the aperture Goal: To find the field for y > 0 ( ) ? , , , = t z y x E r ECE 303 – Fall 2005 – Farhan Rana – Cornell University H-field and Surface Current Density Boundary Condition ( ) K H H n r r r = × 1 2 ˆ 1 H r 2 H r K r First recall the surface current boundary condition for the H-field (now in vector form): For a left-right symmetric problem: K H n r r = × 2 2 ˆ 1 H r 2 H r K r 2 1 H H r r = n ˆ n ˆ
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4 ECE 303 – Fall 2005 – Farhan Rana – Cornell University Principle of Equivalence (Huygens Principle) Principle of equivalence says that if one knows the radiation E- and H-fields at every point on an imaginary closed surface, then the radiation outside the
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lecture35 - Lecture 35 Diffraction and Aperture Antennas In...

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