module 3c - 2/24/2009 Module 3 Lecture 3 Uniform plane...

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2/24/2009 1 Page 1 Material from “Applied Electromagnetics: Early Transmission Lines Approach” By Wentworth (Wiley) Module 3 Lecture 3 Uniform plane waves: supplementary topics Page 2 A little bit of history. ..
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2/24/2009 2 Page 3 On the program today… Wave polarization: •What is it that “turns” in circularly polarized waves? •What is the difference between circularly polarized light and randomly polarized light? •Polarized light in nature (LHC and RHC) •Quarter wave plates and half wave plates •Power flow in uniform plane waves •Sheet resistance of metal surfaces Page 4 What are those blue lines that describe wave polarization? They indicate the location of the end point of the E-field vector, as a function of time, on a plane of constant z (i.e. the x-y plane). For light, this rotary motion occurs at 100 THz.
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2/24/2009 3 Page 5 What is the “meaning” of circular, elliptical, or linear polarization? • Think of a point charge, place somewhere and what happens to it when the wave arrives. At various moments, the field points in all directions at a given point But this is a uniform plane wave, so these changes of direction happen everywhere Page 6 Circular vs random polarization • In circularly polarized waves, the direction of the E-field vector changes in a very precise manner as a function of time (at the wave frequency). • In randomly polarized waves, the direction of the E-field vector changes direction in random jumps, irregular in time, on a time scale determined by the source of the waves.
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2/24/2009 4 Page 7 What is the state of polarization of waves in the real world? • Most EM waves at radio-frequencies and below are generated by humans, using antennas: from currents flowing in wires or transmission lines. They are most often at least partially polarized. • Most EM waves at optical frequencies (1-200 THz range) are generated by randomly oriented dipoles, at the molecular level, in materials. They are most often un-polarized (randomly polarized) Page 8 Electromagnetic wave spectrum ln = c THz
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2/24/2009 5 Page 9 Detecting EM waves • For frequencies up to 100 GHz, antennas are required: antenna design must take polarization into account • For frequencies above a few tens of THz (optical domain), waves are detected using either heat or carriers generated in materials: usually polarization-in sensitive. Page 10 Manipulating the state of polarization The determining factor for getting linear, elliptical or circular polarization is the phase difference between the E x and E y components: – 0 or 180 degrees for linear – +90 or -90 degrees for circular – Anything else for elliptical For linear polarization, the ratio of the amplitudes (E x /E y ) determines the angle of the polarization vector relative to the x axis. For elliptical, the angle of the ellipse axes depends on both the ration and on the phase difference.
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This note was uploaded on 09/27/2011 for the course ENGINEERIN 3600 taught by Professor Victor during the Spring '11 term at Carleton CA.

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module 3c - 2/24/2009 Module 3 Lecture 3 Uniform plane...

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