Notes 11 Spring 2005

Notes 11 Spring 2005 - Notes 11 Spring 2005.doc Plane Waves...

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Notes 11 Spring 2005.doc Plane Waves at Boundaries In the previous material we discovered that when incident upon a good conductor the electric and magnetic fields of an EM wave attenuate very quickly at the boundary surface. We saw that the figure of merit for this attenuation is the skin depth δ , where µσ π = δ f 1 . It is clear that δ varies inversely with the frequency and/or the conductivity, σ , of the material. This allows us to state that if a material is a perfect conductor ( σ ) the skin depth is zero and no time-varying electric or magnetic fields will exist within the perfect conductor (recall E G G σ = c J ); in this case all of the electromagnetic energy is “guided” along the surface by the conductor. Alternately we could state that for a conductor of finite conductivity at high frequencies the skin depth is proportionally small and the currents driven in the conductor exist within the outer “skin” of the conductor. In fact because the skin depth in silver is very small, the difference in performance between a pure silver component and a silver-plated brass component is negligible, so silver plating is often used to reduce material cost of waveguide components. For the same reason, hollow tubular conductors are used instead of solid conductors in outdoor television antennas. The key element of this discussion, however, is the tenet that
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Notes 11 Spring 2005 - Notes 11 Spring 2005.doc Plane Waves...

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