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Waveguides An Introduction P Meyer Department of Electrical and Electronic Engineering University of Stellenbosch December 2008

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Outline Introduction Parallel Plate Guide Rectangular Guide Circular Guide Ridge Guide

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Introduction - these days, most people are used to electronic circuits looking like this:

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Introduction - or this:

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Introduction - at microwave frequencies, things look quite different, though...

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Introduction Waveguide Coaxial Line

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Introduction Even for electronic engineers, waveguide remains a strange medium to work with.

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Introduction Dual Ridged Waveguide Quad Ridged Waveguide

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Introduction

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To understand the characteristics of waveguide, we have to do some maths...

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General Equations MaxwellHelmholz - we therefore have a 3-variable differential equation that needs to be solved.

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General Equations A wave travelling in the z-direction according to cos(wt- βz) can be represented as e -jβz, with β called the propagation constant - once we have solved e z and h z we can calculate all the other fields

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Parallel Plate Waveguide the most basic of transmission lines is simply two parallel plates separated by an isolating medium

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Parallel Plate Waveguide Solution 1 [kc=0] Boundary condition is that electric field tangential to the conductor must be zero. - this solution is called the TEM solution, as e z and h z are both zero

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Parallel Plate Waveguide Ideal Practical

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Parallel Plate Waveguide

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However, we have other solutions as well... Each value of n is a separate solution, with a different field pattern, and is known as a mode Modes are denoted TEM, TM (h z =0) and TE (e z =0)

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Parallel Plate Waveguide

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Cut-off in Waveguide

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Losses in Parallel Plate Waveguide

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Parallel Plate Waveguide Why is the possibility of different modes in a waveguide a problem?

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Parallel Plate Waveguide Normally, we use waveguide in a single propagating mode configuration. The useful frequency range is then limited by: low side: the exponentially increasing loss close to cut-off high side: the cut-off frequency of the next mode

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Rectangular Waveguide

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Each m,n combination form a specific waveguide mode. ie the TM10, or TE10 There is no TEM mode

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Cut-off in Rectangular Waveguide

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Mode Patterns in Rectangular Waveguide

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Losses in Rectangular Waveguide

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Circular Waveguide

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Modal Patterns in Circular Waveguide

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Losses in Circular Waveguide

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Ridged Waveguide It is clear that the possibility of higher order modes limits the useful frequency range of waveguide systems severely. Ridged guide can be used to extend this range significantly

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Ridged Waveguide

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Let’s Play... Once we understand how waveguides work, we can use their peculiar characteristics to our advantage, by using them as natural high-pass filters using overmoded guides to build more than one device in the same physical space add modes to create aperture distributions of our choice, and thus specified radiation patterns [Madelé van der Walt] build wideband transitions from coaxial line to waveguide tot antenna [Dirk de Villiers] Thank you

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