waveguides an introduction p meyer department of electrical and electronic engineering university of...
TRANSCRIPT
WaveguidesAn Introduction
P MeyerDepartment of Electrical and Electronic Engineering
University of Stellenbosch
December 2008
Outline
Introduction Parallel Plate Guide Rectangular Guide Circular Guide Ridge Guide
Introduction
- these days, most people are used to electronic circuits looking like this:
Introduction
- or this:
Introduction
- at microwave frequencies, things look quite different, though...
Introduction
WaveguideCoaxial Line
Introduction
Even for electronic engineers, waveguide remains a strange medium to work with.
Introduction
Dual Ridged Waveguide
Quad Ridged Waveguide
Introduction
Introduction
To understand the characteristics of waveguide, we have to do some maths...
General Equations
2k
Maxwell Helmholz
- we therefore have a 3-variable differential equation that needs to be solved.
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 ez and hz we can calculate all the other fields
Parallel Plate Waveguide
the most basic of transmission lines is simply two parallel plates separated by an isolating medium
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 ez and hz are both zero
Parallel Plate Waveguide
Ideal
Practical
Parallel Plate Waveguide
Parallel Plate Waveguide
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 (hz=0) and TE (ez=0)
Parallel Plate Waveguide
Cut-off in Waveguide
Losses in Parallel Plate Waveguide
Parallel Plate Waveguide
Why is the possibility of different modes in a waveguide a problem?
4 6 8 10 12 14 16 18-120
-100
-80
-60
-40
-20
0
Frequency [GHz]
|S11
, S
21
|dB
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
Rectangular Waveguide
Rectangular Waveguide
•Each m,n combination form a specific waveguide mode. ie the TM10, or TE10
•There is no TEM mode
Cut-off in Rectangular Waveguide
Mode Patterns in Rectangular Waveguide
Losses in Rectangular Waveguide
Circular Waveguide
Modal Patterns in Circular Waveguide
Losses in Circular Waveguide
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
Ridged Waveguide
Ridged Waveguide
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