Prof. David R. Jackson Notes 19 Waveguiding Structures Waveguiding Structures ECE Spring 2013
Waveguiding Structures A waveguiding structure is one that carries a signal (or power) from one point to another. There are three common types: Transmission lines Fiber-optic guides Waveguides 2
Transmission lines Lossless: Has two conductors running parallel Can propagate a signal at any frequency (in theory) Becomes lossy at high frequency Can handle low or moderate amounts of power Does not have signal distortion, unless there is loss May or may not be immune to interference Does not have E z or H z components of the fields (TEM z ) Properties (always real: = 0 ) 3
Fiber-Optic Guide Properties Has a single dielectric rod Can propagate a signal at any frequency (in theory) Can be made very low loss Has minimal signal distortion Very immune to interference Not suitable for high power Has both E z and H z components of the fields (“hybrid mode”) 4
Fiber-Optic Guide (cont.) Two types of fiber-optic guides: 1) Single-mode fiber 2) Multi-mode fiber Carries a single mode, as with the mode on a waveguide. Requires the fiber diameter to be small relative to a wavelength. Has a fiber diameter that is large relative to a wavelength. It operates on the principle of total internal reflection (critical angle effect). 5
Multi-Mode Fiber 6 Higher index core region
7 Multi-Mode Fiber (cont.) At left end of rod: Assume cladding is air
8 Multi-Mode Fiber (cont.) At top boundary with air:
Waveguide Has a single hollow metal pipe Can propagate a signal only at high frequency: > c The width must be at least one-half of a wavelength Has signal distortion, even in the lossless case Immune to interference Can handle large amounts of power Has low loss (compared with a transmission line) Has either E z or H z component of the fields (TM z or TE z ) Properties 9 Inside microwave oven
Waveguides (cont.) Cutoff frequency property (derived later) (wavenumber of material inside waveguide) (definition of cutoff frequency) (propagation) (evanescent decay) In a waveguide: We can write 10
Field Expressions of a Guided Wave All six field components of a guided wave can be expressed in terms of the two fundamental field components E z and H z. Assumption: (This is the definition of a guided wave.) A proof of this statement is given next. Statement: "Guided-wave theorem" 11
Field Expressions (cont.) Proof (illustrated for E y ) or Now solve for H x : 12
Field Expressions (cont.) Substituting this into the equation for E y yields the result Next, multiply by 13
Field Expressions (cont.) Solving for E y, we have: This gives us The other three components E x, H x, H y may be found similarly. 14 or
Field Expressions (cont.) Summary of Fields 15
TEM z Wave To avoid having a completely zero field, Assume a TEM z wave: TEM z Hence, 16
TEM z Wave (cont.) Examples of TEM z waves: In each case the fields do not have a z component! A wave in a transmission line (no conductor loss) A plane wave E H Coax x y z E H S Plane wave 17
TEM z Wave (cont.) Wave Impedance Property of TEM z Mode Faraday's Law: Take the x component of both sides: The field varies as Hence, Therefore, we have 18
TEM z Wave (cont.) Now take the y component of both sides: Hence, Therefore, we have Hence, 19
TEM z Wave (cont.) These two equations may be written as a single vector equation: The electric and magnetic fields of a TEM z wave are perpendicular to each other, and the amplitudes of them are related by . Summary: 20
TEM z Wave (cont.) Examples E H Coax x y z E H S Plane wave Microstrip The fields look like a plane wave in the central region. E H 21 “Quasi-TEM” (TEM-like at low frequency)
Waveguide In a waveguide, the fields cannot be TEM z. (property of flux line) (Faraday's law in integral form) contradiction! 22 Proof: Assume a TEM z field y x waveguide E PEC boundary A B C
Waveguide (cont.) In a waveguide (hollow pipe of metal), there are two types of fields: TM z : H z = 0, E z 0 TE z : E z = 0, H z 0 23