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Wave Guides

 

A structure may be a conductor or hollow metal pipe through which e.m. waves can be transmitted from point-to-point and within which fields are confined to a certain extent is called wave guide and the wave is called guided wave. They are employed at and above 3000 mc/s.


Types of Waves

 
  1. TE (Transverse electric) wave : when wave is travelling in Z-direction.
    EZ = 0 i.e. electric field is purely transverse to the plane of propagation.
    but HZ ≠ 0 i.e. electric field is not purely transverse.
  2. TM (Transverse Magnetic) wave : when wave is propagating in Z-direction.
    HZ = 0 i.e. magnetic field is purely transverse to the plane of propagation.
    but EZ ≠0 i.e. electric field is not purely transverse.
  3. TEM (Transverse electromagnetic) wave : when wave is propagating in Z-direction,
    EZ = 0 and
    HZ = 0
    i.e. Both electric field and magnetic field are purely transverse to the plane of propagation.
  4. Hybrid wave : In this neither EZ nor HZ are purely transverse to the direction of propagation.
    i.e. EZ ≠0
    HZ ≠0

Modes in Rectangular Waveguides

When the cross-section of the metal tube or waveguide is rectangular, it is called rectangular wavelength.


Boundary Conditions for Rectangular Waveguide

Etangential = 0 i.e. Ex = EZ = 0 at y = 0 and y = b

Etangential = 0 i.e. Ey = EZ = 0 at x = 0 and x = a


Modes in Lossless Waveguide

 
  1. TE (Transverse electric) mode
    Designed as TEmn
  2. TM (Transverse magnetic) mode
    Designed as TMmn

Here, m-number of half waves of electric or magnetic intensity in the x-direction and

n-numbers of half waves in the x-direction if the wave is propagated in the positive Z-direction.

For TEmn modes, cut-off wave number,
 

730.png 
 

where a and b are in meters.


For TEmn modes, cut-off frequency,

 

736.png
 

Propagation constant (or phase constant),
 

741.png 
 

Phase velocity in the positive z direction for the TEmn modes

746.png 





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