7. what are modes in waveguides

Sunday, 23 February 2014

7. what are modes in waveguides


Lets explain what are modes in waveguide mean.For every type of waveguide the electromagnetic waves inside the waveguide can have an infinite number of distinct electromagnetic field patterns or configurations, these distribution of electric and magnetic fields in a waveguide is called modes.

The characteristics of these modes depend upon the cross-sectional dimensions of the conducting waveguide, the type of dielectric material inside the waveguide, and the frequency of operation. Waveguide modes are typically classed according to the nature of the electric and magnetic field components Ez and Hz.These components are called the longitudinal components of the fields.So the types of modes in a waveguide are

TE modes. Transverse-electric modes, sometimes called H modes. These modes have Ez = 0 and Hz ≠ 0 at all points within the waveguide, which means that the electric field vector is always perpendicular (i.e., transverse) to the waveguide axis. These modes are always possible in waveguides with uniform dielectrics.
TM modes. Transverse-magnetic modes, sometimes called E modes. These modes have Hz = 0 and Ez ≠ 0 at all points within the waveguide, which means that the magnetic field vector is perpendicular (i.e., transverse) to the waveguide axis. Like TE modes, they are always possible in waveguides with uniform dielectrics.
EH modes. EH modes are hybrid modes in which neither Ez nor Hz are zero, but the characteristics of the transverse fields are controlled more by Ez than Hz . These modes are often possible in waveguides with inhomogeneous dielectrics.
HE modes. HE modes are hybrid modes in which neither Ez nor Hz are zero, but the characteristics of the transverse fields are controlled more by Hz than Ez . Like EH modes, these modes are often possible in waveguide with inhomogeneous dielectrics.
TEM modes.Transverse-electromagnetic modes, often called transmission line modes. These modes can exist only when a second conductor exists within the waveguide, such as a center conductor on a coaxial cable. Because these modes cannot exist in single, closed conductor structures, they are not waveguide modes.

A waveguide will have a definite cut off frequency for each mode.Also it is possible to propagate several modes within a waveguide.

The different modes are named with subscript m and n, for example TEmn where m is the number of half wave variations across x axis and n is the number of half wave variations across y axis.


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