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What is the Cutoff of a Waveguide?

March 22, 2023

A waveguide is a medium for the transport of RF signals. The medium can support many Transverse Electric (TE) or Transverse Magnetic (TM) modes. Normally, only the lowest order mode between the cutoff and the beginning of the next higher order mode is used, which is TE10 or TM11. Using a waveguide at a frequency where more than one mode is possible is not advised, as different modes have different group velocities.

The low frequency cutoff of a rectangular waveguide for mode TEmn in terms of dimensions a and b is given by:

The formula is the same for TM modes, but TM10 is not realizable. The first transverse magnetic mode is TM11.

The component of the wave vector in the z-axis direction of propagation is given by:

And the phase velocity is:

And group velocity is:

For example, a WR-112 waveguide has dimensions, a = 2.85 cm and b = 1.26 cm. For TE10 mode, Fc is simply c/2a or 5.26 GHz.

The insertion loss of the WR-112 waveguide is shown below in Figure 1:

Figure 1 -WR-112 Waveguide Response Showing Cutoff

Typically, this waveguide is used from 7.05 to 10 GHz.

See this and more important radio frequency charts and formulas in this comprehensive application note