Rectangular waveguides are important for two reasons. First of all, the Laplacian operator separates nicely in Cartesian coordinates, so that the boundary value problem that must be solved is both familiar and straightforward. Second, they are extremely common in actual application in physics laboratories for piping e.g. microwaves around as experimental probes.

In Cartesian coordinates, the wave equation becomes:

(10.72) |

This wave equation separates and solutions are products of sin, cos or
exponential functions in each variable separately. To determine which
combination to use it suffices to look at the BC's being satisfied. For
TM waves, one solves for subject to , which is
automatically true if:

(10.73) |

However, the wavenumber of any given mode (given the frequency) is
determined from:

(10.74) |

(10.75) |

(10.76) |

If we repeat the argument above for TE waves (as is done in Jackson,
which is why I did TM here so you could see them both) you will be led
by nearly identical arguments to the conclusion that the lowest
frequency mode cutoff occurs for , and to produce
the
solution to the wave equation above. The
cutoff in this case is:

(10.77) |

(10.78) |

Note well that this mode and cutoff corresponds to exactly one-half a
free-space wavelength across the long dimension of the waveguide. The
wave solution for the right-propagating TE mode is:

(10.79) | |||

(10.80) | |||

(10.81) |

We used and to get the second of these, and ) to get the last one.

There is a lot more one can study in Jackson associated with waveguides, but we must move on at this time to a brief look at resonant cavities (another important topic) and multipoles.