Transverse electric mode of a rectangular waveguide (${\mathrm{TE}}_{mn}$)

A transverse electric mode of a rectangular waveguide has $E_z=0$ while its axial magnetic field is generally nonzero. For time dependence $e^{-i\omega t}$ and axial dependence $e^{i\beta z}$, set $\Delta =(\omega /c{)}^2-{\beta }^2$. The Maxwell equations reconstruct the transverse components from $B_z$:The axial complex amplitude satisfies a transverse Helmholtz equation, $({\partial }_x^2+{\partial }_y^2+\Delta )B_z=0$. Zero tangential electric field at each perfect conductor wall becomes a Neumann boundary condition on $B_z$. Separation of variables givesThe index $m$ counts half-periods across $a$, and $n$ across $b$. Either index can vanish, allowing ${\mathrm{TE}}_{10}$; both cannot vanish. Indeed a spatially constant axial oscillating magnetic field would give nonzero changing magnetic flux through the cross-section, whereas the line integral of tangential electric field around its metal boundary is zero. The integral form of Faraday's law excludes this ${\mathrm{TE}}_{00}$ candidate. Dividing the reconstruction formulas by $\Delta =0$ would be invalid and cannot produce the separate transverse electromagnetic mode.

For $m=1,n=0$, choose $B_{\ast }=-i(\pi /a)E_0/\omega$. The formulas recover exactly the $E_y$, $B_x$ and $B_z$ in rectangular electromagnetic waveguide. In particular, the transverse magnetic field accompanies $B_z$ and ensures zero divergence; an oscillating longitudinal magnetic field on its own is not the guided solution.

Below the lowest waveguide cutoff frequency, the axial wave number of the ${\mathrm{TE}}_{10}$ mode is $i\kappa$, with $\kappa >0$. For a real peak scale $E_0$ and time convention $e^{-i\omega t}$, $E_y=E_0\sin (\pi x/a)e^{-\kappa z}$, $B_x=-i\kappa E_y/\omega$ and $B_z=-i{E}_0(\pi /a)\cos (\pi x/a)e^{-\kappa z}/\omega$. The electric field and magnetic field are in quadrature. A single decaying mode in a lossless semi-infinite guide consequently has zero time-averaged Poynting vector, although its instantaneous field stores energy.