Which TM mode in rectangular waveguide has lowest cutoff frequency?

E_x = ± Z_TM H_y

E_y = ± Z_TM H_x

where E₀ is the amplitude of the wave.

The expressions for β, λ_c, f_c, λ_g, a are the same as for TE waves.

For TM wave the lowest cut off frequency in rectangular wave guide is for TM₁₁ mode.

If , the cutoff frequency for TM₁₁ mode is about 12% more than that for TE₂₀ mode.