 In rectangular waveguide, the TE10 [q] mode (shown in figure 2) is the dominant mode. There is a band of frequencies between the dominant mode cutoff and the next highest mode cutoff in which the waveguide can be operated without any possibility of generating spurious modes. The next highest cutoff modes are TE20, [r] at exactly twice the TE10 mode, and TE01 [s] which is also twice TE10 if the waveguide used has the commonly used aspect ratio of 2: 1. The lowest cutoff TM mode is TM11 [t] (shown in figure 2) which is <formula> times the dominant mode in 2: 1 waveguide. Thus, there is an octave over which the dominant mode is free of spurious modes, although operating too close to cutoff is usually avoided because of phase distortion.
