The $\vec{E}$ field in a rectangular waveguide of inner dimensions $a \times b$ is given by
\[ \vec{E}=\frac{\omega \mu}{h^{2}}\left(\frac{\pi}{a}\right) H_{0} \sin \left(\frac{2 \pi x}{a}\right) \sin (\omega t-\beta z) \hat{y}\]
where $H_{0}$ is a constant, and $a$ and $b$ are the dimensions along the $x$-axis and the $y$-axis respectively. The mode of propagation in the waveguide is
- $\mathrm{TE}_{20}$
- $\mathrm{TM}_{11}$
- $\mathrm{TM}_{20}$
- $\text{TE}_{10}$