A rectangular waveguide cavity is made from a piece of copper WR - $187 \mathrm{H}$-band waveguide, with $a=4.755 \mathrm{~cm}$ and $b=2.215 \mathrm{~cm}$. The cavity is filled with polyethylene $\left(\epsilon_r=2.25, \tan \delta=0.0004\right)$. If the resonance is to occur at $f=5 \mathrm{GHz}$, find the required length $d$, and the resulting unloaded $Q$ for the $l=1$ and $l=2$ resonant modes.

Solution:

We first calculate the wavenumber $k$ at $5 \mathrm{GHz}$ $$ k=\frac{2 \pi f \sqrt{\epsilon_r}}{c}=157.08 / m $$

The dominant mode is $\mathrm{TE}_{101}$ so $m=1, n=0$. Then we can find the resonance for $l=1$ and 2

$$ d=\frac{l \pi}{\sqrt{k^2-\left(\frac{\pi}{a}\right)^{\wedge} 2}} $$

For $l=1$,

$$ d=2.20 \mathrm{~cm} $$

For $l=2$,

$$ …