rectangle waveguide
calculate the cutoff frequencies for the lower modes.
Parameters
width
a
\(m\)
height
b
\(m\)
Output
cutoff frequency 10
`fc=c/2 sqrt((m/a)^2+(n/b)^2)`
fc 01
fc 20
fc 11
`fc=sqrt((m fc_(01))^2+(n fc_(10))^2)`