Hello,the hollow waveguide bellow has a=109mm b=43mm L=100mm Er=1
from the formula bellow
For TE21 i get cutoff frequency of 4.4Ghz as shown bellow.
I have defined both port on waveguide ends as shown bellow
but as you can see from simulation result is get m=2 n=1 at cutoff frequency of 2.8Ghz.
Where did i go wrong?
Thanks.

 

I've no idea what you have done, but the fields you have plotted look more like a \(TE_{20}\) mode to me, which would have a cutoff around 2.75GHz.

 

you are correct i got it around 4.4
which is very bad ,the simulation is not accurate at all.

 

I'm a bit rusty on this, but years ago when I was calculating waveguide cutoffs, we used to use the Rayleigh Quotient to get a stationary estimator (second order accurate) of the cutoff frequency. You could get reasonably accurate estimates of the cutoff with pretty rough estimates of the fields.