I have been playing around on CST to create a rectangular 2-port waveguide with a block of material at the centre and plot s-parameters. I have done this successfully, however I cannot get my head around what the shape of the plots show. (our class was given a very brief introduction to the subject of s-parameters and our lecturer advised us to start using CST for students to help visualise electromagnetic fields.)
My main questions are:
Why is the peak reached after the cutoff-frequency? From my understanding of waveguides, this is due to the fact that below the cutoff-frequency the waveguide is too small to guide the waves hence, they scatter more whilst at the cutoff-frequency, there is resonance.
Why does the s-parameter for reflection plummet suddenly afterwards and at what frequency does this happen? I think that the number of sudden drops signifies the number of modes, however I do not know why this is. I am especially confused when it comes to the frequencies at which this occurs.
I am also quite unsure about what the phase plots represent.
What properties of the material inside the waveguide affect the s-parameters?