Circular directional coupler

I have a circular direction couple with one hole with diameter 1mm at center(w.r.t height). I have to couple this hole with another rectangular wave guide(WR-22) and have to calculate its coupling factor and directivity(s-parameters).

And another thing is I have to design the same type of circular directional coupler with single hole whose coupling factor should be -40dB.
can anyone suggest how to design this. Actually iam new to microwave designing so iam not getting any idea..

Thanks
Ejaz