You need to have a probe to couple all you waveguide energy to you microstrip. There are few papers to get you started with your design, but most of you work (tuning) will be running sp in HFSS monitoring your insertion loss (S21). If time permits I will locate some references for you.
Good luck
Bouchy
initially set R= lamda_g/4 (calculated according to ssl) at the center frequency of operation.
Theta is over 45 degs. This is a tuning parameter. Select as 60 degs.
L is around lamda_g/4 (calculated according to waveguide parameters.) at the center frequency.
Put the microstripline in to simulation but deembed this line in the simulation.
Select the total length of the waveguide to be somewhat around 3 – 4 lambda_g(calculated according to waveguide parameters.)
Tune R (%10 stdev), theta (+/- 45degs) and l, select for the best Zin (seen at the junction of microstrip and the ssl). By tuning I mean parameter sweep option in HFSS.
You can also look at IEEE papers, you can scale the values according to your frequencies.