Dependence of farfield from monitor position
I've been simulating a grating coupler in Lumerical FDTD and I'm interested in the farfield diagram of the field irradiated by the grating. To realize one, I put a DFT monitor at the top of the grating that collects all the light emitted and then I use it to plot the farfield. However I noticed, that my farfield diagram is really sensitive to the distance of the monitor from the grating.
Do you have any tips or suggestion where to place a monitor correctly for this kind of work?
Thanks in advance