From your description, it might be due to the fields at the monitor edge are clipped. Please first check monitor data with visualizer. and then using the farfield in the visualizer do the farfield projection. Please turn on the farfield filter :
If the fields are clipped you will need to increase the simulation area and the monitor size, and test again.
If not, please send some screenshot of the simulation settings and the results so I can understand better.
Make sure the simulation is long enough and it reaches the autoshutoff min before it is terminated.
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