When I try to measure the transmission of a plane source, it is sometimes greater than 1?

I attempting to measure the transmission, where the source is plane wave at the bottom of the unit cell. Placing the monitor above the top of the cell I get a transmission greater 1. I have changed the mesh size and the range of wavelengths tested but no luck.

I read about re-normalizing the transmission and I would like to try but I do not know how. If someone can tell me how to re-normalize or why I am measuring transmission over 1 I would appreciate it.

for periodic structure,this is due to the effect of the evanescent wave and the reflection of the pml(perfect match layer) which is not so perfect.
so the countermeasure are :
1 when using pml boundary condition ,the disttace between the monitor and structure is larger than 0.5 max wavelength(the larer the better)
2 increase the number of the layers of pml layer.