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

transmission
greaterthanone

#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.


Steps of Creating a Compact model for a Taper Waveguide
#2

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.