Reflection losses for normal incidence in HFSS

I am new in HFSS and I have a project in which I have to study the performance of a small cage lined with ferrite in the walls. The dimension of the cage are 55x55x52cm. This cage is used to make EMC measurements. The first step is analyze the reflection losses of the ferrites at normal incidence an compare the results of the simulation with those of the manufacturer's datasheet. The range of frequency is 100MHz-1GHz. The problem is that I don't get the plot of the manufacturer and I tried to check my model by solving the reflection loss at normal incidence upon a dielectric of 2.6 of relative permittivity. I was expected to get a horizontal line along the whole of the range since there is no variation of the parametters with the frequency. By using the equation of the reflection coeffcient, this value is 0.231 and in db it would be -12.71. I did no get anything close to those values. I am doing the following in my model:

1) the dielectric is a square of 10x10mm per edge and 7mm of thickness
2) The excitation is a plane wave
3) I put an airbox on the top of the dielectric of 10mm of heigh
4) I put a PML boundary on the top of the airbox
5) I put another PML boundary on the bottom of the dielectric to let the wave propagates to the infinite
6) I define master and slave boundaries on the sides of the airbox

I think my problem is with the use of the PML boundaries..

Please somebody help me!!

I will apreciate a lot any help