radiation boundary about HFSS!

when I modal a open transition such as microstrip,I found that the result of S21 of the modal with radiation boundary around is smaller than that with a non-radiation air box around.
for example.
radiation boundary existing: S21 equal to -2.4dB
non-radiation air box existing: S21 equal to -1.0dB

which result is correct?who can give me some ideas?
thank you

Here are a couple of ideas:

1. Move the radiation boundary further away from the transmission line so that it is not interfering with any reactive fields. You don't want it leaching off the fields.

2. Move the PEC boundary further away so that it isn't interfering with the reactive fields. You don't want fields bouncing off the metal boundary.

3. Examine the fields for cutplanes normal to the transmission line. Verify that the fields are showing what you expected.