I have been designing a Yagi Uda antenna using CST Microwave Studio.
For a half-wave dipole the directivity (at 1.3GHz) was 2.116dBi.
After I added a Reflector, the directivity improved to 6.074dBi.
This is an increase of 3.958dB - more than double!
I am trying to understand how this is possible? How can the gain more than double? If I understand correctly, the reflector reflects the fields from the dipole back to the opposite direction. Thus in the best case scenario I will get complete super-position of the fields. Therefore I should find a maximum increase of 3dB - not more!