How can adding a reflector to a half-wave dipole (in a Yagi Uda antenna) increase the gain by more than 3dB?
The plain dipole has a &quot;beam width&quot; of 360° — it radiates equally well in all directions in the normal plane.
When you add the reflector, the -3 dB beam width is not simply reduced to 180° — it's actually significantly less than that, more like 120°. Since gain is more or less inversely proportional to beam width, this accounts for the numbers you're getting.