Design of offset satellite dish – is it half a parabola?

An offset satellite dish is not literally “half a parabola,” though it is related to a parabola.

Here’s the distinction:

• A prime-focus satellite dish is a full paraboloid of revolution (a 3D parabola spun around its axis). The feed horn sits right in front of the dish at the focal point, blocking a bit of the aperture.
• An offset satellite dish is made by “cutting out” a section of a larger paraboloid, usually from below the axis. This means:
  • The feed horn and LNB can be placed off to the side rather than in front of the dish.
  • The aperture looks like a squashed ellipse rather than a full circle.
  • The reflector still has the same parabolic geometry, so all incoming parallel rays still focus at the same point.

So, instead of being “half a parabola,” an offset dish is a section of a paraboloid displaced from the axis.