seeker radar
This is a very complex question with involve much system consideration.
Just to share some pointers:
[1] Radar seeker must be compact and agile, with stringent aerodynamic requirement. The antenna could be a mechanically scanned type or electronically scanned type. In all cases, it must be light weight, and compact, in order to reduce the weight of the seeker. Some form of Radome is also necessary to protect it.
Mechanical slot array antenna had been used in some fighter planes and active radar seekers. They are very light weight (hence lighten the load of the Gimbal system), high gain and low sidelobes (with weighting).
Suggest you take a look at the book " Air and Spaceborne Radar Systems An introduction" which is available in this forum. You should have more insight to this topic. 8)