Why does the radome on the AWACS rotate? I'm not sure I understand why the radome on the AWACS aircraft is so large, and could somebody explain the design and why it has to rotate?
Basically, the radar antenna of the AWACS system is just like this (source):



Rotating waveguide antenna (picture source)

The round shape is only an aerodynamic fairing. Proof:



Boeing E-3A AWACS cutaway (picture source)

A surveillance antenna can also be fixed. Electronic beam steering helps to view a sector of approximately ±60°, but in order to get a 360° sweep the airplane needs to fly circles. Beam steering is also used on the rotating antenna to rapidly switch between several targets, so this is not unique to a fixed antenna:



Pakistani AWACS based on EMB-145 (picture source)