I was wondering about the spiral antennae on the HARM seeker - ie. why do it this way instead of just four simple dipoles. I wonder if the varying width of the antenna helps it capture multiple wavelengths simultaneously, which would certainly help the seeker cope with radars of various wavelengths.
At the same time, my naive understanding of radio mechanics tells me that it would be blind to a bunch of wavelengths because of this as well (but you get a lot of bandwidth to help you deal with actual threat/guidance radars, so it's not all bad).
It is interesting that the British Sea Dart naval SAM missile has four such dipoles protruding from the front. I guess this is exactly the kind of system you wrote about, a narrowband interferometer-based anti-radiation homing system for use against jammer aircraft.
