My off axis certainly dropped using supercup.
Radar Manufacturers recommend that their antennas be placed as close to the windshield as possible for best performance.
This is one of the dynamics that I thought effected the 2009 GOL test of off axis performance. Their modified swivel mount put detectors well back from the windshield which is not the normal mounting position......and the windshield will in fact work as a lense in front of the antenna.
Even though glass (like plastic) is a low loss medium, it still represents a barrier to which microwaves must pass through. Actually, it does three things.......it reflects part of the energy, it refracts part of the energy (changes direction) and it induces a phase shift on those microwaves that make it through.
Now I'm not smart enough to say WHY pulling the detector back away from the windshield has a negative effect, but I believe it does. Obviously one possible reason is you have a smaller patch of glass between the detector and source signal when it's butted up against the glass?
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