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You need to turn it every tick, it's just a question of clockwise or counter-clockwise. The area covered by the sweep of the radar from last tick to this tick is the area that is scanned. So once you scan, you want to reverse the direction you turn the radar, not stop turning it.

Edit: Also, there's an option in Robocode to display the radar arcs, which would help you see exactly what's going on.