A rectangular magnetron placed at the back of a rectangular sputtering target for coating a rectangular panel and having magnets of opposed polarities arranged to form a gap therebetween corresponding to a plasma track adjacent the target which extends in a closed serpentine or spiral loop. The spiral may have a large number of wraps and the closed loop may be folded before wrapping. The magnetron has a size only somewhat less than that of the target and is scanned in the two perpendicular directions of the target with a scan length of, for example, about 100 mm for a 2 m target corresponding to at least the separation of the gap between parallel portions of the loop. A central ferromagnetic shim beneath some magnets in the loop may compensate for vertical droop. The magnetron may be scanned in two alternating double-Z patterns rotated 90° between them.