Shortly after takeoff the pilot experienced “severe vibration” which lasted for about 30 seconds after which the rudder jammed in the neutral position. The pilot was able to maintain control of the aircraft and landed back on the departure runway. The vibration had been caused by a rudder 'flutter' event which was caused by the seizure of the rudder anti-balance tab's spring hinge.  Two modifications have been developed by the Light Aircraft Association to prevent a recurrence.