The helicopter was being used as an platform for test equipment. Part of the test equipment included an aerial that was mounted vertically below the helicopter. This aerial could be retracted and stowed in a horizontal position for landing by operating a control which was positioned in front of the technician. On this occasion the pilot inadvertently attempted to land the helicopter with the aerial extended. Just prior to touch down the helicopter began to vibrate, the pilot lowered the collective and the helicopter rolled onto its right side. Following the completion of each test it was normal for the technician to raise the aerial prior to landing. On this occasion, the technician became engrossed with the transmission of test data and forgot to retract the aerial. The pilot was concentrating on the landing and neglected to ensure that the aerial had been retracted. Normally if a landing is attempted with the aerial extended a weak link in the system fails and the aerial is retracted by a spring. However, it is believed that because the helicopter touched down with little forward speed the weak link did not fail at the required time in the landing sequence.