The pilot departed Moorabbin and conducted a practise instrument flight rules flight, including an instrument landing system approach at Essendon and a practise non-direction beacon approach at Cowes, before returning to Moorabbin. For most of the flight the pilot experienced communications problems with his very high frequency radio. He called inbound to Moorabbin Tower at Carrum and advised of his radio problem. Moorabbin Tower noted that the aircraft's transmissions were readable but there was a loud background squeal. Moorabbin Tower cleared the aircraft for a straight-in approach for runway 35 left and instructed the pilot to report at three miles. At the three miles inbound call the radio appeared to be working fine. At about two miles the pilot was given a clearance to land on runway 35 left. The pilot thought that he checked that the landing gear had been selected down during the final approach but he cannot remember whether or not the three green "gear down and locked" lights were illuminated. He first became aware of a problem when he heard a grinding noise during the touch down. Engineers have subsequently found nothing wrong with the landing gear systems. The aircraft was equipped with a landing gear extender which could have automatically extended the gear under low airspeed/low power conditions but the pilot had disengaged this system. Significant Factors The following factors were considered relevant to the development of the incident: 1. For most of the flight the pilot had been distracted by radio problems. 2. The pilot forgot to select the undercarriage down prior to landing. 3. The pilot had disengaged the backup, automatic landing gear extension system.