On May 10, 2005, at 1737 central daylight time, an experiemental, amatuer-built Mattson Zenith 601XL, N824M, collided with a ditch during an off airport forced landing in Dike, Iowa, following a loss of engine power. The private pilot was not injured. The airplane was substantially damaged. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions without a flight plan. The local flight originated from Waterloo, Iowa, at 1700. The pilot reported that 15 minutes into the flight, the engine rpm started decreasing and he thought he smelled smoke in the cockpit. He reported that he turned off the battery switches and the engine rpm began to increase. The pilot reported he set the engine power to 2,500 rpm and began spiraling down over a private airstrip from an altitude of 4,500 feet above mean sea level (msl) when the engine lost all power. He stated he needed additional power to complete the last 360 degree turn so he elected to land in a field. The airplane struck a fence and a ditch during the landing. Inspection of the airplane and engine by an inspector from the Federal Aviation Administration Flight Standards District Office in Des Moines, Iowa, failed to reveal any mechanical failure/malfunction which would have resulted in a fire, smoke, or the loss of engine power. The engine was equipped with an electronic ignition system. The pilot reported the accident could have been avoided by making a "better landing on power off."