On August 23, 2003, about 1430 eastern daylight time, a homebuilt Hyperbipe, N258HB, was substantially damaged following a loss of engine power while departing Marion Municipal Airport, Marion, Ohio. The certificated private pilot and passenger were not injured. Visual meteorological conditions prevailed for the personal flight destined for Beaver County Airport, Beaver Falls, Pennsylvania. No flight plan had been filed for the flight that was conducted under 14 CFR Part 91. The pilot reported that the airplane lost engine power during the initial climb after takeoff. He further stated: "...At approx 250 to 400 feet of altitude, the engine began to loose power slowly. I could not climb or maintain altitude due to the steady loss of power and began to sink rapidly. I picked a field I thought I could make an emergency landing on straight ahead. I pulled the throttle to idle and pulled the mixture to cut off just before touch down as I flared for a landing in a farm field of soy beans. With no injuries to my self or passenger, we exited the plane as quickly as possible...." During a follow-up telephone interview, the pilot reported that from the air, he thought the height of the vegetation in the field he had picked for a landing was about 6 inches; however, it turned out be about 40 inches high. A mechanic who examined the airplane reported that the air filter screen had separated from the support bracket. The filter element was found in the intake to the fuel injection servo and obstructed the air flow to the engine. The mechanic said he had given the airplane a condition inspection about 30 hours before the accident, and an air filter was not installed at that time. The air filter was "home-made" and installed by the pilot. The filter screen was attached to the support bracket by the use of pop rivets.