On August 11, 1996, at 1000 central daylight time, a Ramesbotham CH701 homebuilt, collided with a powerline and the terrain during a forced landing following a loss of engine power. The loss of power and subsequent impact with the terrain occurred while on final approach to land at Graham Field, N. Sioux City, South Dakota. The airplane was substantially damaged. The pilot was not injured. Visual meteorological conditions prevailed and no flight plan was filed. The flight originated from N. Sioux City, South Dakota, at about 0915 cdt. The pilot stated that the fuel system has a return line to the right tank for excess fuel pumped out of the left tank. Both fuel tanks have an independant On-Off selector. He stated that he usually flew on the left tank and would switch tot he right tank when the fuel level in the left tank got low. He stated that each tank holds 7 gallons and it usually took about 30 minutes to pump 5 gallons from the left tank. The pilot stated he departed and flew the flight using the left fuel tank. The pilot reported that when the left tank got low, he switched to the right tank; however, the engine lost power. He suspects that there was air in the fuel lines from the right tank to the engine and that is why he was unable to regain power. The airplane was at an altitude of approximately 700 feet above the ground when the loss of power occurred. The airplane contacted a powerline during the forced landing descent to a soybean field. The airplane then impacted the terrain in the soybean field. Post accident inspection of the airplane revealed the left fuel tank was empty and the right tank contained approximately 7 gallons of fuel.