After construction of a home built Rans S 6 was completed, the pilot (owner, builder) elected to practice taxiing around the 1300 ft grass strip. After a series of taxi tests, in which the airplane became airborne and was landed, he allowed his wife aboard, intending only to taxi the plane. According to him, as he became more comfortable in the airplane, he increased the taxi speed to about 40 mph, and the airplane inadvertently lifted off. The pilot stated that he did not have enough airstrip to stop, so he elected to remain airborne, circle the field, and land. He cleared trees at the end of the strip and turned crosswind at about 75 ft AGL. He indicated he turned downwind at 70 mph. Although the engine was at full throttle, the airplane began to lose altitude. The pilot noticed powerlines ahead and pulled up while using full flaps. Subsequently, the airplane hit a tree at an airspeed of about 40 mph and crashed. The pilot reported no mechanical malfunction, but said there was turbulence. The calculated density altitude was about 2500 ft.