THE PLT HAD BUILT THE ULTRALIGHT VEHICLE AT HOME BY USING THE PLANS OF A WEEDHOPPER. REPORTEDLY, HE HAD USED MATERIALS THAT WOULD HAVE BEEN SIMILAR TO OR STRONGER THAN THOSE THAT WOULD HAVE BEEN SUPPLIED BY A WEEDHOPPER KIT. A FRIEND, WHO HAD FLOWN THE VEHICLE, STATED THAT THE VEHICLE WAS EXTREMELY WELL BUILT & HE ENJOYED FLYING IT. PRIOR TO THE FATAL FLT, THE OWNER HAD TAXIED THE ULTRALIGHT, BUT HAD NEVER FLOWN IT NOR ANY OTHER KNOWN ACFT. ON THE DAY OF THE ACCIDENT, HE REPORTLY SAID 'WELL, I HAD THIS PLANE FOR 6 MONTHS. TODAY, EITHER I AM GOING TO FLY IT OR SELL IT.' AFTER TAKING OFF, HE CLIMBED TO APRX 200 TO 300 FT & WAS OBSERVED TO ENTER A LEFT TURN. AFTER ENTERING THE TURN, THE VEHICLE ENTERED A STEEP, NOSE DOWN ATTITUDE & CRASHED. NO EVIDENCE OF AN ENG POWER LOSS OR PREIMPACT MECHANICAL FAILURE WAS EVIDENT.