C172 REPEATEDLY HARASSED INFLT BY PLT OF UNIDENTED HOMEBUILT ACFT. REPEATED PASSES WITHIN 50 FT VERTLY AND 200 FT HORIZLY QUALIFIES AS PURPOSEFUL NMAC.

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ON MAR/MON/03; I WAS PIC OF A CESSNA 172; RENTED BY A PHOTOGRAPHER I HAVE FLOWN MANY TIMES. I WAS ORBITING; AT 2300 FT MSL; A BUILDING ABOUT 1/2 MI W OF ARPT; 0A6; IN ORDER FOR THE PHOTOGRAPHER TO TAKE HIS PICTURES. WX WAS GOOD VFR WITH NO CEILING; AND BETTER THAN 10 MI VISIBILITY WITH SOME HAZE. I HAD CALLED 0A6 ON UNICOM; 123.0; AND ADVISED OF MY INTENTIONS AS I APCHED THE PHOTO SITE. AS I WAS FLYING N ALONG THE W SIDE OF THE BUILDING; A SMALL WHITE EXPERIMENTAL ACFT; ALSO HEADED N; PASSED MY ACFT VERY CLOSE ON MY R SIDE. ONCE AHEAD OF MY AIRPLANE; THIS SMALL AIRPLANE MADE A TURN TO WBOUND AND PASSED IN FRONT OF MY AIRPLANE. THE AIRPLANE THEN TURNED S AND PASSED OFF MY L SIDE; AGAIN FAIRLY CLOSE. THE SMALL WHITE ACFT APPARENTLY TURNED BACK NBOUND BEHIND ME AND AGAIN PASSED OFF MY R SIDE AND AGAIN TURNED ACROSS MY FLT PATH IN FRONT OF ME. THE PLANE MADE 1 MORE CIRCLE AROUND MY AIRPLANE; AND ON THE LAST PASS ACROSS MY FLT PATH; TURNED VERY STEEPLY AND WAS HEADED STRAIGHT AT MY PLANE. THE PLANE PULLED UP SHARPLY AND PASSED; SBOUND; OVER MY PLANE. AFTER THAT LAST PASS; I DID NOT SEE THE PLANE ANYMORE. MY PAX; A PHOTOGRAPHER IS A FRIEND. HE IS NOT A GOOD PAX IN THAT ROUGH HANDLING MAKES HIM NAUSEATED; SO I TREAT HIM WITH KID GLOVES WHILE FLYING. HE WAS SO UNNERVED BY THIS AIRPLANE'S HARASSMENT THAT HE TOOK OUT A SICK-SACK. WHILE HE NEVER USED THE RECEPTACLE; HE WAS VERY UNCOMFORTABLE DURING THE REST OF OUR FLT. THE OFFENDING ACFT WAS A LOW WING; WHITE EXPERIMENTAL; VERY SLICK SHAPED LIKE A FIGHTER PLANE; WITH A TINTED CANOPY; AND VERY HIGH PERFORMANCE. THERE WERE NO REGISTRATION NUMBERS VISIBLE ON THE PLANE. AS THIS PLANE FLEW THE CIRCLES AROUND ME; I WOULD ESTIMATE SEPARATION AT TIMES AROUND 200 FT OR LESS; EXCEPT WHEN HE CLBED ABOVE ME. ON HIS LAST PASS; SEPARATION DETERIORATED TO AROUND 50 FT OR LESS.