AN M20J SINGLE PLT HAD AN NMAC WITH A BEECH 1900 WHO ENTERED THE PATTERN AT JHW; UNANNOUNCED; WHICH CAUSED THE RPTR TO GO AROUND.

Narrative

I WAS ON AN IFR FLT PLAN INTO JHW (JAMESTOWN) AN UNCTLED ARPT. I WAS GIVEN A VISUAL APCH TO RWY 31. RADAR SVC WAS TERMINATED AND I WAS ADVISED TO SWITCH TO UNICOM BY ERIE APCH. I ANNOUNCED MY INTENTIONS ON UNICOM AND DUE TO CONFLICTING TFC ON DOWNWIND; I RADIOED THAT I WOULD REENTER THE PATTERN ON A L DOWNWIND FOLLOWING THE ACFT I HAD IN SIGHT. THE PLT OF THIS ACFT COMMUNICATED WITH ME WELL AND THERE WAS NO PROB. I HAD HIM IN SIGHT AND HE HAD A VISUAL ON ME. I REENTERED THE PATTERN ON A L DOWNWIND FOR RWY 31 AND RADIO SAME ON UNICOM. I DID THE SAME FOR L BASE FOR RWY 31. HALFWAY THROUGH MY L BASE I NOTICED A COMMUTER PLANE (BEECH 1900 I BELIEVE) IN FRONT OF ME (ABOUT 1/2 TO 2/3 OF A MI IN FRONT OF ME) AND ABOUT 200 FT BELOW ME. AT THE SAME TIME THE PLT OF THIS BEECH 1900 RADIOED TO ME 'DO YOU HAVE US IN SIGHT?' I STATED YES AND EXECUTED A GAR. PRIOR TO THIS; I DID NOT HEAR THIS BEECH 1900 CALL THAT HE WAS ON FINAL APCH UNTIL HE ASKED ME IF I HAD HIM IN SIGHT. AT THIS POINT HE WAS ON ABOUT A 1/2 MI TO 3/4 MI FINAL APCH. I DO BELIEVE HE WAS ON A LONG FINAL VISUAL APCH TO RWY 31 AT JHW. MORE POS RPTS BY THIS COMMUTER PLANE WOULD HAVE ALERTED ME SOONER THAT THERE WAS CONFLICTING TFC ON A LONG FINAL APCH.