NMAC. TFC PATTERN DEV.

Narrative

I WAS DEPARTING KOA FROM RWY 17 DEP FULL LENGTH. AT 3/4 LENGTH RWY LENGTH; STILL ON RWY HDG; I TURNED XWIND AT 400 FT AGL. KOA TWR CAME ON 120.3 AND RPTED TO A ROBINSON HELI ENTERING THE PATTERN ON A L DOWNWIND (I WAS ON A L TURNOUT) THAT I WAS DEPARTING AND TO LOOK FOR ME. THIS WAS THE FIRST I HEARD FROM TWR THAT ANOTHER ACFT WAS IN THE PATTERN AND I HAD SEEN NO ONE ELSE NEAR THE ARPT WHEN I LINED UP FOR DEP. WHEN I HEARD TWR TALK TO THE HELI I LOOKED FOR HIM AND HE WAS TAKING EVASIVE L TURN AND I TOOK EVASIVE L TURN BECAUSE WE WERE BOTH AT APPROX 500 FT AGL. I TOLD TWR THAT I HAD TFC IN SIGHT AND THAT THAT WAS A CLOSE CALL. THE HELI NEVER SAID ANYTHING. THE TWR THEN TOLD ME THAT I SHOULD HAVE NOTIFIED THEM THAT I WAS GOING TO MAKE AN EARLY L TURN FOR DOWNWIND DEP. I FELT THAT THE TWR SHOULD HAVE TOLD ME OF THE OTHER TFC PRIOR TO DEP SO I WOULD HAVE KNOWN TO BE LOOKING. I ALSO KNOW THAT I SHOULD HAVE STAYED ON RWY HDG UNTIL DEP RWYS 35 HAD BEEN MADE AND THEN HAVE INITIATED MY TURNOUT. REGARDLESS OF ALT; IT IS EXTREMELY IMPORTANT TO MAINTAIN RWY HDG UNTIL END OF RWY BEFORE TURNING XWIND. IT IS ALSO EXTREMELY IMPORTANT TO ABSOLUTELY CLR THE PATTERN ON TKOF VISUALLY SO NEAR MISSES WILL NEVER BE A FACTOR. I FAULT MYSELF FOR BOTH OF MY MISTAKES AND AM THANKFUL TO LEARN FROM THEM.