AN ATX HELI DEPARTED A NON TWR ARPT VFR IN IMC CONDITIONS AND HAD AN NMAC WITH A COMMUTER ACFT THAT WAS ON APCH. THE COMMUTER ACFT TOOK EVASIVE ACTION AND ASKED THE ARTCC CTLR WHO THE TFC WAS. THE HELI FLC THEN CALLED THE CTLR FOR AN IFR CLRNC AND WAS GIVEN TFC INFO ON THE COMMUTER.

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COMMUTER WAS IFR APCH INBOUND TO DUJ; NO OTHER TFC; SO RADAR SVC WAS TERMINATED AND FREQ CHANGE TO ADVISORY WAS APPROVED. COMMUTER WAS LESS THAN 2 MI FROM ARPT WHEN I SAW TFC DEPART DIRECTLY IN FRONT. PREVIOUSLY THE S336 MADE AN INST APCH; FOLLOW BY 2 OTHER COMMUTERS; SINCE AUTO RPTING SYS SAID FIELD WAS BELOW VFR. AS THE HELI CLBED THROUGH 2000 FT HE CALLED AND ASKED FOR HIS IFR CLRNC. I TOLD HIM OF THE TFC; NOW WITHIN 200 FT VERTLY AND LESS THAN 1 MI. HE SAID HE WAS 'LOOKING.' UNABLE TO ISSUE AN IFR CLRNC; I WAS CONTACTED BY THE COMMUTER IN A STRAINED VOICE ASKING IF I HAD RELEASED TFC OFF DUJ. I STATED I HAD NOT; THAT THE TFC HAD DEPARTED VFR. I NOTICED THAT THE COMMUTER'S MODE C INDICATED A CLB OF AT LEAST 200 FT; AS THEY PASSED MODE C'S BOTH INDICATED 2400 FT. NEEDLESS TO SAY; THIS WAS A NEAR MIDAIR COLLISION. I CAN ONLY SURMISE THAT TCASII ALERTED THE COMMUTER TO TFC. THE COMMUTER DID NOT STATE AS MUCH; BUT THE CLB WAS MOST LIKELY AN RA. I CANNOT BE CERTAIN. MY CONCERN IS THE HELI. HE WAS 1 OF 4 INST APCHS; AND DECIDED TO DEPART WITHOUT CLRNC. WX WAS RPTED 700 FT BROKEN; SO THE COMMUTER HAD VERY LITTLE CHANCE TO SEE THE TFC. LOOKING AT THE COLLISION AT AN UNCTLED ARPT IN ILLINOIS RECENTLY; THIS IS DEFINITELY AN AWARENESS ITEM.