NMAC.

Narrative

AFTER RECEIVING VECTORS AROUND THE N SIDE OF CHICAGO; WE WERE CLRED DIRECT TO AURORA; IL; ARPT AT 3000 FT MSL. AT APPROX 7 MI NW OF DUPAGE ARPT; BOTH CREWMEMBERS SPOTTED A CESSNA CARDINAL R/G AT OUR 4 O'CLOCK RELATIVE POS. OUR IAS WAS 200 KTS; AND THE PF ADDED PWR TO AVOID THE CESSNA. (WE WERE CERTAIN THERE WAS VERY LITTLE AIRSPD CHANGE.) I SAW THE CESSNA MAKE A L TURN AWAY FROM US. WE ASKED THE CTLR IF HE HAD THE TFC. THE AIR TFC CTLR DID NOT PICK UP THE CESSNA UNTIL THE CESSNA WAS OVER DUPAGE ARPT AS A PRIMARY TARGET. WE CONTINUED THE FLT TO AURORA ARPT WITH NO FURTHER INCIDENT. CAUSE OF PROB: EXTENSIVE VECTORING AT LOW ALTS AROUND MAJOR TERMINAL AREAS. INCREASES EXPOSURE TO LOW LEVEL VFR ACFT. PREVENT RECURRENCE: ALT ENCODING XPONDERS. GREAT HELP. WE SPOTTED ALL OTHER TFC POINTED OUT BY ATC THIS MORNING. TCASII. SUPPLEMENTAL INFO FROM ACN 267586: POSSIBLE CAUSES OF THIS NEAR COLLISION ARE: ABSENCE OF A SECONDARY RADAR RETURN FROM THE CESSNA. HE WAS WITHIN THE 30 NM VEIL OF THE ORD CLASS B AIRSPACE.