VFR PA28 PLT WITH MCO APCH RECEIVING TA'S EXPERIENCED NMAC WITH UNRPTED TFC AT 5500 FT.

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WHILE ENRTE FROM LEESBURG REGIONAL (LEE) TO MELBOURNE INTL (MLB) AT 5500 FT I WAS RECEIVING VFR TA'S FROM ORLANDO APCH ON 119.4. I WAS TOLD TO FLY A COURSE OF 160 DEGS AND HAD BEEN DOING SO FOR SEVERAL MINS. OUT OF THE CORNER OF MY EYE I CAUGHT A BRIGHT LIGHT AND STROBES OF ANOTHER ACFT; I SUSPECT THAT HE MAY HAVE BEEN EITHER CLBING OR DSNDING AS THAT WOULD EXPLAIN WHY I DIDN'T SEE HIM UNTIL THE LAST MOMENT. AS SOON AS I SPOTTED THE ACFT HE FLEW DIRECTLY OVER THE TOP OF MY ACFT AT NO MORE THAN 50 FT. THE EVENT HAPPENED SO FAST THAT I HAD NO TIME TO REACT. I CALLED APCH IMMEDIATELY AFTER TO RPT A PLANE THAT PASSED WITHIN 50-100 FT. THE CTLR REPLIED THAT THE TFC HAD 'JUST POPPED UP; BUT WAS RPTING NO ALT.' I REPLIED THAT THE ACFT WAS DEFINITELY AT MY ALT; 5500 FT. UNFORTUNATELY; I BELIEVE THAT I DID EVERYTHING I COULD TO AVOID THE SIT; RECEIVING TA'S AND SCANNING FOR TFC. HOWEVER; IT IS IMPORTANT TO KNOW THAT EVEN IN A RADAR ENVIRONMENT TFC CAN STILL GO UNDETECTED; ESPECIALLY WHEN SCANNING FOR TFC VISUALLY AGAINST A BRIGHTLY LIGHT BACKGROUND.