VFR CLIMB HAD A POTENTIAL CONFLICT WITH UNREPORTED TRAFFIC TRANSITING AREA.

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AFTER TAKE OFF FROM A PRIVATE STRIP IN CONCORD; NC ENRTE TO CHAPEL HILL; NC. I WAS CLBING OUT. I WAS IN CONTACT WITH CHARLOTTE DEP 126.15 AND HAD BEEN ASSIGNED A XPONDER CODE. I WAS TAKING CARE OF NORMAL CLBOUT AND NAV DUTIES WHEN I OBSERVED AN SMT PASS ABOVE AND BEHIND ME AT A DISTANCE CLOSER THAN COMFORTABLE; BUT I WOULD NOT CONSIDER IT A NEAR MISS. I HEARD THE SMT PLT TALK TO THE CTLR ABOUT THE TFC NOT BEING WHERE HE WAS TOLD TO LOOK FOR HIM. I THEN TOLD THE CTLR I HAD SEEN THE PLANE BUT IT WAS LATE (MEANING I HAD JUST SEEN HIM WHEN WE PASSED). THE CTLR THEN TOLD ME THAT HE HAD JUST SEEN MY XPONDER COME ON (IT HAD BEEN TURNED ON ALL ALONG SO I'M NOT SURE EXACTLY WHAT HE MEANT). THE FLT THEN CONTINUED UNEVENTFULLY TO CHAPEL HILL VFR AT 7500'. I ALSO HAD ROUTINE CONVERSATIONS WITH GREENSBORO APCH AND RALEIGH APCH ON THE WAY TO MY DEST. SEVERAL YRS AGO I WAS INVOLVED IN A SIMILAR SITUATION NEAR COLUMBIA; SC WHERE I WAS THE PLANE AT ALT OF 10500' AND A PLANE CLBED OUT FROM BELOW AND PASSED NEAR ME. I WAS IN CONTACT WITH ZJX BUT HE DID NOT SEE THE OTHER PLANE (A TURBO PROP) UNTIL WE PASSED AND SEPARATED. IF YOU HAVE QUESTIONS PLEASE GIVE ME A CALL. I LIVE IN THE GREENSBORO; CHARLOTTE RALEIGH-DURHAM AREA AND THE AIRSPACE IS BECOMING QUITE CONGESTED. THE CTLRS HERE SEEM QUITE HELPFUL AND EFFICIENT AND DO ALL THEY CAN TO ASSIST EVERYONE IN THE SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING. RPTR WAS USING CODE 1200 ON HIS XPONDER. SUSPECT THE SMT PLT WAS CONFUSED AS TO THE POS OF THE RPTED TFC AND WAS LOOKING THE WRONG SIDE. WAS NOT TOO CONCERNED BUT ADVISED IS FINDING IT MORE AND MORE DIFFICULT TO GET TA'S ACCOUNT THE INCREASED TFC IN THE CLT AREA. IN GENERAL IS SATISFIED WITH THE SYS BUT IS MAKING EVERY EFFORT TO STAY CLR OF THE NEW ARSA AREAS; INCLUDING AVOIDING FLYING OVER THE TOP.