PA28 PLT HAD LESS THAN LEGAL SEPARATION FROM AN ACFT FLYING VFR IN IMC CONDITIONS.

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THIS RPT DOCUMENTS THE INTRUSION INTO CTLED AIRSPACE OF A NON-CTLED ACFT FLYING IN CLOUD; WHICH ENDANGERED MY ACFT DURING AN INST APCH. AT THE CONCLUSION OF AN IFR FLT FROM ROANOKE; VA (ROA); TO MARK ANTON ARPT; DAYTON; TN (2A0) I WAS IN RADAR AND RADIO CONTACT WITH CHATTANOOGA APCH CTL. AWOS AT MARK ANTON RPTED THE WX TO BE 500 FT BROKEN; 1000 FT OVCST WITH 2 MI VISIBILITY. WHILE E AND S OF THE ARPT I WAS CLRED BY CHATTANOOGA TO DSND TO 3000 FT AND GIVEN VECTORS TO FINAL FOR AN NDB APCH TO RWY 3. I WAS INSTRUCTED TO MAINTAIN 3000 FT UNTIL ESTABLISHED ON THE FINAL APCH COURSE; CLRED FOR AN NDB RWY 3 APCH TO THE DAYTON ARPT; AND RELEASED FOR A CHANGE TO ARPT ADVISORY FREQ. AFTER BEGINNING MY DSCNT; I CONTACTED MARK ANTON UNICOM TO ADVISE THEM OF MY POS AND THAT I WAS INBOUND ON AN NDB RWY 3 APCH. DAYTON UNICOM RESPONDED IMMEDIATELY AND TOLD ME THAT THERE WAS ANOTHER ACFT FLYING IN THE VICINITY OF THE ARPT. I RPTED THAT I WAS FLYING IN SOLID IMC AND ASKED WHERE THE OTHER ACFT WAS. THE OTHER PLT RPTED THAT HE WAS AT 2500 FT MSL ABOUT 2 MI E OF THE ARPT. I SAID THAT HE MUST BE IN THE CLOUDS; AND HE ACKNOWLEDGED THAT HE WAS. BY THAT TIME; I WAS DSNDING THROUGH 2300 FT MSL ABOUT 2.5 MI S OF THE NDB. I ADVISED THE OTHER PLT OF MY CURRENT POS AND THAT I INTENDED TO CONTINUE THE APCH TO MDA OF 1460 FT MSL. THE OTHER PLT RPTED THAT HE WOULD REMAIN E OF THE ARPT AT 2500 FT MSL. I BROKE OUT OF THE CLOUDS AT 1500 FT MSL OVER THE RWY AND WAS ABLE TO CIRCLE TO THE W AND LAND NORMALLY. WHILE ON DOWNWIND; I NOTED THAT THE CLOUDS AT 500 FT MSL WERE MOSTLY E AND S OF THE ARPT. HOWEVER; IF I HAD NOT BEEN ABLE TO SEE THE RWY AND LAND; I WOULD HAVE BEEN REQUIRED TO EXECUTE A MISSED APCH WHICH WOULD HAVE TAKEN ME THROUGH THE AIRSPACE OCCUPIED BY THE OTHER; UNCTLED ACFT. LATER; THE OFFENDING PLT LANDED AN I HAD THE OPPORTUNITY TO TALK WITH HIM ON THE RAMP. HE SAID THAT HE HAD 'GONE UP TO TAKE A LOOK' AT THE WX AND HAD INADVERTENTLY FLOWN INTO THE CLOUDS. HE HAD THEN CLBED IN CLOUD TO 2500 FT MSL TO OBTAIN SAFE TERRAIN CLRNC UNTIL HE COULD FIND HIS WAY BACK TO THE ARPT. RECOMMENDATION: CHATTANOOGA APCH HAS A MINIMUM VECTORING ALT OF 3000 FT MSL NEAR DAYTON DUE TO RADAR LIMITATIONS. RELOCATION OF THIS RADAR SITE TO A HIGHER ELEVATION NEARBY WOULD SIGNIFICANTLY IMPROVE RADAR COVERAGE IN THE AREA AND COULD HAVE MADE IT POSSIBLE TO DETECT THE INTRUDING ACFT AND PROVIDE RADAR SEPARATION.