A C172 PLT EXPRESSED CONCERN REGARDING MODE C RECEPTION BY PCT S OF EMI VOR.

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UPON INITIAL CONTACT WITH POTOMAC; I INFORMED THEM I WAS 5 MILES NE OF THE WESTMINISTER VOR; LEVEL AT 2000 FT AND WAS REQUESTING MY ADIZ AND BRAVO AIRSPACE CLRNC BACK INTO BWI. AFTER RECEIVING A XPONDER CODE OF XXXX; I WAS TOLD TO PROCEED DIRECT TO BWI. THE TIME WAS XA:00 PM LOCAL AS MY ADIZ FLT PLAN BACK TO MY ORIGINATING ARPT (BWI) WAS PREVIOUSLY FILED FOR XA:00 PM. AT APPROXIMATELY 5 MILES S OF THE VOR; I WAS GIVEN A VECTOR OF 120 DEGS AND TOLD TO CONTACT POTOMAC NOW ON FREQ 119.00. UPON INITIAL CALL-UP; ATC QUERIED MY ALT AND AFTER I RESPONDED 'LEVEL AT 2000 FT' HE INFORMED ME THAT MY MODE C READOUT WAS INTERMITTENT. I INFORMED ATC THAT I WOULD RECYCLE MY XPONDER AND AFTER I DID SUCH; ATC RESPONDED THAT THE MODE C CAPABILITY HAD BEEN LOST. I WAS THEN GIVEN A HDG OF 360 DEGS AND DIRECTED TO EXIT THE ADIZ; WHICH; I COMPLIED WITH IMMEDIATELY. AFTER EXITING THE ADIZ; THE CTLR TOLD ME TO 'SQUAWK VFR; SORRY ABOUT THIS AND HAVE A GOOD DAY.' NOW OUTSIDE THE ADIZ; I REESTABLISHED CONTACT WITH POTOMAC ON 119.70 AND ASKED FOR A XPONDER CHECK. THE CTLR REPLIED THAT HE INDEED HAD MY 1200 CODE ON RADAR; 3 MILES E OF THE WESTMINSTER VOR; BUT NO MODE C. UPON TAKING THE ACFT TO AN AVIONICS SHOP LOCATED ON THE FIELD; THE TECHNICIANS INFORMED ME OF TWO POINTS OF INTEREST. 1) THE XPONDER MODE C BENCH TEST WAS ONLY OPERATING INTERMITTENTLY AT BEST AND THAT THEY NEEDED TO KEEP THE ACFT OVER THE WEEKEND FOR FURTHER TESTS. 2) THAT I WAS PERHAPS THE 40TH CUSTOMER WITHIN SEVERAL MONTHS STOPPING AT THE REPAIR STATION FOR THE SAME COMPLAINT. APPARENTLY; THE TECHNICIANS VOICED THEIR SUSPICIONS FOR A 'DEAD AREA' JUST S OF THE WESTMINSTER VOR WHERE EVEN THEY HAD EXPERIENCED FIRST-HAND; POTOMAC'S INABILITY TO PROPERLY PICK UP MODE C XPONDER CAPABILITIES AT AND BELOW 2000 FT MSL. ALTHOUGH MY XPONDER WAS RPTED AS 'INTERMITTENT OPERATION OF MODE C AT BEST;' I THOUGHT THIS UNUSUAL OCCURRENCE SHOULD BE NOTED.