A C172 PLT IS INFORMED AFTER LNDG AT 3W3 THAT PCT TRACON WAS NOT ABLE TO RECEIVE HIS XPONDER.

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ON JUN/SUN/04 AT APPROX XA00Z; I DEPARTED ESN TO RETURN TO 3W3. THE XPONDER WAS SET TO 1200 AND WAS TESTED ON THE GND PRIOR TO DEP FROM ESN USING THE TEST FEATURE. THE XPONDER TEST WAS SUCCESSFUL. I DEPARTED RWY 4 AT ESN AND TURNED TO THE N FOLLOWING UNITED STATES RTE 50 AND CLBED TO APPROX 1800 FT MSL. I CONTINUED ON THIS HDG FOR APPROX 8 NM AND TURNED W APPROX 1-2 NM S OF GRACO AND PROCEEDED DIRECT TO 3W3; DSNDING TO 1000 FT MSL. PRIOR TO TURNING TO THE W; I CHANGED THE XPONDER CODE TO ASSIGNED CODE AND; USING THE TEST FEATURE ON THE XPONDER; GOT A POSITIVE ACKNOWLEDGEMENT (TEST LIGHT ILLUMINATED) THAT THE XPONDER WAS FUNCTIONING PROPERLY. I TUNED IN POTOMAC TRACON ON 124.55 TO MONITOR THE FREQ AND PROCEEDED DIRECT TO 3W3. I ARRIVED AT 3W3; ENTERED A DOWNWIND FOR RWY 28 AND LANDED. I PARKED THE AIRPLANE. I WAS IMMEDIATELY INFORMED BY THE ARPT MGR TO CALL POTOMAC TRACON BECAUSE THEY HAD BEEN UNABLE TO RECEIVE MY XPONDER CODES. I TELEPHONED POTOMAC TRACON PROVIDED THE REQUESTED INFO TO THEM REGARDING THIS INCIDENT. I THEN RETURNED TO THE ACFT AND TESTED THE XPONDER A THIRD TIME AND RECEIVED A POSITIVE TEST RESULT. I ALSO ENGAGED THE 'IDENT' BUTTON AND RECEIVED A POSITIVE INDICATION THAT THE XPONDER WAS IDENTING ITSELF. THE NEXT DAY; I CONTACTED THE FBO AT ESN AND EXPLAINED THE SIT. THE FBO HAS AN AVIONICS SHOP THAT HAD RECENTLY INSTALLED NEW AVIONICS IN THE ACFT. FBO MAINT CAME AND TESTED THE XPONDER USING AN IFR MODEL ATC-601-A TEST APPARATUS. MAINT STATED THE XPONDER WAS SENDING OUT A SIGNAL; BUT IT WAS SO WEAK THE XPONDER FAILED THE TEST. HE REMOVED THE XPONDER IN ORDER TO PERFORM A BENCH TEST. THE XPONDER WAS BENCH TESTED AND FOUND TO BE IN OPERATING ORDER. MAINT INFORMED ME THAT THIS IS AN INDICATION THAT THE XPONDER ANTENNA OR CABLE HAS FAILED AND REQUIRES REPLACEMENT. THE REPLACEMENT OF THE XPONDER CABLE AND ANTENNA IS SCHEDULED FOR JUN/WED/04. PRIOR TO MY DEP AT ESN; I REVIEWED THE ENTRY PROCS IN FAA PUB ATP-200. I FOLLOWED EACH AND EVERY GUIDELINE DESCRIBED IN THE PUB.