AEROBATIC ACFT ALLEGED TO HAVE VIOLATED CLASS B AIRSPACE. DISPUTED.

Narrative

I HAD BEEN PRACTICING AEROBATICS NEAR LEXINGTON RESERVOIR 30-35 MI SE OF SFO. THE ACFT; A PITTS SPECIAL S1A; IS BASED AT SJC. SJC WAS VERY BUSY AND I HAD A HARD TIME RAISING BAY APCH TO ENTER SJC'S CLASS C AIRSPACE WHEN I WAS FINISHED PRACTICING. I DECIDED TO PROCEED TO PAO WHERE I LANDED; AND THEN TO PROCEED TO SJC. THIS WOULD BE EASIER SINCE PAO TWR WOULD HAND ME OFF DIRECTLY TO SJC TWR. BEFORE PAO CLRED ME TO TKOF; THEY ASKED ME TO PHONE BAY APCH AFTER LNDG. I GOT BACK TO SJC; PARKED THE AIRPLANE IN THE HANGAR AND CALLED BAY. THE CTLR TOLD ME THAT MY RADAR TRACK SHOWED ME INSIDE THE SFO CLASS B AT 6300 FT. THERE WAS NO DISCIPLINARY ACTION; JUST A WARNING AND A REQUEST TO BE MORE CAREFUL. PLAYING BACK THE EVENTS IN MY MIND; I BELIEVE MY MODE C WAS INCORRECT AND THAT I WAS ACTUALLY WELL CLR OF THE CLASS B. I HAD BEEN PRACTICING AEROBATICS IN THE 4000-6500 FT ALT RANGE; FINISHING OFF WITH A LOMCEVAK. I THEN DSNDED TO ABOUT 4000 FT OVER LEXINGTON RESERVOIR IN PREPARATION FOR RETURNING TO SJC. THIS WAS 30 MI FROM SFO; BEYOND THE LATERAL LIMIT OF THE CLASS B. I WAS CERTAINLY NOT 3000 FT ABOVE THE NEARBY MOUNTAINS. I THEN FLEW ALONG THE W SIDE OF THE SANTA CLARA VALLEY; DSNDING INTO PAO. WHEN I CALLED UP SJC TWR; 1 STATIC-FILLED EXCHANGE INCLUDED 'SQUAWK' AND '4600.' I DIALED THIS INTO THE XPONDER. THEY CALLED BACK AND SAID I WAS SQUAWKING MY ALT AND TO SQUAWK XXXX. AT THIS POINT I WAS BTWN 2000-2500 FT. I THINK THE XPONDER AND ENCODER WAS TELLING APCH THAT I WAS AT LEAST 2000 FT HIGHER THAN I ACTUALLY WAS AND THAT THEY THOUGHT I WAS AT 4600 FT. BAY HAD NOTED NO PROBS WHEN I ORIGINALLY DEPARTED SJC. THE ONLY EXPLANATION I CAN THINK OF IS THAT THE HARD AEROBATICS CONFUSED THE ENCODER A BIT OR THAT A HAND-HELD RADIO THAT CAME LOOSE FROM ITS HOLDER HAD HIT AND DAMAGED THE ENCODER. I NEED TO GET THIS CHKED OUT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES HE WAS AT 6000 FT DURING HIS AEROBATICS; THEN STARTED DSNDING ON ATTEMPT TO RETURN TO SJC. HE WAS WELL BELOW THE 4000 FT FLOOR TO ARRIVING AT PAO. HE HAS SINCE FLOWN THE ACFT WITH CHKS FROM APCH AND HAS RECEIVED CONFIRMATION OF HIS MODE C BEING ACCURATE. WHY THIS ISOLATED MISREAD HE DOES NOT KNOW.