AFTER TURNING BASE TO FINAL AT LVJ ARPT; C172 PAX ALERTS PLT TO NMAC SIT.

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I FIRST TUNED INTO CLOVER FIELD UNICOM ABOUT 8 MILES SE OF THE FIELD TO DETERMINE TFC SIT. THERE ARE SEVERAL ARPTS WITH 122.8 UNICOM FREQ IN THAT AREA; SO IT WAS HARD TO TELL. I CALLED AT ABOUT 6 MILES FROM THE FIELD DECLARING MY INTENTION TO CROSS MID FIELD AT 1500 FT MSL AND ENTER A L DOWNWIND FOR RWY 32. UPON REACHING MID FIELD; I CALLED MY POSITION; AND RECEIVED AN ADVISORY THAT THERE WERE TWO LOW WING ACFT IN THE PATTERN. I SAW ONE ON BASE AND THE OTHER ON DOWNWIND; NEAR BASE. EVER SINCE I FIRST TUNED THE FREQ; I REPEATEDLY HEARD A GARBLED XMISSION; COMPLETELY UNDISTINGUISHABLE. I CALLED BACK RPTING BOTH ACFT IN SIGHT AND ENTERED THE LEFT DOWNWIND; PROBABLY AT 1100 FT. I FELT LIKE I WAS WIDE; SO I TURNED AND EARLY BASE AT ABOUT 800 FT WHICH WAS HIGH-PATTERN ALTITUDE IS 800 FT. I TURNED FINAL AT ABOUT 500 FT AND PUT IN FULL FLAPS. AT ABOUT 200 FT; ONE OF MY PASSENGERS SAYS 'WHAT THE HELL IS THAT?' AND I SEE THE PIPER CROSS UNDER ME LESS THAN 50 FT. I PULLED RIGHT AND WENT BACK AROUND WHILE HE DID 3 TOUCH-AND-GO'S BEFORE LNDG. IT WAS THEN CLR THAT IT WAS THAT ACFT THAT WAS MAKING THOSE GARBLED RADIO CALLS AND WE NEVER REALIZED HE WAS IN THE PATTERN. NOR DID WHOEVER TOLD ME THAT THERE WERE TWO LOW WINGS IN THE PATTERN. I DID DISTINCTLY HEAR TWO ACFT CALLING THEIR POSITIONS TO CLOVER FIELD SPECIFICALLY; SO I NEVER EXPECTED A THIRD. HAD HIS RADIO BEEN FUNCTIONING PROPERLY; I WOULD HAVE BEEN LOOKING FOR HIM ALSO.