AA5 LNDG NON-CTLED ARPT EXPERIENCES CLOSE ENCOUNTER WITH HELI.

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I MADE A 45 NM FLT FROM N57 TO N76; UNDER RADAR COVERAGE FROM HARRISBURG APCH. 4 NM FROM N76; THE CTLR CANCELLED COVERAGE WITH NO COMMENTS ABOUT TFC IN THE VICINITY OF N76. I SWITCHED TO CTAF 123.0 AND CALLED 4NM OUT; THEN SINCE I WAS IN POSITION TO DIRECTLY ENTER LEFT BASE RWY 29; I DID SO AND CALLED MY POS. I SAW NO TFC IN THE PATTERN; ALTHOUGH MY NEW TPAS WARNED ME OF AN ACFT WITHIN 2NM. AS THE TPAS GIVES ONLY RANGE AND NOT BEARING; I SUSPECTED THE TFC MIGHT BE HEADING TO A NEARBY ARPT SINCE NO ONE WAS RESPONDING ON N76'S CTAF. I TURNED FINAL AND MADE THE RADIO CALL; STILL NO RESPONSE FROM OTHER TFC. NOW; THE TPAS BEGAN TO DISPLAY A RAPIDLY DECREASING RANGE; DOWN TO >0.4NM. SUSPECTING I WAS DSNDING ON ANOTHER PLANE ON FINAL; I LEVELED OFF WENT AROUND; NOT CLIMBING IN CASE THE TFC WAS ABOVE ME. AT MIDFIELD AND AT 100 FT AGL; I HEARD A HELICOPTER MAKE A RADIO CALL OF SHORT FINAL; THEN I SAW HIM AS I TURNED XWIND. HE COMPLETED HIS TOUCH AND GO; THEN FLEW ANOTHER TIGHT AND VERY LOW PATTERN; COMPLETED ANOTHER TOUCH AND GO THEN LEFT THE AREA. NEITHER I NOR THE OTHER PLT COMMENTED OVER THE RADIO ABOUT THIS. I SUSPECT THAT HIS RADIO WAS OFF UNTIL HE SAW ME PASS OVER HIM ON FINAL AT THIS INFREQUENTLY USED RURAL ARPT. I SEE 3 FACTORS CONTRIBUTING TO THIS EVENT AND ONE FACTOR THAT MAY WELL HAVE PREVENTED A TRAGEDY. FIRST; I FAILED TO SEE AND AVOID TFC IN THE PATTERN; ALTHOUGH IT WAS DIFFICULT TO SEE A SMALL HELICOPTER FLYING A NONSTANDARD PATTERN. SECOND; THE CHOPPER PLT WAS NOT USING HIS RADIO; ASSUMING HE WAS THE ONLY ONE AROUND. THIRD; THE HARRISBURG CTLR PROBABLY SAW THE TFC BUT DID NOT MENTION IT. THE MAJOR FACTOR IN AVOIDING A COLLISION WAS THE TPAS; IT MADE ME AWARE OF TFC THAT I OTHERWISE WOULD NOT HAVE SEEN. WHILE IT IS RELATIVELY UNSOPHISTICATED; GIVING ONLY APPROX RANGE WITH NO BEARING AND DEPENDS ON ACTIVE TRANSPONDERS IN THE OTHER ACFT; IT HAS NEVERTHELESS PROVED IT'S VALUE TO ME. STILL; IT IS NOT A SUBSTITUTE FOR 'SEE AND AVOID'; I NEED TO BE MORE OBSERVANT.