C172 PLT ON FINAL APCH AT NON TWR FIELD HAS NMAC WITH ACFT WHICH FLIES BELOW HIM.

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I WAS PERFORMING TOUCH-AND-GOES AT GXY; AN UNCTLED FIELD. IT WAS A BUSY SUNDAY MORNING WITH SEVERAL SURROUNDING ARPTS ALSO USING THE CTAF OF 122.8. THIS MADE THE FREQ CONGESTED AND DIFFICULT TO DIFFERENTIATE TFC AT THE DIFFERENT ARPTS. I HAD PERFORMED SEVERAL PREVIOUS TOUCH- AND-GOES AND WAS MAKING A FULL STOP LNDG. I ADVISED TFC OF MY POS AND INTENTIONS UPON EACH LEG OF THE PATTERN. HEARING NO OTHER TFC FOR GXY I PROCEEDED TO TURN BASE AND FINAL; AGAIN CALLING MY LEGS. AT ABOUT 1/2 MI FINAL AND 300-400 FT; A LOW WING SINGLE ENG T-TAIL ACFT PASSED UNDERNEATH MY ACFT AND THEN PROCEEDED DOWN THE FINAL APCH BEFORE ADDING PWR AND CLBING OUT AS IF ON A GAR. THE ACFT SHOWED NO SIGN OF SEEING MY ACFT AND NEVER MADE ANY RADIO CALLS THAT I CAN REMEMBER. I CONTINUED MY APCH AND LANDED WITHOUT INCIDENT. THIS IS THE MOST DANGEROUS OF MANY IN RECENT HISTORY AT GXY THAT HAVE EITHER HAPPENED TO MYSELF OR FELLOW PLTS. I FEEL THE AMOUNT OF TFC AT THIS ARPT IS SUCH THAT MAINTAINING THE FIELD AS UNCTLED IS A MAJOR SAFETY RISK. EVEN ASSIGNING SEPARATE FREQS TO THE ARPTS IN THE VICINITY WOULD HELP TO PREVENT CONFUSION AND CONGESTION OF 122.8. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES HIS ACFT WAS A C172 AND THE OTHER WAS A SEMINOLE. THERE WAS NO ONE ELSE AROUND WHEN THE INCIDENT OCCURRED AND SO HE HAD NO IDEA WHETHER THE OTHER ACFT MADE A STRAIGHT-IN APCH OR NOT. ANALYST RECOMMENDED THE USE OF THE FAA HOTLINE.