A LNDG C172 IS OVERTAKEN AND ALMOST HIT ON SHORT FINAL BY ANOTHER C172. INSTRUCTOR PLT RPTR CITES THE FREQ OVERLAP FROM NEARBY ARPTS AS BLOCKING RADIO XMISSIONS OF ACFT IN THE TFC PATTERN.

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WHILE ON THE FINAL APCH FOR RWY 8R AT THE NORTH PALM BEACH COUNTY ARPT; THE PLT OF THE OTHER AIRPLANE FRANTICALLY SHOUTED ON THE UNICOM FREQ THAT ANOTHER AIRPLANE WAS IN HIS WAY. I WAS INSTRUCTING A STUDENT PLT ON HOW TO LAND THE AIRPLANE AND DID NOT MAKE AN IMMEDIATE REPLY. I LOOKED BOTH SIDES AND IN FRONT BUT DID NOT SEE THE OTHER AIRPLANE. WE HAD EXTENDED OUR DOWNWIND TO FOLLOW BEHIND A TWIN PIPER WHICH DSNDED ONTO THE DOWNWIND FROM A HIGHER ALT. I CAN ONLY SURMISE THAT THE OTHER C172 HAD FLOWN A SHORT PATTERN AND HAD CUT IN FRONT OF US AND MADE A STEEP DSCNT WHICH BROUGHT HIM OVER AND BEHIND OUR AIRPLANE. THE PLT OF THE OTHER CESSNA MADE A GAR AND WAS QUITE SHAKEN UP. WITNESSES ON THE GND SAID THE OTHER AIRPLANE CAME WITHIN HALF A WINGSPAN OF OURS ON SHORT FINAL. FORTUNATELY; MY STUDENT WAS NOT AWARE OF WHAT HAD HAPPENED AND HE REMAINED VERY CALM. ALTHOUGH EVERY AIRPLANE MADE POS RPTS IN THE TFC PATTERN; THE COMS WERE FREQUENTLY BLOCKED BY XMISSIONS FROM AIRPLANES AT OTHER ARPTS USING THE SAME CTAF/UNICOM FREQ OF 123.0 MHZ. IT IS NOT UNCOMMON TO HEAR XMISSIONS FROM SEVERAL NEARBY ARPTS USING THE SAME FREQ. WELLINGTON; A PVT FIELD TO THE S; OKEECHOBEE (OBE) AND INDIANTOWN (X58) TO THE NW; DADE COLLIER TRAINING FACILITY TO THE S. ON SOME DAYS WE HEAR XMISSIONS FROM ARPTS SOME 100 MI AWAY; ARTHUR DUNN; NAPLES AND VALKARIA. ALTHOUGH ACFT COM RADIOS ARE NOW REQUIRED TO HAVE 720 CHANNELS; NEITHER THE FCC NOR FAA HAS DESIGNATED UNICOM/CTAF FREQS IN THE 0.05 AND .25 MHZ INTERVALS TO RELIEVE THE CONGESTION ON THE LIMITED FREQS (122.7; 122.8; 122.9 AND 123.0 MHZ) ASSIGNED TO UNCTLED ARPTS AROUND THE COUNTRY. THE EQUIP IS IN PLACE. IT IS TIME TO SPLIT UP THE FREQS TO AVOID CONGESTION ON THE FREQ FOR OBVIOUS SAFETY REASONS.