A C172 FLOWN BY A CFI ON AN INTRODUCTORY FLT WITH A STUDENT PLT ABOARD WAS CLRED TO LAND WHILE ON L BASE FOR RWY 35. THE CFI LOOKED OUT HIS WINDOW AND SAW A BANNER TOW ACFT AT SAME ALT IN CLOSE PROX AND DOVE TO PREVENT A COLLISION. RPTR SAID TWR KNEW OTHER ACFT WAS IN THE AREA AND ALLEGES THAT THE BANNER TOW ACFT DID NOT NEED MODE C TO OPERATE IN THE ARSA. A NMAC RPT WAS FILED WITH THE TWR SUPVR.

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DURING AN INTRODUCTION FLT IN A C172 I RETURNED FROM THE PRACTICE AREA TO MYR. APCH CTL HANDED ME OFF TO TWR AFTER TELLING ME TO ENTER A L BASE FOR RWY 35. I WAS ALREADY AT 1000 FT WHICH IS TFC PATTERN ALT. AFTER I CONTACTED TWR I WAS CLRED TO LAND. WHILE LOOKING OUT THE L WINDOW I SAW ANOTHER ACFT WHO APPEARED TO BE AT THE SAME ALT AND WE APPEARED TO BE CONVERGING. I TOOK EVASIVE ACTION BY 'DIVING' DOWN TO 700 FT. I ASKED THE CTLR IF HE WAS TALKING TO THE OTHER PLT. HE SAID 'YES.' I TOLD THE CTLR THAT WE CAME AWFULLY CLOSE TO EACH OTHER TO WHICH HE REPLIED: 'I DID NOT REALIZE YOU WERE THIS LOW THIS FAR OUT.' AFTER LNDG I CALLED THE ATC SUPVR TO FILE THE PROPER RPT. I HAD MY XPONDER ON AND OPERATING IN MODE C. IN MY OPINION THE TWR CTLR SHOULD HAVE SEPARATED ME FROM THE OTHER ACFT. IN MYR AIRSPACE THERE ARE SOME PROBS WITH THE IMPLEMENTATION OF THE CLASS C AIRSPACE; ESPECIALLY WITH THE NUMEROUS BANNER TWRS (WHICH IS WHAT THE OTHER PLANE WAS). THEY ARE EXEMPT FROM THE MODE C REQUIREMENT. I THINK SAFETY WILL INCREASE IF ALL BANNER TOWING AIRPLANES WILL BE EQUIPPED WITH MODE C XPONDERS.