TCASII TA ALERTS FLC OF A BE02 TO THE PRESENCE OF AN AG5B AND THEY TAKE EVASIVE ACTION.

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FO WAS THE PF. WHILE ON A L BASE AT 1500 FT AGL ON A VISUAL APCH TO RWY 19; WE HAD A NEAR MISS WITH AN ACFT TYPE AG5B. THE TCASII ALERTED US AUDIBLY 'TFC; TFC;' WE ACQUIRED THE ACFT VISUALLY; AND COMMENCED AN EVASIVE MANEUVER TO AVOID A COLLISION. THE FO CLBED 200-300 FT TO AVOID A MIDAIR. THE ACFT PASSED DIRECTLY UNDERNEATH US. THE AG5B WAS ON ANOTHER FREQ. THE APCH FREQ WE WERE ON SAID THEY WERE NOT TALKING TO THAT ACFT. THEY GAVE US A LATE HDOF TO TWR. TWR SAID THEY WERE NOT TALKING TO THAT ACFT; ALTHOUGH THE AG5B WAS RPTED TO BE IN THE PATTERN DOING TOUCH-AND-GOES; AND HAD JUST DEPARTED. CONTRIBUTING FACTORS: 1) I BELIEVE THERE WERE TOO MANY APCH FREQS IN USE. (EGLIN IS A TRAINING FACILITY FOR MIL CTLRS.) 2) INEXPERIENCED AND OVERLOADED CTLRS -- THUS; A LATE HDOF TO TWR. 3) TFC SATURATION (A DC9 WAS ON APCH TO RWY 12; A C310; THE AG5B; US (BE1900D); AND OTHER MIL ACFT -- ALL IN VICINITY). 4) PRIMARY RADAR ONLY. 5) LACK OF DME OR NAVAID AT FIELD; MAKING IT DIFFICULT TO ESTABLISH POS OF OTHER ACFT. THE TCASII WAS INVALUABLE. THE WX CONDITIONS ALLOWED US TO SEE THE OTHER ACFT AND AVOID IT. IF WE HAD NOT CLBED; WE WOULD HAVE MOST ASSUREDLY SUFFERED A MIDAIR COLLISION.