SAAB 340 FLT CREW IS REQUIRED TO TAKE EVASIVE ACTION ON DEP AS T38 TRAINER DOES LOW APCH OVER THEIR ACFT.

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WE WERE CLRED INTO POS AND HOLD ON RWY 18. WHILE I WAS PERFORMING MY BEFORE TKOF DUTIES; I RECALL OVERHEARING THE CTLR ISSUE MISSED APCH INSTRUCTIONS TO THE T-38 ON PRACTICE APCH TO THE SJT ARPT. AT THAT TIME I DID NOT HEAR THE RWY TO WHICH HE WAS FLYING THE APCH; BUT I DO REMEMBER THE LCL CTLR INSTRUCTING THE T-38 TO EXECUTE HIS MISSED APCH 1 NM PRIOR TO THE RWY THRESHOLD AND 'DO NOT OVERFLY THE DEP OR ARR END OF RWY 18; DEPARTING TFC.' WE WERE THEN CLRED FOR TKOF ON RWY 18; RWY HDG. THE CTLR DID NOT PROVIDE US WITH A TA OF THE INBOUND T-38 PRIOR TO TKOF. AFTER LIFTOFF; THE CAPT CALLED FOR 'GEAR UP.' SHORTLY AFTER (APPROX 300-400 FT AGL); I HEARD THE CTLR STATE IN AN ANXIOUS TONE; 'TURN L!' THE CAPT; THEN I; VISUALLY ACQUIRED THE T-38 AT OUR 1 O'CLOCK POS IN LEVEL FLT AT ABOUT 400 FT AGL APCHING THE THRESHOLD TO RWY 3 (WHICH INTERSECTS AT THE DEP END OF RWY 18). THE CAPT MADE A DSNDING L TURN TO EVADE THE T-38. THE CTLR CONTINUED TO ISSUE COMMANDS TO THE T-38 TO MAKE AN IMMEDIATE L TURN; AT WHICH TIME I OBSERVED THE T-38 EXECUTE A CLBING L TURN AND PASS OFF THE R SIDE OF OUR ACFT. THE CAPT HAVING LEVELED THE ACFT AT 300-350 FT AGL; THEN BEGAN A CLB TO THE NE TOWARD BWD ON COURSE; AND THE FLT CONCLUDED WITHOUT INCIDENT. TCASII HAD BEEN SELECTED TO THE AUTO MODE PRIOR TO THE BEFORE START CHKLIST; BUT THE TARGET WAS IDENTED ON THE MFD AS 'TA ONLY' AND NO AURAL ALERT WAS GIVEN DUE TO THE LOW ALT OF OUR ACFT. CONTRIBUTING FACTORS: THE T-38 WAS PERFORMING A PRACTICE APCH; OPPOSITE DIRECTION TO THE FLOW OF TFC; TO AN INTERSECTING RWY WITH A TAILWIND (WIND 190 DEGS AT 12 KTS). THE T-38 WAS OPERATING ON A UHF FREQ. WE WERE ON VHF; THEREFORE; WE COULD NOT OVERHEAR HIS XMISSION TO THE LCL CTLR. A TA WAS NOT ISSUED TO US PRIOR TO TKOF.