A DEP ACR TUNES IN WRONG NAVAID AFTER TKOF AND IS OFF COURSE. CTLR THINKS THAT SIMILAR FREQS ON 2 NAVAIDS ARE TO BLAME.

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I WAS WORKING THE BPT LOW SECTOR; WHICH IS A DEP SECTOR OUT OF HOUSTON; TX. AT XX40Z; ACR X DEPARTED IAH EBOUND ON THE IAH 8 DEP TO BPT. THE PLT CHKED ON FREQ BTWN 10000-11000 FT. I NOTICED THAT THE ACFT WAS HDG S OF THE BPT VOR. MY BELIEF AT THIS TIME THAT HOUSTON APCH HAD LEFT THE ACFT ON A VECTOR. THIS SOMETIMES HAPPENS AND WE (ZHU) RECLR THE ACFT TO BPT. I ASKED THE PLT OF ACR X IF HE WAS HDG TOWARDS BPT. THE PLT ACKNOWLEDGED THAT BOTH VOR NEEDLES WERE CTRED AND HDG DIRECT BPT. I THEN TOLD THE PLT THAT BPT WAS APPROX 15 DEGS TO THE L AND THEN ASKED THE PLT IF HE HAD 115.4 TUNED IN. THE PLT SAID HE DID AND THE MISTAKE WAS FOUND. I GAVE THE PLT THE CORRECT FREQ WHICH IS 114.5 AND HE PROCEEDED TO TURN L DIRECT BPT. THE PROB IS THAT BPT AND SBI (SABINE VOR/DME) ARE 15 MI APART ON A N/S LINE. BPT IS 114.5 AND SBI IS 115.4. THE DEPS PROCEED EBOUND VIA THE IAH 8 DEP. ARRS PROCEED WBOUND OVER SBI VIA THE SBI 5 ARR. THIS EVENT OCCURRED DURING A DEP RUSH AND AN ARR RUSH. THE POTENTIAL FOR A FATAL MIDAIR IS VERY REAL IF A PLT HAS THE WRONG VOR TUNED IN. IF ZHU LOSES THE EFD (ELLINGTON) RADAR WE WILL NOT SEE THE ACFT UNTIL THEY ARE OUT OF 16000-17000 FT. IF A PLT HAS THE WRONG FREQ TUNED IN; IT MAY BE TO LATE TO SEE THE CONFLICT DEVELOP AND STEP IN TO CORRECT THE MISTAKE. SUGGEST THAT EITHER THE BPT OR SBI VOR FREQ BE CHANGED TO PREVENT THIS POTENTIAL PROB.