UNAUTH USE OF OVERWATER OP WHEN MEL REQUIREMENTS ARE NOT ADHERED TO. RADIO EQUIP PROB.

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I HAD NEVER FLOWN THIS RTE FROM DCA TO MIA AS AN OVERWATER FLT. MY COMPANY ONLY RECENTLY HAD STARTED USING THE MLG IN ITS NEWLY CONFIGURED LIMITED OVERWATER VERSION FOR THIS RTE. WHILE REVIEWING THE RTE WITH MY FO DURING PREFLT; I NOTICED THAT THE MAJOR PORTION OF THE RTE S OF THE ORF VORTAC ALL THE WAY TO MIA USED NDBS TO DEFINE THE OVERWATER AIRWAYS. MY ACFT HAD ITS ONLY ADF RECEIVER PLACARDED INOP PER OUR COMPANY MEL. THIS ONLY RESTR THE ACFT FROM FLYING INTO MEXICO AND EXECUTING ANY INST APCH IN WHICH AN NDB WAS NEEDED FOR A MISSED APCH PROC. I TOLD FO THAT I DIDN'T THINK WE WERE LEGAL TO FLY THE RTE AS FILED DUE TO OUR LACK OF ADF RECEIVER. HE SHOWED ME ON THE NAV CHARTS THAT COLLOCATED WITH THE AIRWAY NDBS THERE WERE VORS AVAILABLE TO DEFINE THE AIRWAY AS WELL. ON THE CHART WITH THE NDB FREQS WE SAW 4 DIGIT VOR-LIKE FREQS SUCH AS 117.1. CONVINCED ALL WAS WELL; WE PROCEEDED WITH OUR FLT. HOWEVER; APCHING THE FIRST NDB REQUIRED FOR NAVIGATING THE AIRWAY; WE TUNED UP THE CORRESPONDING VOR- LIKE FREQ AND WERE DISMAYED TO ONLY RECEIVE DME INFO. IT WAS THEN WE REALIZED THAT WE HAD MISREAD THE RADIO AND IT WAS NOT A VOR BUT A TACAN THAT DEFINED THE AIRWAY IN ADDITION TO THE NDB. WITHOUT ANY AZIMUTH; I HAD TO ASK ATC FOR VECTORS TO FLY THE AIRWAY TO FINAL DEST. I CALLED MY COMPANY DISPATCHER THE NEXT DAY TO INFORM HIM THAT HE SHOULD NOT FILE AN OVERWATER FLT PLAN IF THE ACFT DOES NOT HAVE THE PROPER OPERATING EQUIP TO DEFINE THE AIRWAYS. DISPATCH; THE MEL AND MISREAD NAV CHARTS CONTRIBUTED TO THIS ERROR. HOWEVER; AS PIC I SHOULD HAVE MORE THOROUGHLY EXAMINED ALL ASPECTS OF THIS NEW RTE TO ME. I'LL BE MORE CAREFUL NEXT TIME!