B727 FO FINDS DIFFICULTY IN COMMUNICATING THE ACFT'S HDG AND INTENTIONS TO ZME CTLR WHEN CLRED DIRECT TO SYI VOR OFF OF AN ASSIGNED HDG.

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ATC WAS GIVING US RADAR VECTORS WHILE WE WERE ON A FLT DEPARTING MEM TOWARD THE E. DEP CTL ISSUED US AN ASSIGNED HDG OF 090 DEGS AND TOLD US TO CONTACT CTR. CTR THEN GAVE US A CLRNC TO CLB TO AND MAINTAIN 16000 FT. WHILE WE WERE CLBING UP TOWARDS 16000 FT; ATC CLRED US DIRECT TO SYI VOR (OUR FIRST NAV FIX ON OUR RTE). AFTER CTRING OUR HSI NAV NEEDLES; IT APPEARED THAT THIS HDG OF 090 DEGS WOULD TAKE US ON COURSE TO THE VOR. AFTER APPROX 1 MIN; ATC ASKED US WHAT OUR HDG WAS. I IMMEDIATELY RESPONDED 'ASSIGNED 090 DEGS.' AFTER I HAD STATED THIS; I REALIZED I SHOULD HAVE JUST SAID '090' AND LEFT THE WORD 'ASSIGNED' OUT. IN RESPONSE; THE CTLR SAID THAT HE HAD ALREADY GIVEN US A CLRNC DIRECT SYI VOR AND TO TURN TO A HDG OF 075 DEGS. AFTER TURNING TO THIS NEW HDG IT APPEARED THIS WOULD TAKE US A LITTLE MORE DIRECT TO OUR FIRST FIX. THE REMAINDER OF THE FLT WAS NORMAL AND UNEVENTFUL. WHAT I BELIEVE CAUSED THIS PROB WAS THE FACT THAT THIS NAV FIX WAS A LOW ALT VOR AND ITS SIGNAL (STRENGTH) WAS PROBABLY SOMEWHAT WEAK. THIS WAS DUE TO THE FACT THAT WE WERE A FAR DISTANCE AWAY FROM IT (APPROX 150 MI). EVEN THOUGH IT APPEARED WE WERE RECEIVING IT NORMALLY ON OUR NAV INSTS; ITS COURSE DISPLAY WASN'T AS ACCURATE AS IT WOULD HAVE BEEN IF WE WERE CLOSER TO IT. ALSO I USED THE WRONG PHRASEOLOGY IN ANSWERING THE CTLR'S QUESTION. THIS PROBABLY CONFUSED HIM. WHAT I THINK CAN BE DONE TO PREVENT A RECURRENCE OF THIS PROB IS TO USE STANDARD PHRASEOLOGY; NOT BE IN A HURRY AND THINK BEFORE YOU RESPOND TO A CTLR'S QUESTION. ALSO LOOK ON THE NAV CHART TO DETERMINE AN APPROX DIRECTION TO THE NAV FIX TO BACK UP THE COURSE GUIDANCE BEING DISPLAYED ON THE INSTS. AND ALWAYS CHK THE MORSE CODE TO PROPERLY IDENT A FIX AND CHK ITS SIGNAL STRENGTH. IF ITS SIGNAL APPEARS WEAK ASK THE CTLR FOR A RADAR VECTOR HDG TOWARD IT UNTIL YOU CAN RECEIVE IT BETTER.