B737 CREW OVERSHOT THE LOC COURSE.

Narrative

330 DEG HDG TO INTERCEPT LOC RWY 1R. CAPT NAV RADIO TUNED TO LOC; FO TUNED TO AML FOR DME. ATTEMPTING TO OBTAIN VALID IDENT FOR IAD BUT SIGNAL WAS VERY WEAK. FINALLY BEGAN RECEIVING A VERY WEAK; BUT CORRECT; IDENT WITH WARNING FLAGS INTERMITTENTLY GOING OUT OF VIEW. TILLE NDB WAS NOT BEING RECEIVED AT THE TIME AND WAS NOT AVAILABLE AS A BACKUP. WE WERE JUST ABOUT TO QUERY APCH REGARDING STATUS OF RWY 1R LOC WHEN APCH CALLED US TO INFORM US THAT WE DID NOT INTERCEPT THE LOC AND WE WERE NOW XING THE RWY 1L FINAL. I IMMEDIATELY TURNED BACK TO THE R TO RE-ESTABLISH AN INTERCEPT HDG TO THE RWY 1R FINAL AND AT THAT TIME WAS ABLE TO VISUALLY ACQUIRE THE IAD ARPT. RWY 1L WAS BEING USED FOR DEPS SO THERE WAS NO CONFLICT. ONE ACFT FOLLOWING US WAS TOLD TO INTENTIONALLY OVERSHOOT THE RWY 1R LOC TO MAINTAIN SPACING BEHIND US WHILE WE CORRECTED OUR COURSE. DURING OUR COURSE CORRECTION TO RWY 1R; THE FO'S NAV RADIO WAS SELECTED TO THE LOC (VERSUS AML) AND WE STARTED RECEIVING COURSE GUIDANCE. MY LOC CAME ALIVE AT JUST ABOUT THE SAME TIME; AS DID THE TILLE NDB. APCH AND LNDG WAS UNEVENTFUL FROM THIS POINT. THE CAUSE OF THIS PROB; FROM OUR POINT OF VIEW; WAS A WEAK SIGNAL AT THE DISTANCE AND ALT WE WERE FLYING. I'M NOT SURE EXACTLY THE DISTANCE THE SIGNAL BECAME USABLE SINCE NO DME WAS AVAILABLE; BUT I'M GUESSING ABOUT 20 DME. OUR NEXT LEG WAS IAD-MCO; AND BOTH NAV RECEIVERS WORKED NORMALLY WITH STRAY LOC (RWY 17) SIGNALS RECEIVED AND IDENTED OUTSIDE 30 NM.