A B737-500 FLC HAS TO ASK FOR A SECOND APCH WHEN THE ILS LOC AND GS FOR RWY 31L IS SCALLOPING AT MDW; IL.

Narrative

NAV INTERFERENCE. OVER CHT; CLRED '10 DEGS R INTERCEPT LOC RWY 31L PLAN CIRCLE RWY 22L.' UPON TUNING LOC FREQ AND SETTING COURSE; IT APPEARED WE WERE ON THE LOC; ALTHOUGH VISUALLY WE APPEARED S OF COURSE. ATC ASKED IF WE HAD INTERCEPTED AND SAID WE WERE S OF COURSE. THE CDI THEN SWUNG FULL SCALE TO THE OTHER SIDE INDICATING WE WERE N OF COURSE. I TURNED TO CTR THE CDI AND WE SWITCHED TO TWR. MY CDI SWUNG R INDICATING WE WERE S OF COURSE. I NOTICED THE FO'S CDI WAS SWINGING THE SAME DIRECTION AS MINE; BUT MOVING ABOUT HALF AS FAR. WHEN WE SAW THE RWY; WE WERE N OF COURSE WITH CDI'S INDICATING WE WERE S OF COURSE. WE WERE HIGH AND WELL N OF COURSE WHEN TWR ASKED IF WE COULD GET DOWN FROM THERE. WE ASKED TO BE TURNED OUT TO RE-ENTER THE PATTERN. UPON TURNING OUTBOUND WE MADE A PA ASKING PEOPLE TO PLEASE MAKE SURE THEIR CELL PHONES AND OTHER EQUIP WERE TURNED OFF. THE CDI'S IMMEDIATELY BECAME STEADY AND WE COMPLETED A NORMAL ILS RWY 31C CIRCLE RWY 22L WITH NORMAL INDICATIONS AND THE FLT ATTENDANTS RPTED THAT A WOMAN IN THE FORWARD LOUNGE WAS TALKING ON HER CELL PHONE. AS SOON AS SHE TURNED HER PHONE OFF; OUR CDI INDICATED NORMALLY.