What was found
CREW REPORTED THAT ON APPROACH, GLIDE SLOPE DEVIATED UP AND DOWN TO RUNWAY 18L. AUTOPILOT WAS ENGAGED AND COUPLED TO BOTH OF THE NAVIGATION RECEIVERS, WHICH CAUSED THE AIRPLANE TO PORPOISE UP AND DOWN. A/P DISCONNECTED BY CREW AND LANDING CONTINUED ON VISUALLY. MX CHECKS FOUND NO SIGN OF FAILURE AND SYS, OPS CHECKED OK ON GROUND. REPLACED GLIDE SLOPE ANTENNA AS PRECAUTION AND FLIGHT CREW FLEW SAME APPROACH A COUPLE DAYS LATER AND REPORTED THAT ALL SYS WORKED CORRECTLY.