AN ACR MD80 FLC RPTS THAT THEY HAD DEFLECTIONS IN THE RWY 4R LOC AT ABOUT 20 NM FROM THE ARPT DURING AN APCH. THE ACFT'S AUTOFLT SYS ATTEMPTED TO FOLLOW THE L AND R DEFLECTIONS.

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EXPERIENCED RWY 4R LOC DEFLECTION AT 20 MI FROM ORD VOR/DME. AUTOPLT RESPONDED TO LOC DEFLECTION AND WAS DISCONNECTED. APPROX DEFLECTION WAS LESS THAN 1/3 DOT DEFLECTION. LOC STEADIED AS APCH CONTINUED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT SAID THAT ALL OF HIS COMPANY'S FLT PLANS HAVE INFO REGARDING THE ARPT AND THE RTE INCLUDED ON THE PLAN. THIS ARPT ALWAYS HAS A NOTE ABOUT THIS APCH ADVISING THE CREW THAT THESE DEFLECTIONS HAVE BEEN OCCURRING. UNLIKE SOME OF THE LATER ACFT THAT HAVE A DAMPING LOGIC IN THE AUTOFLT SYS; THE MD80 SERIES WILL ATTEMPT TO FOLLOW THE RAPIDLY MOVING LOC ON THE APCH AS IT MOVES L TO R ACCORDING TO THE RPTR. HE SAID THAT THE ONLY SOLUTION IS TO DISCONNECT THE AUTOPLT AND HAND FLY IT UNTIL THE LOC SETTLES DOWN. AS THE CREW GOT CLOSER TO THE ARPT THEY SAW A COMPANY MD80 THAT HAD LANDED BEFORE THEM TAXIING IN. THEY FELT THAT THE MD80 HAD CAUSED THE INTERFERENCE WITH THE LOC SIGNAL ALSO AS THE ACFT GOT CLOSER TO THE ARPT THE LOC COURSE DISPLAY SHOWED LITTLE OR NOT RAPID DEFLECTION. THE RPTR SAID THAT HE SUSPECTS THAT THE DEFLECTIONS ARE CAUSED BY ACFT AS THEY ROLLOUT ON THE RWY. HE FEELS THAT THESE ACFT DEFLECT OR OTHERWISE INTERFERE WITH THE SIGNAL AND HE THINKS THAT RELOCATING THE ANTENNA MAY HELP SOLVE THE PROB.