Not enough info.
However, if it is an integral unit (controller and transmitter together) which I suspect it is and it is only squaking mode A then there will only be a few wires:- power, earth, case ground, lighting and the antenna circuit. My advise is to get a VSWR check of the antenna cct c/o. Get your local friendly radio engineer to come and play with it with an ATC 600 and a VSWR test set and you should get a result.