A coupling adapter is insertable in at least one joint of a drill string as the drill string is extended from a drill rig. The coupling adapter includes an arrangement for receiving a data signal that is generated by an inground tool and for electromagnetically coupling the data signal onto at least a portion of the drill string that extends from the adapter to the drill rig such that at least some of the drill pipe sections cooperate as an electrical conductor for carrying the data signal to the drill rig. In another feature, a current transformer is resiliently supported to isolate the current transformer from mechanical shock and vibration that is produced by an inground operation that is performed using the drill string. In another feature, a drill string repeater is described.