A rotary machine includes a rotatable shaft and a retaining ring coupled to, and at least partially extending about, the rotatable shaft. The rotatable shaft defines a longitudinal axis. The rotatable shaft and the retaining ring define an annular cavity. The rotatable shaft and the retaining ring each include a radially outer surface. The rotary machine also includes a monitoring system including a stationary reader-antenna positioned proximate the retaining ring and a radio frequency (RF) coupler. The RF coupler includes at least one flexible antenna band coupled to, and extending over, the radially outer surface of the retaining ring. The flexible antenna band is configured to establish RF coupling with the stationary reader-antenna. A sensor die is coupled to the flexible antenna band. The sensor die extends into the substantially annular cavity and the sensor die is also coupled to the radially outer surface of the rotatable shaft.