A device includes a primary antenna configured to communicate a signal to an antenna of an implantable medical device (IMD). A circulator is coupled to the primary antenna. The circulator enables the signal to pass from a transmitter to the primary antenna. The circulator also enables a backscatter signal from the IMD to pass from the primary antenna to a receiver. A processor coupled to the receiver. The processor configured to determine, based on the backscatter signal, an improved impedance value for a component of the IMD and/or an improved frequency for the signal communicated to the IMD, to improve communication efficiency of the signal to the IMD.