A circuit is designed with a measurement circuit (432). The measurement circuit is coupled to receive a first input signal (903) from a first antenna (128) of a transmitter and coupled to receive a second input signal (913) from a second antenna (130) of the transmitter. Each of the first and second signals is transmitted at a first time. The measurement circuit produces an output signal corresponding to a magnitude of the first and second signals. A control circuit (430) is coupled to receive the output signal and a reference signal. The control circuit is arranged to produce a control signal at a second time in response to a comparison of the output signal and the reference signal.