An aircraft is provided including an airframe, an extending tail, and a counter rotating, coaxial main rotor assembly including an upper rotor assembly with an upper blade and a lower rotor assembly with a lower blade. A first antenna in one of upper blade and the lower blade, and a second antenna in the other of the upper blade and the lower blade. An oscillator to apply an excitation signal to the first antenna. A blade proximity monitor to monitor a magnitude of the excitation signal and an output signal from the second antenna to determine a distance between the upper blade and the lower blade.