Characteristics of Impedance Change of Circular Coil Induced by Rebar

The closed form solutions to the coil impedance change are used to study the characteristics of the circular cover meter probe coil in this paper. The solutions are confirmed by experiment, and the measured data at different frequencies can be used to correct the relative permeability of the rebar and the lift-off (the least distance from the rebar surface to the bottom of the coil). The resistance change and the magnitude of the impedance change are increasing with an increase in rebar diameter, decreasing with an increase in lift-off. But the angle of the impedance change is decreasing with an increase in lift-off slightly, decreasing with an increase in rebar diameter dramatically. These characteristics of the impedance change of circular coil can be used to design cover meter and improve its performance.