As the fill factor decreases, how does the impedance variation of a pickup coil change with a given conductivity change?

The fill factor in the context of a pickup coil refers to the ratio of the area of the coil wire that is occupied or filled to the total area available for the winding. As the fill factor decreases, there is less conductive material occupying the space in the coil, which influences the coil's overall performance.

When the fill factor decreases, the electric field generated by the coil may not couple as effectively with the conductive material being tested. This results in a decrease in the coil’s sensitivity to changes in conductivity. Since the impedance of a coil is dependent on its inductance and resistance, and with reduced sensitivity due to a lower fill factor, there is a decrease in the impedance variation of the pickup coil when faced with a conductivity change.

Thus, it can be understood that a decrease in fill factor leads to a situation where changes in conductivity result in lesser variations in impedance, hence supporting the conclusion that the impedance variation decreases with the given change in conductivity.