The term fill factor is associated with which type of coil?

The term "fill factor" refers to the proportion of the area of the coil that is filled with the conductor material of the coil, relative to the total coil area. This concept is particularly relevant in the context of encircling coils, where the coil design is intended to surround the object being tested to maximize the sensitivity and efficiency of the electromagnetic coupling.

With encircling coils, the fill factor is crucial because a higher fill factor generally leads to improved signal strength and a more uniform magnetic field. This results in better data quality during testing of materials, as the coil interacts more effectively with the entire volume of the object being inspected.

While surface coils and coaxial cables can also have fill factors, the term is most commonly and critically associated with encircling coils, particularly in nondestructive testing applications. Therefore, the association of the fill factor with encircling coils makes it the most accurate choice in this context.