A common-mode current choke can be made by winding coaxial cable on a ferrite toroid. Why is cable with solid dielectric preferred over foam dielectric?

When creating a common-mode choke by winding coaxial cable tightly around a ferrite toroid, strong mechanical stress is applied to the cable. This tight bending causes the inner center conductor to pull tightly against the insulating dielectric layer that separates it from the outer shield. Foam dielectric is relatively soft and compressible. Over time, the constant tension from a tight bend can cause the center conductor to migrate slowly through the foam until it eventually touches the shield, creating a dead short. Solid polyethylene dielectric is much denser and more rigid. It effectively resists the pressure of the bent center conductor, ensuring the spacing between the inner and outer conductors remains intact without risking a short circuit.