The tether is transferring the lifting force from the kite to the ground station. The diameter of the tether is of great importance for kite turbines. Tethers of braided polyethylene fibers provides the highest strength which in turn provides the lowest diameter.
Polyethylene wires also has very good qualities to cope with external influences such as moisture and UV light.
Tether - example
30kW kite turbine will be able to operate with a tether of 4 mm diameter, which draws nominal 7.5 kN (equivalent to a lifting force of about 750 kg).
2 MW kite turbine will be able to operate with a wire of 42 mm which draws nominal 500 kN (equivalent to a lifting force of about 50 tons).