A Sub-Harmonic Arc Detection (SHAD) system has been installed on the transmission lines feeding the JET ICRF ITER-Like-Antenna (ILA). Along with the commissioning of SHAD, extensive measurements of the RF field in the transmission lines were carried-out using fast sampling (125Mb/s) oscilloscopes. The system is described, and the SHAD ability to detect arcs during ILA operation (in particular on ELMy H modes) is discussed. Overall, SHAD proved to be efficient, and in some conditions it can offer extra protection in complement to other arc detection systems.