 After experimenting with port-side islands on two previous carriers, and "Hiryū", the IJN opted to build both "Shōkaku" and her sister ship "Zuikaku" with starboard-side islands.In September 1942, a Type 21 air-warning radar was installed on "Shōkaku"s island atop the central fire control director, the first such device to be fitted on any Japanese carrier.The Type 21 had a "mattress" antenna and the initial prototypes were light enough that no major structural modifications were necessary.