The 3.5mm is rather like a precision ruggedized SMA that is rated to 34ghz. The 2.4mm adapter has a precision interface rated to 50ghz. 3.5mm to 2.4mm adapters are commonly used in test labs, the 3.5 adapter using english threads while the 2.4 adapter uses metric threads. These 2.4mm to 3.5mm adapters are made with stainless steel. The 3.5mm to 2.4mm adapters shown below are offered in different grades of vswr.