Bradtke was born in the southern Adelaide suburb of Noarlunga and began playing basketball in 1984 while attending Redcliffe State High School north of Brisbane in Queensland (by that stage he was already 6'8" (203 cm)). Spotted by Adelaide 36ers coach Ken Cole who signed him to the 36ers as a 16-year-old, Bradtke was tipped to make his NBL debut in 1986, but he decided to attend the Australian Institute of Sport where he attended on a scholarship in 1986 and 1987.