The FuG 227 Flensburg was a German passive radar receiver developed by Siemens AG and introduced into service in Spring 1944. It used wing and tail-mounted dipole antennae and was sensitive to the mid-VHF band frequencies of 170-220 MHz, subharmonics of the Monica radar's 300 MHz transmissions. It allowed Luftwaffe nightfighters to home in on the Monica tail warning radar fitted to RAF bombers. Subsequently, further variants of Flensburg (Flensburg II to Flensburg VI) were developed for detecting Allied radar jammers. Only Flensburg II and III were used operationally.