The FuG 227 Flensburg was a German passive radar receiver developed by Siemens & Halske 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.