Underground danger zone protection system - zone bounded by set frequency field danger signal emitted at deviant frequency

摘要

Underground danger zone protection systems keeps men out by means of HF signal transmitters allocated to the men, a danger zone boundary transmitter and a danger signal receiver to operate switches which give the alarm and/or cut out machinery. The danger zone is closed off by means of an HF field produced by a boundary transmitter working at frequency fl, and the danger signal transmitters are reflection transmitters which respond to incident HF energy at frequency fl. They emit a signal whose frequency or frequency spectrum differs from frequency fl, the danger signal receiver being blocked against frequency fl for this purpose. Preferably the danger zone boundary transmitter should cover the zone with a fan shaped HF field, though it can employ a bundled field likewise.