An RFID reader is a radio frequency device that emits a signal through an antenna. This signal is received by RFID tags that respond to interrogation by the reader. Responses are read by the reader, and through a variety of protocols the reader can communicate with all the RFID tags in its field. Readers generally take three forms. Portal readers are fixed in one location; RFID tags pass through them and communicate with the reader. Handheld readers are portable devices that can communicate with RFID tags. Mounted readers are a special type, placed on mobile equipment such as forklifts and cranes. They often communicate with tags in fixed locations.