When operating at UHF frequencies reflections cause multi-path signals. One signal can go directly to the tag, whereas a second signal may be reflected and arrive at a slightly different time. When these signals arrive in-phase, they combine to increase the power and tags thought to be out of range respond. This process is called ghost readings. When the signals are out-of-phase, they cancel and create reading holes. These holes can be prevented by using multiple antennas or moving the tagged objects while reading.