generally deal in the transmission and reception of radio waves and are a necessary part of all radio equipment. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, cell phones, radar and spacecraft communication. A common antenna is a vertical rod a quarter of a wavelength long. Such antennas are simple in construction, usually inexpensive and both radiate in and receive from all horizontal directions (Omni-Directional). In air, radio signals travel very quickly and with a very low transmission loss. The signals are absorbed when moving through more conductive materials, such as concrete walls or rock. When encountering a surface, the waves are partially reflected and partially transmitted through.