Definition: An antenna is a conductor consistining commonly of a wire or set of wires by which electromagnetic waves are sent out or received. Anything that needs to recieve or send the types of waves use an antenna, a few examples including cell phones, laptops, radios, and satellites. Antennas come in all shapes and sizes and function in different ways, depending on the frequency the antenna is trying to recieve.



Details: The size of an antenna is related to the frequency of the signal that the antenna is trying to transmit or receive. The larger the frequency, the smaller the antenna. The reason for this relationship has to do with the speed of light, and the distance electrons can travel as a result. This explains why car radio antennas are longer compared to cell phone antennas; their wave frequencies are less than that of waves being intercepted by cell phone antennas. Other types of antennas include WiFi and TV. Some antennas transmit waves while others recieve them (ex- radio station and radios). Others can do both (cell phones). All electromagnetic waves that correspond to antennas have a different frequency based on what they are trying to do (ex- make a cell phone call or broadcast a radio signal). Also, it is important to know that as current enters the antenna, it creates a magnetic field around the antenna.

Sources:
http://www.howstuffworks.com
http://www.dictionary.reference.com