Antennas are electronic devices which are used to convert alternating electrical currents (AC) into radio frequency radiation (RF), or radio waves. An efficient antenna is one which converts most the electrical energy which is applied to it into radio frequency radiation. An inefficient antenna is one which "wastes" much of (or in some cases, all of) the electrical energy which is applied to it. Primarily this "wasted" energy is converted to heat. 

 

There are a wide variety of antenna designs which have been created, each is designed to adhere to certain environmental conditions or constraints, as well as the unique requirements of the end-user or operator. Some types of antennas favor the generation of radio waves in a particular direction, these are referred to as "directional antennas," or "beam antennas" because the radiated energy is concentrated in a beam. Other antennas radiate evenly in all directions, these antennas are referred to as "omnidirectional antennas."

In general, an antenna is only useful for a particular range of frequencies. This is because antennas can only effectively convert electrical energy into RF energy at frequncies which the antenna is resonant at.

 

Similar to a guitar-string, an antenna must be at or near resonance for a particular frequency in order to perform its function. The resonant frequency of an antenna is directly related to the physical properties of the antenna, primarily length. In this sense, an antenna can be aptly compared to a guitar string: when the string is struck, it vibrates causing a sound to be heard. The sound is produced because the string vibrates at a particular frequency. If the length of the string was reduced, a higher-pitched sound would be produced as a result of the string vibrating at a higher frequency. If the string was elongated, a lower-pitched sound would be produced as a result of the string vibrating at a lower frequency. 

 

Antennas likewise must be designed with the operating parameters in mind.