Loop antenna

A loop antenna is a radio antenna consisting of a loop (or loops) of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line (or a balun). Within this physical description there are two very distinct antenna designs: the small loop (or magnetic loop) with a size much smaller than a wavelength, and the much larger resonant loop antenna with a circumference close to the intended wavelength of operation.

Small loops have a poor efficiency and are mainly used as receiving antennas at low frequencies. Except for car radios, almost every AM broadcast receiver sold has such an antenna built inside it or directly attached to it. These antennas are also used for radio direction finding. In amateur radio, loop antennas are often used for low profile operating where larger antennas would be inconvenient, unsightly, or banned. Loop antennas are relatively easy to build.

A small loop antenna, also known as a magnetic loop, generally has a circumference of less than one tenth of a wavelength, in which case there will be a relatively constant current distribution along the conductor. As the frequency or the size is increased, a standing wave starts to develop in the current, and the antenna starts to acquire some of the characteristics of a resonant loop (but isn't resonant); these intermediate cases thus cannot be analyzed using the concepts developed for the small and resonant loop antennas described below.