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. 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 resonant loop antenna with a circumference approximately equal to the wavelength. Self-resonant loop antennas are larger. They are typically used at higher frequencies, especially VHF and UHF, where their size is manageable. They can be viewed as a form of folded dipole and have somewhat similar characteristics. The radiation efficiency is also high and similar to that of a dipole.
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 of it or directly attached to it. These antennas are also used for radio direction finding. As the frequency or the size are increased, a standing wave starts to develop in the current, and the antenna starts to have some of the characteristics of a folded dipole antenna or a self-resonant loop. A technically small loop, also known as a magnetic loop, should have a circumference of one tenth of a wavelength or less. This is necessary to ensure a constant current distribution round the loop.
The large or self-resonant loop antenna can be seen as a folded dipole which has been reformed into a circle (or square, etc.). This loop has a circumference approximately equal to one wavelength (however it will also be resonant at odd multiples of a wavelength). Compared to the dipole or folded dipole, it transmits less toward the sky or ground, giving it a somewhat higher gain (about 10% higher) in the horizontal direction.