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A loop antenna is an antenna that creates a continuous conducting path between one conductor of a two-wire transmission line to the other. There are three categories of loop antenna: small, medium, and large. A small loop is considered so if its circumference is less than one forth of a wavelength. Most directional receiving loops are approximately one tenth of a wavelength.
A small loop is sometimes referred to as a magnetic loop, due to its increased sensitivity to the magnetic components of an electromagnetic wave. It is also less sensitive to nearby electric noise than other antennas, when properly shielded. If a small loop is brought into resonance with a tuning capacitor, its received voltage can be greatly increased. When a signal arrives along the loop's axis, an equal amount of voltage will be induced in each limb to account for symmetry. Since the loop's output is the difference in voltage between the two limbs, it should always be zero. If a signal arrives in the plane of the loop, it creates a phase difference between the limb voltages. These signals produce the maximum output for a small loop.
There are two specific cases when a loop antenna is classified as a medium loop. As such, these cases have specific characteristics. The first case occurs when a half-wavelength loop is mounted in a horizontal plane. This creates a horizontally polarized omnidirectional antenna, an antenna which radiates power uniformly in the horizontal plane. The second case is that of a full-wavelength loop that radiates on its axis. These loops are often found as elements of the quad antennas used by amateur radio operators. They are polarized according to the position of their feed point.
A large loop antenna is essentially a dipole antenna that has been connected at the ends to form either a circle, a triangle, or a square. Those formed into triangles are called delta loop antennas. The circular loops gets about ten percent higher gain than any of the other forms, but they can be difficult to support because of their shape. It is for this reason that triangle and square large loops are so much more common. Large loops usually have much stronger signal in the plane of the loop, rather than along the axis. This remains true as long as the loop is not too large.
Some loop antennas can also be tuned to the AM broadcasting band. An AM loop antenna is usually tuned with a capacitor. They may also be wound around a ferrite rod to increase aperture. Ferrite is a ceramic material used in applications ranging from magnetic components to microelectronics. Antennas wound around these rods are commonly referred to as loopstick antennas, due to the shape of the wire. These antennas work well for receiving but are inefficient and impractical for transmitting.
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