thumb|A ferrite loopstick antenna, a small loop used for [[amplitude modulation|AM reception in a portable radio, consisting of a wire wound around a ferrite core; the most common type of loop antenna today.]]
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Loop antennas can be divided into three categories:
Large loop antennas: Also called self-resonant loop antennas or full-wave loops; they have a perimeter close to one or more whole wavelengths at the operating frequency, which makes them self-resonant at that frequency. Large loop antennas have a two-lobe dipole like radiation pattern at their first, full-wave resonance, peaking in both directions perpendicular to the plane of the loop.
Halo antennas: Halos are often described as shortened dipoles that have been bent into a circular loop, with the ends not quite touching. Some writers prefer to exclude them from loop antennas, since they can be well-understood as bent dipoles, others make halos an intermediate category between large and small loops, or the extreme upper size limit for small transmitting loops: In shape and performance halo antennas are very similar to small loops, only distinguished by being self resonant and having much higher radiation resistance. (See discussion below)
Small loop antennas: Also called magnetic loops or tuned loops; they have a perimeter smaller than half the operating wavelength (typically no more than to wave). They are used mainly as receiving antennas because of low efficiency, but are sometimes used for transmission; loops with a circumference smaller than about become so inefficient they are rarely used for transmission. A common example of small loop is the ferrite (loopstick) antenna used in most AM broadcast radios. The radiation pattern of small loop antennas is maximum at directions within the plane of the loop, so perpendicular to the maxima of large loops.
Large, self-resonant loop antennas<span id="large_loop_anchor" class="anchor"></span>
For the description of large loops in this section, the radio's operating frequency is assumed to be tuned to the loop antenna's first resonance. At that frequency, one whole free-space wavelength is slightly smaller than the perimeter of the loop, which is the smallest that a "large" loop can be.
At the lower shortwave frequencies, a full loop is physically quite large, and its only practical installation is "lying flat", with the plane of the loop horizontal to the ground and the antenna wire supported at the same relatively low height by masts along its perimeter. This results in horizontally polarized radiation, which peaks toward the vertical near the lowest harmonic; that pattern is good for regional NVIS communication, but unfortunately is not generally useful for making continental-scale contacts.
Above about 10 MHz, the loop is approximately 10 meters in diameter, and it becomes more practical for the loop to be mounted "standing up" – that is, with the plane of the loop vertical – in order to direct its main beam towards the horizon. If the frequency is high enough, then the loop might be small enough to attach to an antenna rotator, in order to rotate that direction as desired. Compared to a dipole or folded dipole, a vertical large loop wastes less power radiating toward the sky or ground, resulting in about 1.5 dB higher gain in the two favored horizontal directions.
Additional gain (and a uni-directional radiation pattern) is usually obtained with an array of such elements either as a driven endfire array or in a Yagi configuration – with only one of the loops being driven by the feedline and all the remaining loops being "parasitic" reflectors and directors. The latter is widely used in amateur radio in the "quad" configuration (see photo).
Low-frequency one-wavelength loops "lying down" are sometimes used for local NVIS communication. This is sometimes called a lazy quad. Its radiation pattern consists of a single lobe straight up (radiation toward the ground which is not absorbed is reflected back upward). The radiation pattern and especially the input impedance is affected by its proximity to the ground.
If fed with higher frequencies, then the antenna input impedance will generally include a reactive part and a different resistive component, requiring use of an antenna tuner. As the frequency increases above the first harmonic, the radiation pattern breaks up into multiple lobes which peak at lower angles relative to the horizon, which is an improvement for long-distance communication for frequencies well above the loop's second harmonic.
Halo antennas<span class="anchor" id="halo_anchor"></span>
A halo antenna is often described as a half-wave dipole antenna that has been bent into a circle. Although it could be categorized as a bent dipole, it has the omnidirectional radiation pattern very nearly the same as a small loop. The halo is more efficient than a small loop, since it is a larger antenna at in circumference with its disproportionately larger radiation resistance. Because of its much greater radiation resistance, a halo presents a good impedance match to 50-Ohm coaxial cable, and its construction is less demanding than a small loop, since the maker is not compelled to take such extreme care to avoid losses from mediocre conductors and contact resistance.
At wave, the halo antenna is near or on the extreme high limit of the size range for "small" loops, but unlike most oversized small loops, it can be analyzed with simple techniques by treating it as a bent dipole.
Practical use
thumb|Car-roof-mounted 6-meter halo antenna for mobile amateur radio [[Harry Garland|(). Notice the triple-loop.]]
On the VHF bands and above, the physical diameter of a halo is small enough to be effectively used as a mobile antenna.
The horizontal radiation pattern of a horizontal halo is nearly omnidirectional – to within 3 dB or less – and that can be evened out by making the loop slightly smaller and adding more capacitance between the element tips. Not only will that even out the gain, it will reduce upward radiation, which for VHF is typically wasted by radiating into space.
Halos pick up less nearby electrical spark interference than monopoles and dipoles, such as ignition noise from vehicles.
Electrical analysis
Although it has a superficially different appearance, the halo antenna can conveniently be analyzed as a dipole (which also has a half-wave radiating part with a high voltage and zero current at its ends) that has been bent into a circle. Simply using dipole results greatly simplifies the calculations and for most properties are the same as a halo. Halo performance can also be modeled with techniques used for similar, moderate-sized "small" transmitting loops, but for brevity, that complicated analysis is often skipped in introductory articles on loop antennas (unfortunately, this typical omission leaves otherwise well-read persons unaware of the properties of "large" small loops).
The halo's gap
Some writers mistakenly consider the gap in the halo antenna's loop to distinguish it from a small loop antenna, since there is no DC connection between the two ends. But that distinction is lost at RF; the close-bent high-voltage ends are capacitively coupled, and the RF current crosses the gap as displacement current. The gap in the halo is electrically equivalent to the tuning capacitor on a small loop, although the incidental capacitance involved is not nearly as large.
Small loops<span class="anchor" id="small_loop_anchor"></span><span class="anchor" id="small_loop_ants"></span>
thumb|right|200px| Although a full in diameter, this receiving antenna is a "small" loop compared to the [[Low frequency|LF and MF wavelengths it is used with.]]
Small loops are "small" in comparison to their operating wavelength. Contrary to the pattern of large loop antennas, the reception and radiation strength of small loops peaks inside the plane of the loop, rather than broadside (perpendicular) to it. The ability to increase the radiation resistance by using multiple turns is analogous to making a dipole out of two or more parallel lines for each dipole arm ("folded dipole").
Small loops have advantages as receiving antennas at frequencies below 10 MHz. Although a small loop's losses can be high, the same loss applies to both the signal and the noise, so the receiving signal-to-noise ratio of a small loop may not suffer at these lower frequencies, where received noise is dominated by atmospheric noise and static rather than receiver-internal noise. The ability to more manageably rotate a smaller antenna may help to maximize the signal and reject interference. Several construction techniques are used to ensure that small receiving loops' null directions are "sharp", including adding broken shielding of the loop arms and keeping the perimeter around wavelength (or wave at most). Small transmitting loops' perimeters are instead made as large as feasibly possible, up to wave (or even if possible), in order to make the best of their generally poor efficiency, although doing so sacrifices sharp nulls.
The small loop antenna is also known as a magnetic loop since the response of an electrically small receiving loop is proportional to the rate of change of magnetic flux through the loop. At higher frequencies (or shorter wavelengths), when the antenna is no longer electrically small, the current distribution through the loop may no longer be uniform and the relationship between its response and the incident fields becomes more complicated.
thumb|right|400px|Amount of atmospheric noise for [[Low frequency|LF, MF, and HF spectrum according CCIR 322.
For example, at 1 MHz, the man-made noise might be 55 dB above the thermal noise floor. If a small loop antenna's loss is 50 dB (as if the antenna included a 50 dB attenuator), then the electrical inefficiency of that antenna will have little influence on the receiving system's signal-to-noise ratio. In contrast, at quieter frequencies at about 20 MHz and above, an antenna with a 50 dB loss could degrade the received signal-to-noise ratio by up to 50 dB, resulting in terrible performance.
However, as frequency rises, there is no need to suffer bad performance: At the higher, quieter frequencies, the wavelengths become short enough that a halo antenna is small enough to be feasible – at 20 MHz it is a little less than in diameter, and proportionally shrinks as the frequency increases. So the quieter the rising frequency gets, the more convenient it is to replace a small receiving loop with a larger, but still relatively compact, halos. It is mostly a direct substitute for a small receiving loop, but with superior signal reception.
Radiation pattern and polarization
thumb|right|200px|[[Radiation patterns of loop antennas. Distance from the origin is proportional to the power density in that direction. The full wave loop (left) emits maximum power broadside to the wires with nulls off the sides, the small loop (right) emits maximum power in the plane of its wires with nulls broadside to the wires.]]
Surprisingly, the radiation and receiving pattern of a small loop is perpendicular to that of a large self resonant loop (whose perimeter is close to one wavelength). Since the loop is much smaller than a wavelength, the current at any one moment is nearly constant round the circumference. By symmetry it can be seen that the voltages induced in the loop windings on opposite sides of the loop will cancel each other when a perpendicular signal arrives on the loop axis. Therefore, there is a null in that direction. Instead, the radiation pattern peaks in directions lying in the plane of the loop, because signals received from sources in that plane do not quite cancel owing to the phase difference between the arrival of the wave at the near and far sides of the loop. Increasing that phase difference by increasing the size of the loop causes a disproportionately large increase in the radiation resistance and the resulting antenna efficiency.
Another way of looking at a small loop as an antenna is to consider it simply as an inductive coil coupling to the magnetic field in the direction perpendicular to plane of the coil, according to Ampère's law. Then consider a propagating radio wave also perpendicular to that plane. Since the magnetic (and electric) fields of an electromagnetic wave in free space are transverse (no component in the direction of propagation), it can be seen that this magnetic field and that of a small loop antenna will be at right angles, and thus not coupled. For the same reason, an electromagnetic wave propagating within the plane of the loop, with its magnetic field perpendicular to that plane, is coupled to the magnetic field of the coil. Since the transverse magnetic and electric fields of a propagating electromagnetic wave are at right angles, the electric field of such a wave is also in the plane of the loop, and thus the antenna's polarization (which is always specified as being the orientation of the electric, not the magnetic field) is said to be in that plane.
Thus, mounting the loop in a horizontal plane will produce an omnidirectional antenna which is horizontally polarized; mounting the loop vertically yields a vertically polarized, weakly directional antenna, but with exceptionally sharp nulls along the axis of the loop. Size criteria that favor loops with a perimeter of or smaller ensure the sharpness of the loop's receiving null. Small loops intended for transmitting (see below) are designed as large as feasible to improve the marginal radiation resistance, sacrificing the sharp null by using perimeters as large as to
Receiver input tuning
Since a small-loop antenna is essentially a coil, its electrical impedance is inductive, with an inductive reactance much greater than its radiation resistance. In order to couple to a transmitter or receiver, the inductive reactance is normally canceled with a parallel capacitance. Since a good loop antenna will have a high factor (narrow bandwidth), the capacitor must be variable and is adjusted to match the receiver's tuning.
Small-loop receiving antennas are also almost always resonated using a parallel-plate capacitor, which makes their reception narrow-band, sensitive only to a very specific frequency. This allows the antenna, in conjunction with a (variable) tuning capacitor, to act as a tuned input stage to the receiver's front-end, in lieu of a preselector.
Direction finding with small loops
thumb|right|300px|Loop antenna, receiver, and accessories used in [[amateur radio direction finding at wavelength (3.5 MHz).]]
As long as the loop perimeter is kept below about wave, the directional response of small loop antennas includes a sharp null in the direction normal to the plane of the loop, so small loops are favored as compact radio direction finding antennas for long wavelengths.
The procedure is to rotate the loop antenna to find the direction where the signal vanishes – the "null" direction. Since the null occurs at two opposite directions along the axis of the loop, other means must be employed to determine which side of the antenna the nulled signal is on. One method is to rely on a second loop antenna located at a second location, or to move the receiver to that other location, thus relying on triangulation.
Instead of triangulation, a second dipole or vertical antenna can be electrically combined with a loop or a loopstick antenna. Called a sense antenna, connecting and matching the second antenna changes the combined radiation pattern to a cardioid, with a null in only one (less precise) direction. The general direction of the transmitter can be determined using the sense antenna, and then disconnecting the sense antenna returns the sharp nulls in the loop antenna pattern, allowing a precise bearing to be determined.
AM broadcast receiving antennas<span class="anchor" id="tuned_RX_loops"></span>
Small-loop antennas are lossy and inefficient for transmitting, but they can be practical receiving antennas in the mediumwave (520–1710 kHz) broadcast band and below, where wavelength-sized antennas are infeasibly large, and the antenna inefficiency is irrelevant, due to large amounts of atmospheric noise.
AM broadcast receivers (and other low frequency radios for the consumer market) typically use small-loop antennas, even when a telescoping antenna may be attached for FM reception. A variable capacitor connected across the loop forms a resonant circuit that also tunes the receiver's input stage as that capacitor tracks the main tuning. A multiband receiver may contain tap points along the loop winding in order to tune the loop antenna at widely different frequencies.
In AM radios built prior to the invention of ferrite in the mid-20th century, the antenna might consist of dozens of turns of wire mounted on the back wall of the radio – a planar helical antenna – or a separate, rotatable, furniture-sized rack looped with wire – a frame antenna.
Ferrite loop antenna<span class="anchor" id="ferriteloop"></span>
thumb|right|300px|Ferrite loopstick antenna from an AM radio having two windings, one for [[long wave and one for medium wave (AM broadcast) reception. About long. Ferrite antennas are usually enclosed inside the radio receiver.]]
Ferrite loop antennas are made by winding fine wire around a ferrite rod. They are almost universally used in AM broadcast receivers.
<span class="anchor" id="small_TX_loop_anchor">Small transmitting loops</span>
Small transmitting loops are "small" in comparison to a full wavelength, but considerably larger than a "small" receive-only loop. They are typically used on frequencies between 14 and 30 MHz. Unlike receiving loops, small transmitting loops' sizes must be scaled-up for longer wavelengths, in order to keep radiation resistance from falling to unusably low levels; their larger sizes blur or erase the otherwise especially sharp nulls that small receiving loops provide.
Size, shape, efficiency, and pattern
thumb|right|200px|A loop antenna for [[amateur radio under construction]]
Transmitting loops usually consist of a single turn of large-diameter conductor; they are typically round or octagonal to maximize the enclosed area for a given perimeter, hence maximizing radiation resistance. The smaller of these loops are much less efficient than the extraordinary performance of full-sized, self-resonant loops, or the moderate efficiency of monopoles, dipoles, and halos, but where space for a full wave loop or a half-wave dipole is not available, small loops can provide adequate communications with low-but-tolerable efficiency.
A small transmitting loop antenna with a perimeter of 10% or less of the wavelength will have a relatively constant current distribution along the conductor,