Loop antenna
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.
Small loop antennas: Also called magnetic loops or tuned loops; they have a perimeter smaller than half the operating wavelength. 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 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
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.Self-resonant loop antennas for so-called "short" wave frequencies are relatively large, with a perimeter just greater than the intended wavelength of operation, hence for circular loops diameters between roughly at the largest, around 1.8 MHz. At higher frequencies their sizes become smaller, falling to a diameter of about at 30 MHz.
Large loop antennas can be thought of as folded dipoles whose parallel wires have been split apart and opened out into some oval or polygonal shape. The loop's shape can be a circle, triangle, square, rectangle, or in fact any closed polygon, but for resonance, the loop perimeter must be slightly larger than a wavelength.
Shape
Loop antennas may be in the shape of a circle, a square, or any other closed geometric shape that allows the total perimeter to be slightly more than one wavelength. The most popular shape in amateur radio is the quad antenna or "quad", a self-resonant loop in a square shape so that it can be constructed of wire strung across a supporting -shaped frame. There may be one or more additional loops stacked parallel to the first as "parasitic" director or reflector element, creating an antenna array which is unidirectional with gain that increases with each additional parasitic element. This design can also be turned 45 degrees to a diamond shape supported on a -shaped frame. Triangular loops have also been used for vertical loops, since they can be supported from a single mast. A rectangle twice as high as its width obtains slightly increased gain and also matches 50 Ω directly if used as a single element.Unlike a dipole antenna, the polarization of a resonant loop antenna is not obvious from the orientation of the loop itself, but depends on the placement of its feedpoint. If a vertically oriented loop is fed at the bottom, then its radiation will be horizontally polarized; feeding it from the side will make it vertically polarized.
Radiation pattern
The radiation pattern of a first-resonance loop antenna peaks at right angles to the plane of the loop. As the frequency progresses to the second and third resonances, the perpendicular radiation fades and strong lobes near the plane of the loop arise.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 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.
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. 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
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
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 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.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
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 to it.As with all antennas that are physically much smaller than the operating wavelength, small loop antennas have small radiation resistance which is dwarfed by ohmic losses, resulting in a poor antenna efficiency. They are thus mainly used as receiving antennas at lower frequencies. Like a short dipole antenna, the radiation resistance is small. The radiation resistance is proportional to the square of the area:
where is the area enclosed by the loop, is the wavelength, and is the number of turns of the conductor around the loop.
Because of the higher exponent than linear antennas, the fall in with reduced size is more extreme. 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.
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. Small transmitting loops' perimeters are instead made as large as feasibly possible, up to wave, 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, 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. In the case of transmission, the fields produced by an electrically small loop are the same as an "infinitesimal magnetic dipole" whose axis is perpendicular to the plane of the loop.
Because of their meager radiation resistance, the properties of small loops tend to more often be intensively optimized than are full-size antennas, and the properties optimized for transmitting are not quite the same as for receiving. With full-size antennas, the reciprocity between transmitting and receiving usually makes the distinctions unimportant, but since a few RF properties important for receiving differ from those for transmitting – particularly below about 10~20 MHz – small loops intended for receiving have slight differences from [|small transmitting loops]. They are discussed separately in following two subsections, although many of the comments apply to both.