Loop antenna
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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. Within this physical description there are two (possibly three) distinct types:
- Large loop antennas
- Large loops are also called self-resonant loop antennas or full-wave loops; they have a perimeter close to one or more whole efficient, by an order of magnitude, of all antenna designs of similar size.
- Halo antennas
- Halos are often explained 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
- Small loops are also called magnetic loops or tuned loops; they have a perimeter smaller than half the operating wavelength (typically no more than 1 /3~ 1 /4 efficiency; loops with a circumference smaller than about 1/ 10 wavelength become so inefficient they are rarely used for transmission.[c] A common example of small loop is the ferrite (loopstick) antenna used in most AM broadcast radios.[d]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.
- Small loops divide into two sub-types, depending on the purpose they are optimized for:
- Receiving
- Small direction-finding, better than most moderately large antennas, and as good as many huge antennas.
- Transmitting
- Small ) makes the small loop better for transmitting, although it sacrifices or outright loses the precise "null" direction of smaller small 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.[2]
Self-resonant loop antennas for so-called
Large loop antennas can be thought of as a
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
Unlike a
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.[3](p 235)
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.[2] 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, 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
Low frequency one wavelength loops "lying down" are sometimes used for local
If fed with higher frequencies 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
At 1/ 2 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
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
Halos pick up less nearby electrical spark interference than monopoles and dipoles – ignition noise from vehicles for example.[5]
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.[g]
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 (perpendicular) to it.[3]: 235
As with all antennas that are physically much smaller than the operating wavelength, small loop antennas have small
where A is the area enclosed by the loop, λ is the wavelength, and N is the number of turns of the conductor around the loop.
Because of the higher exponent than linear antennas (loop area squared ≈ perimeter to the 4th power, vs. dipole & monopole length squared = 2nd power) the fall in Rrad with reduced size is more extreme.[6]: 5‑11 The ability to increase the radiation resistance Rrad 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.[7] 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 1/ 10 wavelength (or 1 /4 wave at most). Small transmitting loops' perimeters are instead made as large as feasibly possible, up to 1 /3 wave (or even 1 /2 if possible), in order to make the best their generally poor efficiency, although doing so sacrifices sharp nulls.
The small loop antenna is also known as a magnetic 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.
Small receiving loops
If the perimeter of a loop antenna is much smaller than the intended operating wavelengths – say 1 /8 to 1/ 100 of a wavelength – then the antenna is called a small receiving loop, since loop antennas that small are only practical for receiving. Several performance factors, including received power, scale in proportion to the loop's area. For a given loop area, the length of the conductor (and thus its net loss resistance) is minimized if the perimeter is circular, making a circle the optimal shape for small loops. Small receiving loops are typically used below 3 MHz where human-made and natural atmospheric noise dominate. Thus the signal-to-noise ratio of the received signal will not be adversely affected by low efficiency as long as the loop is not excessively small.
A typical diameter of receiving loops with "air centers" is between 30 and 100 cm (1 and 3.5 feet). To increase the magnetic field in the loop and thus its efficiency, while greatly reducing size, the coil of wire is often wound around a
Small loop antennas are also popular for
The
Wasted power is undesirable for a transmitting antenna, however for a receiving antenna, the inefficiency is not important at frequencies below about 15 MHz. At these lower frequencies,
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) 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.
Radiation pattern and polarization
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
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
Thus mounting the loop in a horizontal plane will produce an omnidirectional antenna which is horizontally polarized; mounting the loop vertically yields a vertically polarizated, weakly directional antenna, but with an exceptionally sharp nulls along the axis of the loop.[i] Size criteria that favor loops with a perimeter of 1 /4 wave 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 1 /3~ 1 /2 wave.
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.[j] Since a good loop antenna will have a high Q 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
As long as the loop perimeter is kept below about 1 /4 wave, the directional response of small loop antennas includes a sharp
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
Small loop antennas are lossy and inefficient for transmitting, but they can be practical receiving antennas in the
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.
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
Inclusion of a magnetically permeable core increases the radiation resistance of a small loop,
Small transmitting loops
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 14–30 MHz. Unlike receiving loops, small transmitting loops’ sizes must be scaled-up for longer wavelengths, in order to maintain adequate radiation resistance, and their larger size blurs or erases the especially sharp null found in small receiving loops.
Size, shape, efficiency, and pattern
A transmitting loop usually consists of a single turn of large diameter conductor; they are typically round or octagonal to provide maximum enclosed area for a given perimeter, hence maximizing
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,[1] and the main lobe will be in the plane of the loop, so they will show the strong null familiar in the radiation pattern of small receiving loops. Loops of any size between 10% and 30% of a wavelength in perimeter, up to almost exactly 50% in circumference, can be built and tuned with series capacitor to resonance but their non-uniform current will reduce or eliminate the small loops' pattern null. A capacitor is required for a circumference less than a half wave, an inductor for loops more than a half wave and less than a full wave.
Loops in the small transmitting loops' size range may have neither the uniform current of very small loops, nor the sinusoidal current of large loops, and thus cannot be analyzed using the assumptions useful for the small receiving loops nor full-wave loop antennas. Performance is best determined with NEC analysis. Antennas within this size range include the halo (see above) and the G0CWT (Edginton) loop. For brevity, introductory articles on small loop antennas sometimes confine discussion to loops smaller in circumference than 1/ 10 wavelength, since for loops with circumferences larger than 1/ 10 wave the simplifying assumption of uniform current around the entire loop becomes untenably inaccurate. Since the larger halo also has a simple analysis, moderate-sized small loop antennas and their complicated analysis are often omitted, leaving many otherwise well informed antenna builders in the dark regarding the performance obtainable with moderately small loops.
Use for land-mobile radio
Vertically aligned small loops are used in military land-mobile radio, at frequencies between 3~7 MHz, because of their ability to direct energy upwards, unlike a conventional whip antenna. This enables near vertical incidence skywave (NVIS) communication up to 300 km (190 miles) in mountainous regions. For NVIS a typical radiation efficiency of around 1% is acceptable, because signal paths can be established with 1 W of radiated power or less – feasible when a 100 W transmitter is used.
In military use, the antenna may be built using a one or two conductors 2.5–5 cm (1–2 inches) in diameter. The loop itself is typically 1.8 m (6 feet) in diameter.
Power limits and RF safety
One practical issue with small loops as transmitting antennas is that a small transmitting loop will not only have a very large current going through it, but also has a very high voltage across the capacitor – typically thousands of
Making the loop larger in diameter will lower the gap voltage, as well as improving efficiency, however all other efficiency improvements will tend to increase the gap voltage: Efficiency may be increased by making the loop from a thicker conductor; other measures to lower the conductor's loss resistance include welding or brazing the connections, rather than soldering. But because reducing loss resistance increases the antenna's Q the consequence of better efficiency is even greater voltage across the capacitor at the loop's gap. For a given frequency, a smaller small loop is more dangerous than a larger small loop, and perversely, a comparatively efficient small transmitting loop is more dangerous than an inefficient one.
The
Feeder loops
In addition to other common
If both the main and the feeder loops are single-turn, the impedance transformation ratio of the nested loops is almost exactly the ratio of the areas of the two loops separately, or the square of the ratio of their diameters (assuming they have the same shape). Typical feed loops are 1 /8 to 1 /5 the size of the antenna's main loop, which gives transform ratios of 64:1 to 25:1, respectively. Adjusting the proximity of the feeder loop to the main loop and distorting its shape both make small to moderate changes the transform ratio, and allows for fine adjustment of the feedpoint impedance. For main loops with multiple turns, more often used for
Antenna-like non-antenna loops
Some so-called "antennas" look very much like genuine loop antennas, but are designed to couple with the inductive near-field, over distances of 1–2 meters (3–7 feet), rather than to transmit or receive long-distance electromagnetic waves in the
Likewise, coupling coils used for inductive charging systems, regardless of whether they are used at low or high radio frequencies, are excluded from this article, since they are not (or ideally, shouldn't be) radio antennas.
RFID coils and induction heating
Although they are not radio antennas, these systems do operate at radio frequencies, and they involve the use of small magnetic coils, which are called "antennas" in the trade. However, they are more usefully thought of as analogs to the windings in loosely coupled
Footnotes
- ^ The antenna can be described as "self-resonant" in the sense that if you short the antenna terminals, then a current in the loop will be created in response to an electromagnetic wave, and the relative magnitude of that current will be greatly increased around the resonant frequency. The antenna being "resonant" also implies that the input impedance of the antenna, which is reactive at most frequencies, becomes purely resistive (resonant) at this frequency.
- ^ For loops larger than 1 wavelength perimeter, the directive gain increases slightly up to a perimeter of 1.4 wavelengths,[1] but for larger circular loops the radiation pattern becomes multilobed and the perpendicular radiation vanishes or is greatly diminished.
- ^ Small loops with circumferences up to 1 /3~ 1 /4 wavelength are used for transmitting antennas, although their construction requires fastidious efforts to minimize loss resistance; the practical lower size-limit is somewhere around 1 /7~1/ 10 wave.
- ^ a b
An important exception is that radios built for installation inside metal car bodies cannot contain antennas, since their AMreception would be blocked by of the metal of the chassis and the dashboard. Car radios must use external antennas, which are essentially never ferrite loops.
- ^
An antenna's feedpoint is the place where its feedline (RF transmission line) attaches to the radiating part of the antenna.
- ^ A halo antenna has very roughly 10×~500× greater radiation resistance than 1/ 4 ~1/ 10 wave loops, respectively.
- ^
A halo antenna does not need capacitive end-loading, since the nearly 1/ 2 wave circumference halo antenna is already self-resonant. However, since end-capacitance is present even if not needed, to restore resonance the dipole-sized arms must each be trimmed back from the conventional 97% of a quarter-wave.
- ^ The loss resistance includes not only the DC resistance of the conductor but also its increase due to the skin effect and proximity effect. The loss resistance also includes losses in the ferrite rod, if one is used.
- ^ Since AM broadcast radio is conventionally vertically polarized, the internal antennas of AM radios are loops in the vertical plane (that is, with the loopstick core, around which the loop is wound, horizontally oriented). One can easily demonstrate the directivity of such an antenna by tuning to an AM station (preferably a weaker one) and rotating the radio in all horizontal directions. At a particular orientation (and at 180 degrees from it) the station will be in the direction of the ‘null’, that is, in the direction of the loopstick (normal to the loop). At that point reception of the station will fade out.
- ^
Although a series capacitor could also be used to cancel the reactive impedance, doing so results in the receiver (or transmitter) seeing a very small (resistive) impedance. A parallel capacitor creates a parallel-type resonance, on the other hand, leads to a very large impedance seen at the feedpoint when the capacitor's susceptance cancels the antenna's susceptance, and thus produces an increased voltage which is directly available for the receiver's input stage.
References
- ^ ISBN 0-471-66782-X.
- ^ ISBN 978-1-62595-044-4.
- ^ ISBN 978-1-118-64206-1.
- QST Magazine. pp. 37–39.
- QST Magazine. pp. 11–13.
- ISBN 978-0-87259-987-1.
- ^ Karlquist, Rick (17 Oct 2008). Low band receiving loops (PDF). PacifiCon 2008. Retrieved 2018-04-29 – via n6rk.com.
- ^ .
- ISBN 0-7506-5612-3.
- ^ a b CCIR 258; CCIR 322.[full citation needed]
- ISBN 0-86341-569-5.
- ^ a b Dean, Charles E. (1959). Henney, Keith (ed.). Radio Engineering Handbook. New York: McGraw-Hill. ch. 19 p. 21.
- ISBN 0-408-02760-6.
- ^ Siwiak, Kai, KE4PT; Findling, Amir, K9CHP (Summer 2012). "How efficient is your loop antenna?" (PDF). The QRP Quarterly – via qsl.net/k4fk.
{{cite magazine}}
: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link) - ISBN 978-0-87259-974-1.
- ^ a b Austin, B.A.; Boswell, A.; Perks, M.A. (1 August 2014). Loss mechanisms in the electrically small loop antenna (PDF). mpoweruk.com (Report).
External links
- Yates, Steve (AA5TB). "Small transmitting loop antennas". Magnetic loop antennas. AA5TB.com. Fort Worth, TX. Retrieved 2022-10-14.
{{cite web}}
: CS1 maint: numeric names: authors list (link)
- "Small transmitting loop". Antenna calculators. 66pacific.com. Retrieved 2022-10-14. — Online calculator that solves the "Basic equations for a small loop" using formulas from The ARRL Antenna Book, 15th ed.
- "An overview of the Underestimated Magnetic Loop HF Antenna" (PDF). Magnetic loop antennas. www.nonstopsystems.com. Retrieved 2024-01-01. — Extensive Paper by Leigh Turner VK5KLT (SK) on HF Magnetic Loop Antennas.
- "Interactive Magnetic Loop Calculator". Magnetic Loop Calculator. miguelvaca.github.io. Retrieved 2024-01-01. — Interactive Magnetic Loop Calculator by Jose Vaca VK3CPU.