Loop antenna
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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 (possibly via a balun). 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 much larger resonant loop antenna with a circumference close to the intended wavelength of operation.
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 it or directly attached to it. These antennas are also used for radio direction finding. In amateur radio, loop antennas are often used for low profile operating where larger antennas would be inconvenient, unsightly, or banned.[1] Loop antennas are relatively easy to build.[2]
A small loop antenna, also known as a magnetic loop, generally has a circumference of less than one tenth of a wavelength, in which case there will be a relatively constant current distribution along the conductor. As the frequency or the size is increased, a standing wave starts to develop in the current, and the antenna starts to acquire some of the characteristics of a resonant loop (but isn't resonant); these intermediate cases thus cannot be analyzed using the concepts developed for the small and resonant loop antennas described below.
Resonant loop antennas are relatively large, governed by the intended wavelength of operation. Thus they are typically used at higher frequencies, especially VHF and UHF, where their size is manageable. They can be viewed as a folded dipole deformed into a different shape, and have rather similar characteristics such as a high radiation efficiency.
Similar and dissimilar devices
Although a resonant loop may be in the shape of a circle, distorting it into a somewhat different closed shape does not greatly alter its characteristics. For instance, the quad antenna (see illustration) popular in amateur radio consists of a resonant loop (and usually additional parasitic elements) in a rectangular shape, so that it can be constructed of wire strung across a supporting ‘X’ frame. Or a large loop can be completely collapsed into a line, in which case it is termed a folded dipole. In the case of large loops, such as the quad or a folded dipole, the antenna’s resonant frequency is determined by the circumference of the loop. On the other hand a small loop antenna is used for wavelengths much bigger than the loop itself; its radiation resistance and efficiency are instead dependent on the area enclosed by the loop (and number of turns). For a given loop area, the length of the conductor (and thus its net loss resistance) is minimized if the shape is a circle, making that case optimum for small loops.
Although it has a superficially similar appearance, the so-called halo antenna is not technically a loop since it possesses a break in the conductor opposite the feed point. Its characteristics are unlike that of either sort of loop antenna here described.
Also outside the scope of this article is the use of coupling coils for inductive (magnetic) transmission systems including LF and HF (rather than UHF) RFID tags and readers. Although these do use radio frequencies, and involve the use of small loops (loosely described as "antennas" in the trade) which may be physically indistinguishable from the small loop antennas discussed here, such systems are not designed to transmit radio waves (electromagnetic waves). They are near field systems involving alternating magnetic fields only, and may be analyzed as poorly coupled transformer windings; their performance criteria are dissimilar to radio antennas as discussed here.
Resonant loop antennas
The large or resonant loop antenna can be seen as a folded dipole which has been reformed into a circle (or square, etc.). In order to be resonant (having a purely resistive driving point impedance) the loop requires a circumference approximately equal to one wavelength (however it will also be resonant at odd multiples of a wavelength).
Contrary to the small loop antenna, this design radiates in the direction normal to the plane of the loop (thus in two opposite directions). Therefore these loops are normally installed with the plane of the loop in the vertical direction, and may be rotatable. Compared to a dipole or folded dipole, it then transmits less toward the sky or ground, giving it a slightly higher gain (about 10% higher) in the horizontal direction. Further directionality can be obtained by using a loop whose circumference is not one but 3 or 5 wavelengths. However it is more common to increase gain using an array of driven loops or a Yagi configuration including parasitic loop elements. The latter is widely used in amateur radio where it is referred to as a quad antenna (see earlier photo), with the loops being square as they are usually constructed with wires held taut in between the rigid "X" structures.
The polarization of such an antenna is not obvious by looking at the loop itself, but depends on the feed point (where the transmission line is connected). If a vertically oriented loop is fed at the bottom it will be horizontally polarized; feeding it from the side will make it vertically polarized.
Small loops
Small loop antennas are much less than a wavelength in size, and are mainly (but not always) used as receiving antennas at lower frequencies.[3]
Magnetic vs. electrical antennas
The small loop antenna is also known as a magnetic loop since it behaves electrically as a coil (inductor) with a limited but non-negligible radiation resistance due to its small size compared to one wavelength. It can be analyzed as coupling directly to the magnetic field (opposite to the principle of a Hertzian dipole which couples directly to the electric field) in the near field, which itself gives rise to an electric field through Faraday's law of induction and a full blown electromagnetic wave in the far field.
Radiation pattern and polarization
Surprisingly, the radiation and receiving pattern of a small loop is quite opposite that of a large loop (whose circumference 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 along the sides of the loop will cancel each other when a signal arrives along the loop axis. Therefore there is a null in that direction.[4] 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 side and far side of the loop. Increasing that phase difference by increasing the size of the loop has a large impact in increasing 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 normal to plane of the coil according to Ampère's law. Then consider a propagating radio wave normal to that plane. Since the magnetic (and electric) fields of an electromagnetic wave in free space are transverse (with no component in the direction of propagation), it can be seen that this magnetic field and that of a small loop antenna will be orthogonal, and thus uncoupled. For the same reason, an electromagnetic wave propagating in the plane of the loop, with its magnetic field normal 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 in the plane of the loop, and thus the antenna’s polarization (which is always specified as being that of the electric, not 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 weakly directional antenna with vertical polarization.[5]
Small loop receiving antennas
AM broadcast radios (and other consumer low frequency receivers) typically use small loop antennas; a variable capacitor connected across the loop forms a tuned circuit that also tunes the receivers 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 older (and physically larger) AM radios, the small loop might consist of dozens of turns of wire in a loop on the back side of the radio (a frame antenna). In modern radios, the loop antenna often is wound on a ferrite rod; the ferrite rod allows a physically small antenna to have a larger effective antenna area. The resulting coil and core is called a loopstick antenna, a ferrite rod antenna, a ferrite rod aerial, a Ferroceptor, a ferrod antenna,[6] or a ferrite antenna.[7] The term loopstick refers to the underlying loop antenna and the stick shape of the ferrite rod.[8]
Small loop antennas are lossy and inefficient, but they can make practical receiving antennas in the medium-wave (520–1610 kHz) band and below, where the antenna inefficiency is masked by large amounts of atmospheric noise. Loop antennas are often wound with litz wire to reduce skin effect losses.
Antenna efficiency
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.[9] Since a good loop antenna will have a high Q factor, this capacitor must be variable and is adjusted along with the receiver's tuning.
The radiation resistance RR of a small loop is generally much smaller than the loss resistance RL due to the conductors comprising the loop, leading to a poor antenna efficiency.[10] Consequently, most of the transmitted or received power will be dissipated as heat.
So much wasted signal power is a disaster for a transmitting antenna, however in a receiving antenna the inefficiency may not be of great concern. At frequencies below about 10 MHz atmospheric noise and man-made noise dominate over the thermal (Johnson) noise generated inside the receiver itself. Any increase in signal strength increases both the signal and the noise in equal proportion, leaving the signal-to-noise ratio unchanged. (CCIR 258; 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) the electrical inefficiency of that antenna will have little influence on the receiving system’s signal-to-noise ratio. In contrast, at quieter frequencies above about 20 MHz an antenna with a 50 dB loss could degrade the received signal-to-noise ratio by up to 50 dB, resulting in terrible performance. Copper losses are often minimized by the use of spiderweb or basket winding construction and Litz wire.
Insensitivity to locally generated interference
Due to its direct coupling to the magnetic field, unlike most other antenna types, the small loop is relatively insensitive to electric-field noise from nearby sources. No matter how close the electrical interference is to the loop, its effect will not be much greater than if it were a quarter wavelength away.[11] This is valuable inasmuch as most sources of interference with radio frequency content, such as sparking at commutators or corona discharge, produce electric, not magnetic fields.
Small loops are especially used in the AM broadcast band and generally at lower frequencies where resonant antennas are of an impractical size. At those frequencies the near-field is physically quite large. This provides a considerable advantage for loop antennas which are relatively insensitive to the main interference sources encountered.
The same principle makes a small loop particularly sensitive to sources of magnetic noise in its near field. Likewise, a Hertzian (short) dipole couples directly with the electric field and is relatively immune to locally produced magnetic noise. However at radio frequencies nearby sources of magnetic interference are generally not an issue. In either case the small antenna's immunity does not extend to noise sources outside of the near field: Noise sources over one wavelength distant, whether originating as an electric or magnetic field, are received simply as electromagnetic waves. Noise from outside any antenna’s near field will be received equally well by any antenna sensitive to a radio transmitter from the direction of that noise source.
Receiver input tuning
Small loop receiving antennas are also almost always resonated using a parallel 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 coil.
Small loops as transmitting antennas
Due to their small radiation resistance and consequent electrical inefficiency, small loops are seldom used as transmitting antennas, where one is trying to couple most of the transmitter's power to the electromagnetic field. Nevertheless small loops are sometimes used in applications in which a resonant antenna (with elements around a quarter of a wavelength in size) would simply be too large to be practical. Since any antenna much smaller than a wavelength suffers from inefficiency, a loop might not be the worst choice for medium wave and lower frequencies. Small loops (typically 18 to 39 inches in diameter when used from 29.7-7 MHz) are becoming popular as transmitting (as well as receiving) antennas.
The radiation efficiency is greatly boosted by making the outer loop larger (compared to one only used for receiving) insofar as that is possible in a given application, with circumferences ideally greater than 1/10 of a wavelength. Note that the increased size of the now not-so-small loop alters its radiation pattern, as the assumption of currents being totally in phase along the circumference of the loop begins to break down. In addition to making the geometric loop larger, efficiency is also increased by using larger conductors in order to reduce the loss resistance, and plating the outer conductor surfaces with silver or non-anodized aluminum.
Small loops are used in land-mobile radio (mostly military) 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 in mountainous regions. In this case a typical radiation efficiency of around 1% is acceptable because signal paths can be established with 1 Watt of radiated power or less when a transmitter generating 100 Watts is used. In military use, the antenna elements can be 2-3 inches in diameter.
One practical issue with small loops as transmitting antennas is that the loop not only has a very large current going through it, but also has a very high voltage on its terminals, typically kilo-Volts when fed with only a few Watts of transmitter power. This requires a rather expensive and physically large resonating capacitor with a large breakdown voltage, in addition to having minimal dielectric loss (normally requiring an air-gap capacitor).
It might be pointed out that a short (compared to a wavelength) vertical or dipole antenna matched using a loading coil also has a high voltage present at its base. The difference being that for a coil which is already physically large in order to reduce loss and carry high current, high voltage breakdown is not usually as much of an issue.
As for any antenna system, efficient electrical coupling requires impedance matching. For a small loop tuned with a parallel capacitor the resulting large (resistive) impedance (or for a small loop tuned with a series capacitor the resulting small impedance) will not be a good match to a standard transmission line or transmitter. In addition to other common impedance matching techniques, this is sometimes accomplished by connecting the transmission line not directly to the loop but to a smaller feed loop, typically 1/8 to 1/5 the size of the loop antenna. Essentially acting as a step-up transformer, power is inductively coupled from the feed loop to the main loop which itself is connected to the resonating capacitor and is responsible for radiating most of the power.
Direction finding with loops
Since the directional response of small loop antennas includes a sharp null in the direction normal to the plane of the loop, they are used in radio direction finding at longer wavelengths. The loop is thus rotated to find the direction of the null. Since the null occurs at two opposite directions, other means must be employed to determine which side of the null the transmitter 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.
A second dipole or vertical antenna known as a sense antenna can be electrically combined with a loop or a loopstick antenna. Switching the second antenna in obtains a net cardioid radiation pattern from which the general direction of the transmitter can be determined. Then switching the sense antenna out returns the sharp nulls in the loop antenna pattern, allowing a precise bearing to be determined.
References
- ↑ Low Profile Amateur Radio (A. Brogdon, W1AB)
- ↑ A Great Shortwave Loop http://antenadx.com.br/?page_id=99
- ↑ Ian Poole, Newnes guide to radio and communications technology Elsevier, 2003 ISBN 0-7506-5612-3, pages 113-114
- ↑ Handbook of Antenna Design Vol 2, Rudge A.W., Milne K., Olver A.D. & Knight, P. pp688
- ↑ Since AM broadcast radio is normally 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.
- ↑ Graf, Rudolf F. (1999), Modern Dictionary of Electronics, Newnes, p. 278
- ↑ Snelling 1988, p. 303
- ↑ Snelling 1988, p. 303
- ↑ Although a series capacitor could likewise be used to cancel the reactive impedance, doing so results in the receiver (or transmitter) seeing a very small (resistive) impedance. A parallel resonance, on the other hand, provides a very large radiation resistance as seen at the feedpoint, and thus an increased voltage which can directly be applied to the base of a transistor (for instance) at a receiver's input stage.
- ↑ Note that the calculated loss resistance must account not only for the DC resistance of the conductor, but also its increase due to the skin effect and proximity effect. If a ferrite rod is used, there are additional losses in the core as well as a relative increase in signal strength.
- ↑ Magnetic Loop Antennas Receiving (W8JI) - http://www.w8ji.com/magnetic_receiving_loops.htm
- Snelling, E. C. (1988), Soft Ferrites: Properties and Applications (second ed.), Butterworths, ISBN 0-408-02760-6
- The ARRL Antenna Book (15th edition), ARRL, 1988, ISBN 0-87259-206-5
External links
- Small Transmitting Loop Antenna Calculator - Online calculator that performs the "Basic Equations for a Small Loop" in The ARRL Antenna Book, 15th Edition
- Small Transmitting Loop Antennas - AA5TB