Loop antenna


Loop antenna
A shortwave loop antenna

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. 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 resonant loop antenna with a circumference approximately equal to the wavelength.

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 of it or directly attached to it. These antennas are also used for radio direction finding. A technically small loop, also known as a magnetic loop, should have a circumference of one tenth of a wavelength or less. This is necessary to ensure a constant current distribution round the loop. As the frequency or the size are increased, a standing wave starts to develop in the current, and the antenna starts to have some of the characteristics of a folded dipole antenna or a self-resonant loop.

Self-resonant loop antennas are larger. They are typically used at higher frequencies, especially VHF and UHF, where their size is manageable. They can be viewed as a form of folded dipole and have somewhat similar characteristics. The radiation efficiency is also high and similar to that of a dipole.

Contents

Similar and dissimilar devices

"Quad antenna" including one driven and one parasitic element. Each "X" is a support structure; the squares are the conductive (loop) elements.

Although the loop may be in the shape of a circle, distorting it into a somewhat different closed shape does not qualitatively alter its characteristics. For instance, the quad antenna popular in amateur radio, consists of a resonant loop (and usually additional parasitic elements) in a square shape so that it can be constructed of wire strung in between insulators. In a self-resonant loop antenna the most important characteristic, resonant frequency, is determined by the circumference of the loop. On the other hand, the main characteristics of a small loop antenna are rather determined by the area enclosed by the loop. For a given loop area, the length of the conductor (and thus its net loss resistance) is minimized in the case of a circle, making that shape 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

A loop antenna for amateur radio under construction

The large or self-resonant loop antenna can be seen as a folded dipole which has been reformed into a circle (or square, etc.). This loop has a circumference approximately equal to one wavelength (however it will also be resonant at odd multiples of a wavelength). Compared to the dipole or folded dipole, it transmits less toward the sky or ground, giving it a somewhat higher gain (about 10% higher) in the horizontal direction.

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. 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 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 it is fed at the bottom it will be horizontally polarized; feeding it from the side will make it vertically polarized.

Small loops

Small loop antenna used for receiving, consisting of about 10 turns around a 10cm square.

Small loop antennas are much less than a wavelength in size, and are mainly (but not always) used as receiving antennas at lower frequencies.[1]

The small loop antenna is also known as a magnetic loop since it behaves electrically as a coil (inductor) with a small but non-negligible radiation resistance due to its finite size. 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 is coupled to an electromagnetic wave in the far field through the application of Maxwell's equations. Because of this fact it is somewhat immune to noise affecting the electric field ("static") generated in the near field. Since at low frequencies, such as the AM broadcast band, the near field region is physically quite large, this provides a considerable benefit in relation to static generating devices (such as sparking at the commutator of an electric motor) in the vicinity.[2] Contrary to myth, however, this immunity does not extend to noise sources outside of the near field: such noise is received as an electromagnetic (propagating) wave and would be received equally by any antenna sensitive to a radio transmitter at the location of that noise source.

Since the small loop antenna is essentially a coil, its electrical impedance is inductive, with an inductive reactance much much greater than its radiation resistance. In order to couple to a receiver, that inductance is normally cancelled with a parallel capacitance.[3] In an AM broadcast receiver which must operate over a wide frequency range, this capacitance is therefore varied using a section of the tuning capacitor. Since a good device will have a rather high Q, it is important for the shunt capacitance to be adjusted carefully for electrical resonance.

Surprisingly, the radiation pattern of a small loop is quite opposite that of a resonant loop. Since the loop is much smaller than a wavelength, currents along the conductor are essentially in phase. By symmetry it can be seen that the voltages induced along the various 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 along the plane of the loop. Although a similar argument may seem to apply to signals received in that plane, that voltages generated by an impinging radio wave would cancel along the loop, this is not quite true due to the phase difference between the arrival of the wave at the near side and far side of the loop. Thus increasing that phase delay 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 this is to view the small loop antenna 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 transvserse 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]

Amount of atmospheric noise for LF, MF, and HF spectrum according CCIR 322

The radiation resistance RR of a small loop is often much smaller than the loss resistance RL due to the conductors comprising the loop, leading to a poor antenna efficiency.[6] Consequently, most of the transmitted or received power will be dissipated in loss resistance. However in a receiving antenna, this inefficiency may not be of great concern since atmospheric noise and man-made noise dominate thermal (Johnson) noise at lower frequencies. (CCIR 258; CCIR 322.) For example, at 1 MHz, the man-made noise might be 55dB above the thermal noise floor. If a small loop antenna's loss is 50 dB (in effect, the attenna includes a 50 dB attenuator), the electrical inefficiency of that antenna will have little impact on the receiving system's signal-to-noise ratio. In contrast, at quieter VHF frequencies, an antenna with a 50 dB loss could degrade the received signal-to-noise ratio by up to 50 dB, resulting in terrible performance.

AM broadcast receiver loop antennas

Loopstick antenna having two windings, one for long wave and one for medium wave (AM broadcast) reception. Typically 10cm long, these are usually hidden inside the radio receiver.

AM broadcast radios (and some other receivers used at low frequencies) typically use small loop antennas, tuned using a variable capacitor which tracks the frequency the receiver is tuned to. In older (and physically larger) AM radios, this might consist of dozens of turns of wire in a loop on the back side of the radio. However modern AM radios usually use a loop antenna wound around a ferrite rod to increase its inductance without requiring such a large size. The resulting coil is called a loopstick antenna, a ferrite rod antenna, a ferrod antenna,[7] or a ferrite antenna.[8] The term loopstick refers to the underlying loop antenna and the stick shape of the ferrite rod.[9]

As with all small loops, loopstick antennas are most practical at lower frequencies such as the medium-wave (AM broadcast band - 520–1610 kHz) and long-wave (50–500 kHz) bands, using ferrite materials which are not too lossy at these frequencies. A multiband receiver may contain tap points along the winding in order to tune the loopstick antenna at widely different frequencies.

As with all small loop antennas, loopstick antennas are largely immune to locally generated (within the near field) electrical noise, as they are coupled directly to the magnetic field. Loopstick antennas are also used in radio direction-finding (RDF) applications.

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) is simply too large to be practical. Since any antenna much smaller than a wavelength suffers from inefficiency, a loop might not be the worst choice. The efficiency is greatly boosted by making the 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 breaks down. In addition to making the geometric loop larger, efficiency is also increased by using larger conductors in order to reduce the loss resistance.

Small loops are used in land-mobile radio (mostly military) at frequencies between 3 and 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 are 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 has a very high voltage on its terminals, typically kilovolts 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 such a coil (which is already physically large in order to reduce loss) high voltage breakdown is not usually an issue. As with any antenna design, efficient transmission generally demands additional impedance matching since the (resistive) impedance generated across the small loop when tuned with a parallel capacitor is not likely to match that of a standard transmission line or the transmitter. In addition to other common impedance matching techniques, this is sometimes accomplished by connecting the transmission line to a smaller feed loop, typically 1/8 to 1/5 the size of the loop antenna. Power is inductively coupled from it 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

Loop antenna, receiver, and accessories used in amateur radio direction finding at 80 meter wavelength (3.5 MHz).

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

  1. ^ Ian Poole, Newnes guide to radio and communications technology Elsevier, 2003 ISBN 0750656123, pages 113-114
  2. ^ By the same principle, a small loop is particularly sensitive to sources of magnetic noise in the near field, which a Hertzian dipole would be immune to. However such sources of interference at radio frequencies are generally weak or absent, whereas man-made noise affecting the electric field is commonly produced by sparking or corona discharge from high voltages.
  3. ^ Although a series capactitor 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 great boost to the 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.
  4. ^ Handbook of Antenna Design Vol 2, Rudge A.W., Milne K., Olver A.D. & Knight, P. pp688
  5. ^ 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 directionality 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.
  6. ^ 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 core losses.
  7. ^ Graf, Rudolf F. (1999), Modern Dictionary of Electronics, Newnes, p. 278 
  8. ^ E. C. Snelling, Soft Ferrites, page 303
  9. ^ Snelling, page 303.
  • Snelling, E. C. (1988), Soft Ferrites: Properties and Applications (second ed.), Butterworths, ISBN 0408027606 

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