Loop antenna - AbsoluteAstronomy.com

A loop antenna is a radio antenna

Antenna (radio)

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

consisting of a loop (or loops) of wire, tubing, or other electrical conductor

Electrical conductor

In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

with its ends connected to a balanced

Balanced line

In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the...

transmission line

Transmission line

In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

. 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

Electrically short

An electrically short antenna is an antenna of length 2h, such that2 \pi h\over\lambda \scriptstyle\ll 1.An electrically short transmission line is a transmission line in which all of its effects will complete within one cycle of an impressed alternating current....

, 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.

Similar and dissimilar devices

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

Quad antenna

The quad antenna is an antenna which is frequently used by amateur radio operators. It consists of a driven element and parasitic elements, like a Yagi; however, the elements are square loops. If there is only one parasitic element it is referred to as a 'Cubical Quad' .- History :It was developed...

popular in amateur radio

Amateur radio

Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...

, 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

Copper loss

Copper loss is the term often given to heat produced by electrical currents in the conductors of transformer windings, or other electrical devices. Copper losses are an undesirable transfer of energy, as are core losses, which result from induced currents in adjacent components...

) 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

Halo antenna

A halo antenna, or halo, is a horizontally polarized, omni-directional 1/2 wavelength dipole antenna. It is shaped like a loop with a small break on the side of the loop directly opposite the feedpoint, so that the dipole ends do not meet...

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

Near and far field

The near field and far field and the transition zone are regions of the electromagnetic radiation field that emanates from a transmitting antenna, or as a result of radiation scattering off an object...

systems involving alternating magnetic fields only, and may be analyzed as poorly coupled transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

windings; their performance criteria are dissimilar to radio antenna

Antenna (radio)

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

s as discussed here.

Resonant loop antennas

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

Yagi antenna

A Yagi-Uda array, commonly known simply as a Yagi antenna, is a directional antenna consisting of a driven element and additional parasitic elements...

configuration including parasitic loop elements.

The polarization

Polarization

Polarization is a property of certain types of waves that describes the orientation of their oscillations. Electromagnetic waves, such as light, and gravitational waves exhibit polarization; acoustic waves in a gas or liquid do not have polarization because the direction of vibration and...

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

The small loop antenna is also known as a magnetic loop since it behaves electrically as a coil (inductor

Inductor

An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...

) with a small but non-negligible radiation resistance

Radiation resistance

Radiation resistance is that part of an antenna's feedpoint resistance that is caused by the radiation of electromagnetic waves from the antenna. The radiation resistance is determined by the geometry of the antenna, not by the materials of which it is made...

due to its finite size. It can be analyzed as coupling directly to the magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

(opposite to the principle of a
Hertzian dipole which couples directly to
the electric field) in the near field

Near and far field

,
which itself is coupled to an electromagnetic wave in the
far field

Near and far field

through the application
of Maxwell's equations

Maxwell's equations

Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...

. Because of this fact it is somewhat immune to noise affecting the electric field ("static") generated in the near field

Near and far 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. 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

Electrical impedance

Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

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. 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

Q is the seventeenth letter of the basic modern Latin alphabet.- History :The Semitic sound value of Qôp was , a sound common to Semitic languages, but not found in English or most Indo-European ones...

, 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

Null (radio)

In radio electronics, a null is an area or vector in an antenna's radiation pattern where the signal cancels out almost entirely.This can be an advantage, as nulls in the horizontal plane can be used to protect other transmitters from interference. If not carefully planned however, nulls can...

in that direction. 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

Antenna efficiency

In electromagnetics, antenna efficiency or radiation efficiency is a figure of merit for an antenna. It measures the electrical losses that occur throughout the antenna while it is operating at a given frequency, or averaged over its operation across a frequency band...

.

Another way of looking at this is to view the small loop antenna simply as an inductive coil

Coil

A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...

coupling to the magnetic field in the direction normal to plane of the coil according to Ampère's law

Ampère's law

In classical electromagnetism, Ampère's circuital law, discovered by André-Marie Ampère in 1826, relates the integrated magnetic field around a closed loop to the electric current passing through the loop...

. 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.

The radiation resistance

Radiation resistance

R_R of a small loop is often much smaller than the loss resistance R_L due to the conductors comprising the loop, leading to a poor antenna efficiency. 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

Atmospheric noise

Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. On a worldwide scale, eight million lightning discharges per day — about 100 lightning flashes per second.-History:...

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

Signal-to-noise ratio

Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

. 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

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

Ferrite (magnet)

Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...

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, or a ferrite antenna. The term loopstick refers to the underlying loop antenna and the stick shape of the ferrite rod.

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

Radio direction finder

A radio direction finder is a device for finding the direction to a radio source. Due to low frequency propagation characteristic to travel very long distances and "over the horizon", it makes a particularly good navigation system for ships, small boats, and aircraft that might be some distance...

(RDF) applications.

Small loops as transmitting antennas

Due to their small radiation resistance

Radiation resistance

and consequent electrical inefficiency

Antenna efficiency

, 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

Copper loss

.

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

Whip antenna

A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. The bottom end of the whip is connected to the radio receiver or transmitter. They are designed to be flexible so that they won't break...

. This enables near-vertical-incidence-skywave NVIS

NVIS

NVIS can refer to:*Near Vertical Incidence Skywave*Night Vision Imaging System*NVIS, Inc, manufacturer of virtual reality display systems...

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

Impedance matching

In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....

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

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

Triangulation

In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

.

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

Cardioid

A cardioid is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It is therefore a type of limaçon and can also be defined as an epicycloid having a single cusp...

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.

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

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.