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A Yagi-Uda antenna

An antenna or aerial is an electronic component designed to transmit or receive radio waves. The words "antenna" (plural: antennas ) and "aerial" are used interchangeably throughout this article.

More technically, an antenna is an arrangement of conductors designed to radiate (transmit) an electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or to be placed into an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals.

Overview

There are two fundamental types of antennas, which, with reference to a specific three dimensional (usually horizontal or vertical) plane, are either omni-directional (radiate equally in the plane) or directional (radiates more in one direction than in the other). All antennas radiate some energy in all directions but careful construction results in large directivity in certain directions and negligible power radiated in other directions.

By adding additional conducting rods or coils (called elements) and varying their length, spacing, and orientation, an antenna with specific desired properties can be created, such as a Yagi-Uda Antenna (often abbreviated to "Yagi"). Typically, antennas are designed to operate in a relatively narrow frequency range. The design criteria for receiving and transmitting antennas differ slightly, but generally an antenna can receive and transmit equally well. This property is called reciprocity.

The vast majority of antennas are simple vertical rods a quarter of a wavelength long. Such antennas are simple in construction, usually inexpensive, and both radiate in and receive from all horizontal directions (omnidirectional). One limitation of this antenna is that it does not radiate or receive in the direction in which the rod points. This region is called the antenna blind cone or null.

Antennas have practical use for the transmission and reception of radio frequency signals (radio, TV, etc.), which can travel over great distances at the speed of light, and pass through nonconducting walls (although often there is a variable signal reduction depending on the type of wall, and natural rock can be very reflective to radio signals).

Antenna parameters

There are several critical parameters that affect an antenna's performance and can be adjusted during the design process. These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties.

Resonant frequency

The resonant frequency is related to the electrical length of the antenna. This is usually the physical length of the wire multiplied by the ratio of the speed of wave propagation in the wire. Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on that resonant frequency. However, the other properties of the antenna (especially radiation pattern and impedance) change with frequency, so the antenna's resonant frequency may merely be close to the center frequency of these other more important properties.

Antennas can be made resonant on harmonic frequencies with lengths that are fractions of the target wavelength. Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. The most commonly known type of wide band aerial is the logarithmic or log aerial but its gain is usually much lower than that of a specific or narrower band aerial.

Gain

In antenna design, gain is the logarithm of the ratio of the intensity of an antenna's radiation pattern in the direction of strongest radiation to that of a reference antenna. If the reference antenna is an isotropic antenna, the gain is expressed in units of dBi (decibels over isotropic). For example, a dipole antenna has a gain of 2.14 dBi . Often, the dipole antenna is used as the reference, in which case the gain of the antenna in question is measured in dBd (decibels over dipole).

side lobes in a two-dimensional power diagram (schematic, polar diagram)
side lobes in a two-dimensional power diagram (schematic, polar diagram)

An antenna has gain if it radiates more strongly in one direction than in another. Gain is measured by comparing an antenna to a model antenna, typically the isotropic antenna which radiates equally in all directions. Often a dipole is also used as a practical reference as the isotropic source cannot be realised in practice, but it has 2.1 dB gain over an isotropic source. Most practical antennas radiate more than the isotropic antenna in some directions and less in others. Gain is inherently directional; the gain of an antenna is usually measured in the direction which it radiates best. Gain is one-dimensional. Gain does not mean that the antenna radiates more power than is fed to it, merely that it distributes the power more strongly in some directions than in others.


Aperture, and radiation pattern are closely related to gain.

Aperture is the shape of the "beam" cross section in the direction of highest gain, and is two-dimensional. (Sometimes aperture is expressed as the radius of the circle that approximates this cross section or the angle of the cone.)

Radiation pattern is the three-dimensional plot of the gain, but usually only the two-dimensional horizontal and vertical cross sections of the radiation pattern are considered. Antennas with high gain typically show side lobes in the radiation pattern. Side lobes are peaks in gain other than the main lobe (the "beam"). Side lobes detract from the antenna quality whenever the system is being used to determine the direction of a signal, as in radar systems and reduce gain in the main lobe by distributing the power.

Bandwidth

The bandwidth of an antenna is the range of frequencies over which it is effective, usually centered around the resonant frequency. The bandwidth of an antenna may be increased by several techniques, including using thicker wires, replacing wires with cages to simulate a thicker wire, tapering antenna components (like in a feed horn), and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna. Small antennas are usually preferred for convenience, but there is a fundamental limit relating bandwidth, size and efficiency.

Impedance

Impedance is similar to refractive index in optics. As the electric wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance. At each interface, some fraction of the wave's energy will reflect back to the source, forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system.

Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.

Polarization

The polarization of an antenna or orientation of the radio wave is determined by the electric field or E-plane. The ionosphere changes the polarization of signals unpredictably, so for signals which will be reflected by the ionosphere, polarization is not crucial. However, for line-of-sight communications, it can make a tremendous difference in signal quality to have the transmitter and receiver using the same polarization. Polarizations commonly considered are linear, such as vertical and horizontal, and circular, which is divided into right-hand and left-hand circular.

Efficiency

Efficiency is the ratio of power actually radiated to the power put into the antenna terminals. A dummy load may have a SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates heat but not RF energy, showing that SWR alone is not an effective measure of an antenna's efficiency. Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance. Loss resistance usually results in heat generation rather than radiation, and therefore, reduces efficency.

Overview of antenna parameters

Of the parameters above, SWR is most easily measured. Impedance can be measured with specialized equipment, as it relates to the complex SWR. Measuring radiation pattern requires a sophisticated setup including significant clear space (enough to get into the antenna's far field) or an anechoic chamber designed for antenna measurements, careful study of experiment geometry, and specialised measurement equipment that rotates the antenna during the measurements. Bandwidth depends on the overall effectiveness of the antenna, so all of these parameters must be understood to understand bandwidth. However, typically bandwidth is measured by only looking at SWR, i.e., by finding the frequency range over which the SWR is less than a given value. Bandwidth over which an antenna exhibits a particular radiation pattern might also be considered.

Transmission and reception

All of these parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving antenna, due to reciprocity. Impedance, however, is not applied in an obvious way; for impedance, the impedance at the load (where the power is consumed) is most critical. For a transmitting antenna, this is the antenna itself. For a receiving antenna, this is at the (radio) receiver rather than at the antenna.

Antennas used for transmission have a maximum power rating, beyond which heating, arcing or sparking may occur in the components, which may cause them to be damaged or destroyed. Raising this maximum power rating usually requires larger and heavier components, which may require larger and heavier supporting structures. Of course, this is only a concern for transmitting antennas; the power received by an antenna rarely exceeds the microwatt range.

Antennas designed specificially for reception might be optimized for noise rejection capabilities. This can be done by selecting a narrow bandwidth so that noise from other frequencies is rejected, or selecting a specific radiation pattern to reject noise from a specific direction, or by selecting a polarization different from the noise polarization, or by selecting an antenna that favors either the electric or magnetic field.

For instance, an antenna to be used for reception of low frequencies (below about ten megahertz) will be subject to both man made noise from motors and other machinery, and from natural sources such as lightning. Successfully rejecting these forms of noise is an important antenna feature. A small coil of wire with many turns is more able to reject such noise than a vertical antenna. However, the vertical will radiate much more effectively on transmit, where extraneous signals are not a concern.

Basic antenna models

There are many variations of antennas, but here are a few common models. More can be found in Category:Radio frequency antenna types.

A multiband rotary directional antenna for amateur radio use
  • The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator).
  • The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as reference model for other antennas; gain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the groundplane antenna, the whip, and the J-pole.
  • The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements added with functionality similar to adding a reflector and lenses (directors) to focus a filament lightbulb.
  • Loop antennas have a continuous conducting path leading from one conductor of a two-wire transmission line to the other conductor. "Symmetric" loop antennas have a plane of symmetry running along the feed and through the loop. "Planar" loop antennas lie in a single plane which also contains the conductors of the feed. "Three-dimensional" loop antennas have wire which runs in all of the x,y, and z directions. By definition they are not planar. They may, however, be symmetric about planes which contain the feed.
  • The (large) loop antenna is similar to a dipole, except that the ends of the dipole are connected to form a circle, triangle (delta loop antenna) or square. Typically a loop is a multiple of a half or full wavelength in circumference. A circular loop gets higher gain (about 10%) than the other forms of large loop antenna, as gain of this antenna is directly proportional to the area enclosed by the loop, but circles can be hard to support in a flexible wire, making squares and triangles much more popular. Large loop antennas are more immune to localized noise partly due to lack of a need for a groundplane. The large loop has its strongest signal in the plane of the loop, and nulls in the axis perpendicular to the plane of the loop.
  • The small loop antenna, also called the magnetic loop antenna is a loop of wire (in other words, both ends of the wire connect to the radio) less than a wavelength in circumference. Typically, the circumference is less than 1/10 for a receiving loop, and less than 1/4 for a transmitting loop. Unlike nearly all other antennas in this list, this antenna detects the magnetic component of the electromagnetic wave. As such, it is less sensitive to near field electric noise when properly shielded. The receiving aperture can be greatly increased by bringing the loop into resonance with a tuning capacitor. Due to the small size of the loop, the radiation pattern is 90 degrees from that of the large loop. The radiation pattern is perpendicular to the plane of the loop, with sharp nulls in the plane of the loop.
  • The electrically short antenna is an open-end wire far less than 1/4 wavelength in length - in other words only one end of the antenna is connected to the radio, and the other end is hanging free in space. Unlike nearly all other antennas in this list, this antenna detects only the electric field of the wave instead of the electromagnetic field - think of the free end of the wire as measuring the voltage of that point in space, as opposed to measuring both the voltage and the magnetic field. Its receiving aperture can be greatly increased by increasing the voltage; by adding an inductor or resonator tuned to resonance with the signals of interest. Electrically short antennas are typically used where operating wavelength is large and space is limited, e.g. for mobile transceivers operating at long wavelengths.
  • The microstrip antenna consists of a patch of metalization on a ground plane. These are low profile, light weight antennas, most suitable for aerospace and mobile applications. Because of their low power handling capability, these antennas can be used in low-power transmitting and receiving applications. Microstrip antennas are the most commonly used antennas in mobile communications, satellite links, W-LAN and so on because circuit functions can be directly integrated to the microstrip antenna to form compact transceivers and spatial power combiners.
  • The quad antenna is an array of square loops that vary in size. The quad is related to the loop in exactly the same way the yagi is related to the dipole. Typically, the quad needs fewer elements to get the same gain as a yagi. Variations of the quad include the delta loop antenna which uses a triangle instead of a square, requiring fewer supports for large wavelength antennas.
  • The random wire antenna is simply a very long (greater than one wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency.
  • The endfire helical antenna is a directional antenna suited for receiving signals that are either circular polarized or randomly polarized. These are usually used with satellites, and are frequently used for the driven element on a dish.
  • The Phased array antenna is a group of independently fed active elements in which the relative phases of the respective signals feeding the elements are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. In plain language, this is a directional antenna that can be aimed without moving any parts.
  • Synthetic aperture radar uses a series of observations separated in time and space to simulate a very large antenna. More generally, interferometry allows the combining of signals from several radio receivers or a single moving receiver.
  • A trailing wire antenna is used by submarines when submerged. These antennas are designed to pick up transmissions in the low frequency (LF) and very low frequency (VLF) ranges.
  • An evolved antenna refers to an antenna fully or substantially designed using a computer algorithm based on Darwinian evolution.
  • A dielectric resonator is a variation on the conventional antenna in which an insulator with a large dielectric constant is used to modify the electromagnetic field. It is claimed that the dielectric contains the antenna's near field and therefore prevents it from interfering with other nearby antennas or circuits, making it suitable for miniature equipment such as mobile phones.
  • A feed horn is an antenna system that handles the incoming waveform from the dish to the focal point. It usually comprises of a series of rings with decreasing radius in order to drive the signal to the polarizer.

Combination of multiple antennas

Multiple antennas can be combined in one physical device to save space and weight in mobile applications. E.g. airlanes need antennas for radar, GPS, radio, beacons, and voice radio.

How antennas work

The reactive field

Fundamentally, all electromagnetic fields are created by the existence or movement of electrical charge, and in normal electrical circuits, this charge is exclusively carried by electrons and protons. Since protons tend to be confined within atoms and move very little, it is usually only the movement of electrons that needs to be considered.

Since an electric current in a wire consists of a moving cloud of electrons, it follows that every electric current induces a magnetic field. (Every electron also has its own permanent electric field called its coulomb field, but this is not observable outside the circuit because it is cancelled by the equal but opposite coulomb field of a nearby proton.) If the current is constant, it induces a constant magnetic field, and the magnetic field is proportional to current.

Maxwell's equations predict that a changing magnetic field induces a changing electric field, so we now have both magnetic and electric fields around the circuit, creating an electromagnetic field called the reactive field or inductive field. However, when the current stops, these fields collapse, returning energy to the power supply. The circuit therefore behaves like a reactive component, either a capacitor or an inductor, which stores energy temporarily but periodically returns it to the source.

The radiating field

Now consider a current that periodically reverses direction: an alternating current. This consists of a flow of electrons that must therefore reverse direction, and a change of direction is an acceleration. Because of the way that electromagnetic fields propagate through space at the speed of light, an accelerating electrical charge creates electromagnetic radiation. The result is that energy is continually radiated into space, and must be replenished from the circuit's power supply. The circuit is now behaving as an antenna, and is continually converting electrical energy into a radiating field that extends indefinitely outward.

When the circuit is much shorter than the wavelength of the signal, the rate at which it radiates energy is proportional to the size of the current, the length of the circuit and the frequency of the alternations. In most circuits, the product of these three quantities is small enough that not much energy is radiated, and the result is that the reactive field dominates the radiating field. When the length of the antenna approaches the wavelength of the signal, the current along the antenna is no longer uniform and the calculation of power output becomes more complex.

Practical antennas

Although any circuit can radiate if driven with a signal of high enough frequency, most practical antennas are specially designed to radiate efficiently at a particular frequency. An example of an inefficient antenna is the simple Hertzian dipole antenna, which radiates over wide range of frequencies and is useful for its small size. A more efficient variation of this is the half-wave dipole, which radiates with high efficiency when the signal wavelength is twice the electrical length of the antenna.

One of the goals of antenna design is to minimise the reactance of the device so that it appears as a resistive load. Reactance diverts energy into the reactive field, which causes unwanted currents that heat the antenna and associated wiring, thereby wasting energy without contributing to the radiated output. Reactance can be eliminated by operating the antenna at its resonant frequency, when its capacitive and inductive reactances are equal and opposite, resulting in a net zero reactive current. If this is not possible, compensating inductors or capacitors can instead be added to the antenna to cancel its reactance as far as the source is concerned.

Once the reactance has been eliminated, what remains is a pure resistance, which is the sum of two parts: the ohmic resistance of the conductors, and the radiation resistance. Power absorbed by the ohmic resistance becomes waste heat, and that absorbed by the radiation resistance becomes radiated electromagnetic energy. The greater the ratio of radiation resistance to ohmic resistance, the more efficient the antenna.

References for this section

Footnote

  1. In the context of engineering and physics, the plural of antenna is "antennas", and it has been this way since about 1950 (or earlier), when a cornerstone textbook in this field, Antennas, was published by John D. Kraus of the Ohio State University. Besides the title, Dr. Kraus noted this in a footnote on the first page of his book. Insects may have "antennae" but not in technical contexts.

See also

External links

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