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Antenna (radio)

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A Yagi-Uda antenna
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A Yagi-Uda antenna

An antenna or aerial is an electronic component designed to transceive radio signals (and, more generally, other electromagnetic waves). Antennas are for transmission of radio wave energy through the natural media (i.e., air, earth, water, etc.) for point-to-point communication or for the reception of such transmitted radio wave energy. Antennas are primarily designed for transmission of radio wave energy through free space or any space where the movement of energy in any direction is substantially unimpeded, such as interplanetary space (such as the interplanetary medium or interstellar medium), the atmosphere, the ocean (and other large bodies of water), or the Earth. Antennas are used for communicating and conveying information specifically in larger systems, such as the radio, telephone, and the telegraph.

Physically, 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.

Contents

Terminology

The words "antenna" (plural: antennas 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.) and "aerial" are used interchangeably throughout this article. The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi. In 1895, while testing early radio apparatus in the Swiss Alps at Salvan, Switzerland in the Mont Blanc region, Marconi experimented with early wireless equipment. A 2.5 meter long pole, along which was carried a wire, was used as a radiating and receiving aerial element. In Italian a tent pole is known as l'antenna centrale, and the pole with a wire alongside it used as an aerial was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals. Marconi's use of the word antenna (Italian for pole) would become a popular term for what today is uniformly known as the "antenna". "Salvan: Cradle of Wireless, How Marconi Conducted Early Wireless Experiments in the Swiss Alps", Fred Gardiol & Yves Fournier, Microwave Journal, February 2006, pp. 124-136.

A "hertz antenna" is a set of terminals that does not require the presence of a ground for its operation. A "loaded antenna" is an active antenna having an elongated portion of appreciable electrical length and having additional inductance or capacity directly in series or shunt with the elongated portion so as to modify the standing wave pattern existing along the portion or to change the effective electrical length of the portion. An "antenna grounding structure" is a ground for establishing a reference potential level for operating the active antenna. It can be any structure closely associated with (or act as) the ground which is connected to the terminal of the signal receiver or source opposing the active antenna terminal, (i.e., the signal receiver or source is interposed between the active antenna and this structure.

The use of "antenna" usually excludes non-communication applications involving radio waves where no communications or signalling is involved. The use of "antenna" also usually excludes communication by radiant energy other than radio waves (e.g. by partial rays, compressional waves, etc.). Radiant energy is the partially kinetic and partially potential energy associated with waves produced in free space by a source of energy, such as electromagnetic radiations (including radio wave). Communication systems that utilize electromagnetic waves other than radio waves (e.g. rays of visible light, infrared, ultraviolet, X-rays, cathode rays, ions, gamma rays, beta rays, and inductive coupling) do use this term "antenna" though.

"Radio waves" (or "Hertz waves") are electromagnetic waves whose frequency spectrum extends over a range from somewhat above the frequency of audible sound waves to somewhat below the frequency of heat and light waves. The radio waves are produced by oscillations of electric change in an antenna. Values of 10 kilocyles to 30,000 megacyles have been given as the lower and upper limits of the range for radio waves. It should be noted that other values exist beyond these limits. Radio waves usually exclude compressional waves, light waves, heat waves, infrared waves, ultraviolet waves, X-rays, cathode rays, gamma rays, and ion beams.

Overview

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

By adding additional conducting rods or coils (called elements) and varying their length, spacing, and orientation (or changing the direction of the antenna beam), an antenna with specific desired properties can be created, such as a Yagi-Uda Antenna (often abbreviated to "Yagi"). An "antenna array" is a plurality of active antennas coupled to a common source or load to produce a directive radiation pattern. Usually the spatial relationship also contributes to the directivity of the antenna. By the use of term "active element" it is intended to describe an element whose energy output is modified due to the presence of a source of energy in the element (other than the mere signal energy which passes through the circuit) or an element in which the energy output from a source of energy is controlled by the signal input. The "antenna lead-in" is the conductive means (as for example a transmission line or feed line) for conveying the signal energy between the active antenna and the signal source. It extends directly from the active antenna towards the source. The "antenna feed" refers to the components between the active antenna and an amplifier. The "antenna counterpoise" is the structure of conductive material most closely associated with ground but insulated from (or capacitively coupled to) the natural ground. It aids in the function of the natural ground, particularly where variations (or limitations) of the characteristics of the natural ground interfere with its proper function. Such structure are usually connected to the terminal of the signal receiver or source opposing the active antenna terminal.

A "antenna component" is a portion of the antenna performing a distinct function and limited for use in an antenna, as for example, a reflector, director, or active antenna. "Parasitic elements" are usually metallic conductive structures which reradiates into free space impinging electromagnetic radiation coming from or going to the active antenna. The "electromagnetic wave refractor" is a structure which is shaped or positioned to delay or accelerate transmitted electromagnetic waves, passing through such structure, an amount which varies over the wave front. The refractor alters the direction of propagation of the waves emitted from the structure with respect to the waves impinging on the structure. It can alternatively bring the wave to a focus or alter the wave front in other ways, such as to convert a spherical wave front to a planar wave front (or vice versa). The velocity of the wave radiated have a component which is in the same direction ("director") or in the opposite direction ("reflector") that of the velocity of the impinging wave. A "director" is usually a metallic conductive structure which reradiates into free space impinging electromagnetic radiation coming from or going to the active antenna, the velocity of the reradiated wave having a component in the direction of velocity of the impinging wave. The director modifies the radiation pattern of the active antenna and there is no significant potential relationship between the active antenna and this conductive structure. A "reflector" is usually a metallic conductive structure (e.g., screen, rod or plate) which reradiates back into free space impinging electromagnetic radiation coming from or going to the active antenna. The velocity of the returned wave having a component in a direction opposite to the direction of velocity of the impinging wave. The reflector modifies the radiation of the active antenna. There is no significant potential relationship between the active antenna and this conductive structure.

An "antenna coupling network" is a passive network (which may be any combination of a resistive, inductive or capacitive circuit(s)) for transmitting the signal energy between the active antenna and a source (or receiver) of such signal energy. 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 theroretically travel over great distances at the speed of light (the true velocity depends on the transmission medium over which it passes). These signals can also 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" and "electrical resonance" is related to the electrical length of the antenna. The electrical length 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 periodic, 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 often expressed in units of dBi (decibels over isotropic). For example, a dipole antenna has a gain of 2.14 dBi [link]. Often, the dipole antenna is used as the reference (since a perfect isotropic reference is impossible to build), 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)

The gain of an antenna is a passive phenomena - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has a positive gain in some directions, it must have a negative gain in other directions as energy is conserved by the antenna. The gain that can be achieved by an Antenna is therefore trade-off between the range of directions that must be covered by an Antenna and the gain of the antenna. For example, a dish antenna on a spacecraft has a very large gain, but only over a very small range of directions - it must be accurately pointed at earth - but a radio transmitter has a very small gain as it is required to radiate in all directions.

For dish-type antennas, gain is proportional to the aperture (reflective area) and surface accuracy of the dish, as well as the frequency being transmitted/received. In general, a larger aperture provides a higher gain. Also, the higher the frequency, the higher the gain, but surface inaccuracies lead to a larger degradation of gain at higher frequencies.

"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 analogous 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, depending on the impedance match, 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 is the orientation of the electric field (E-plane) of the radio wave with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation. It has nothing in common with antenna directionality terms: "horizontal", "vertical" and "circular". Thus, a simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally. "Electromagnetic wave polarization filters" are structures which can be employed to acts directly on the electromagnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization.

Reflections generally affect polarization. For radio waves the most important reflector is the ionosphere - signals which reflect from it will have their polarization changed unpredictably. For signals which are reflected by the ionosphere, polarization cannot be relied upon. For line-of-sight communications for which polarization can be relied upon, it can make a large difference in signal quality to have the transmitter and receiver using the same polarization; many tens of dB difference are commonly seen and this is more than enough to make the difference between reasonable communication and a broken link.

Polarization is largely predictable from antenna construction, but especially in directional antennas, the polarization of side lobes can be quite different from that of the main propagation lobe. For radio antennas, polarization corresponds to the orientation of the radiating element in an antenna. A vertical omnidirectional WiFi antenna will have vertical polarization (the most common type). An exception is a class of elongated waveguide antennas in which vertically placed antennas are horizontally polarized. Many commercial antennas are marked as to the polarization of their emitted signals.

Polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the direction of motion of the radio wave. In the most general case, polarization is elliptical (the projection is oblong), meaning that the antenna varies over time in the polarization of the radio waves it is emitting. Two special cases are linear polarization (the ellipse collapses into a line) and circular polarization (in which the ellipse varies maximally). In linear polarization the antenna compels the electric field of the emitted radio wave to a particular orientation. Depending on the orientation of the antenna mounting, the usual linear cases are horizontal and vertical polarization. In circular polarization, the antenna continuously varies the electric field of the radio wave through all possible values of its orientation with regard to the Earth's surface. Circular polarizations, like elliptical ones, are classified as right-hand polarized or left-hand polarized using a "thumb in the direction of the propagation" rule. Optical researchers use the same rule of thumb, but pointing it in the direction of the emitter, not in the direction of propagation, and so are opposite to radio engineers' use.

In practice, regardless of confusing terminology, it is important that linearly polarized antennas be matched, lest the received signal strength be greatly reduced. So horizontal should be used with horizontal and vertical with vertical. Intermediate matchings will lose some signal strength, but not as much as a complete mismatch. Transmitters mounted on vehicles with large motional freedom commonly use circularly polarized antennas so that there will never be a complete mismatch with signals from other sources. In the case of radar, this is often reflections from rain drops.

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 efficiency.

Overview of antenna parameters

Except for polarization, the SWR is the most easily measured of the parameters above. 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 put the sensor into the antenna's far field, or an anechoic chamber designed for antenna measurements), careful study of experiment geometry, and specialized measurement equipment that rotates the antenna during the measurements. The distance is the space between two points, which may be immediately juxtaposed or widely spaced.

Bandwidth depends on the overall effectiveness of the antenna, so all of these parameters must be understood to fully characterize the bandwidth capabilities of an antenna. However, in practice, bandwidth is typically determined by looking only 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 is also important, for in practical use the performance of an antenna at the extremes of an assigned frequency band is important.

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. Tuning is done by adjusting the length of an electrically long linear antenna to alter the electrical resonance of the antenna.

Antenna tuning is done by adjusting an inductance or capacitance combined with the active antenna (but distinct and separate from the the active antenna). The inductance or capacitance provides the reactance which combines with the inherent reactance of the active antenna to establish a resonance in a circuit including the active antenna. The established resonance being at a frequency other than the natural electrical resonant frequency of the active antenna. Adjustment of the inductance or capacitance changes this resonance.

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 specifically for reception might be optimized for noise rejection capabilities. An "antenna shield" is a conductive or low reluctance structure (such as a wire, plate or grid) which is adapted to be placed in the vicinity of an antenna to reduce, as by dissipation through a resistance or by conduction to ground, undesired electromagnetic radiation, or electric or magnetic fields, which are directed toward the active antenna from an external source or which emanate from the active antenna. Other methods to optimized for noise rejection 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 that have various configuartions. These configurations contain space or medium which tends to confine the energy within specified boundaries along a predetermined path (known as "restricted space"), such as wave guides, hollow resonators, and conductive wires. Below are a few common models. More can be found in .

A multiband rotary directional antenna for amateur radio use
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A multiband rotary directional antenna for amateur radio use

How antennas work

Any conducting mass may function as a radiator or collector of radio wave energy and may act as an antenna. Antennas, more specifically, are passive conducting mass, which may be in the form of a metallic current conductor, wave guide, or space discharge. This mass in use is in direct engagement with free space to emit or collect radio wave energy to or from free space, and is coupled or connected to a source of energy or to a load. To act as an antenna the mass usually has a particular shape of dimension, or may have electrical circuit elements, namely, resistance, inductance, or capacity, associated therewith.

"Scanning" an antenna repeated moves the antenna beam over an area in space, such as in radar. "Sweeping" an antenna moves the antenna beam repeatedly along a single line (which may be straight or curved) in space.

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 canceled 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 minimize the reactance of the device so that it appears as a resistive load. An "antenna inherent reactance" includes not only the distributed reactance of the active antenna but also the natural reactance due to its location and surroundings (as for example, the capacity relation inherent in the position of the active antenna relative to ground). 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.

See also

External articles and further reading

General references and footnotes

"Practical antennas" references
General websites
  • "[Antenna]". Hamradio, Hamradio.co.in. (ed. Antenna for Ham / Amateur Radio)
  • "Ham radio guide : [Antennas]". The DXZone.com, 2006. (ed. Amateur radio antenna plans and documents)
  • "[Ham Radio Antennas]" NØHR.com, 2006.(ed. Ham radio antenna sites sorted by band, design, and homebrew vs. commercial antenna products.)
  • Ian Poole (editor), "[Antenna topics]". Adrio Communications Ltd. (ed. Information regarding antennas)
Theory and simulations
  • Hans Lohninger, "Learning by Simulations: Physics: [Coupled Radiators]". vias.org, 2005. (ed. Interactive simulation of two coupled antennas)
  • Justin Smith "[Aerials]". A.T.V (Aerials and Television), 2006. (ed. Article on the (basic) theory and use of TV aerials)
  • Antennas Research Group, "[Virtual (Reality) Antennas]". Democritus University of Thrace, 2005.
  • "Support > Knowledgebase > RF Basics > Antennas / Cables > [dBi vs. dBd detail]". MaxStream, Inc., 2005. (ed. How to measure antenna gain)
    • Patents and USPTO
  • [CLASS 343], Communication: Radio Wave Antenna

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