Arikah Map

Antenna (radio)

For other uses, see Antenna.
Antenna (radio):A Yagi-Uda beam antenna
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A Yagi-Uda beam antenna
Antenna (radio):Short Wave "Curtain" Antenna (Moosbrunn, Austria)
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Short Wave "Curtain" Antenna (Moosbrunn, Austria)

An antenna or aerial is an arrangement of aerial electrical conductors designed to transmit or receive radio waves which is a class of electromagnetic waves. In other words, antennas basically convert radio frequency electrical currents into electromagnetic waves and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.

Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabola, horn antenna) just adapt the free space to another type of antenna.

Antennas were first used in 1889 by Heinrich Hertz (1857-1894) to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. He even placed the emitter dipole in the focal point of a parabolic reflector. He published his work and installation drawings in Annalen der Physik und Chemie (vol. 36, 1889).


Contents

Terminology

The words antenna (plural: antennas[1]) and "aerial" are used interchangeably.

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.[2]

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 capacitance 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 structure for establishing a reference potential level for operating the active antenna. It can be any structure closely associated with (or acting 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).

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 (radiates equally in all directions) or
  2. Directional (radiates more in one direction than in the other).

All antennas radiate some energy in all directions in free space but careful construction results in substantial transmission of energy 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 two or more antennas coupled to a common source or load to produce a directional radiation pattern. The spatial relationship between individual antennas contributes to the directivity of the antenna.

The term active element 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.

An antenna lead-in is the medium, for example, a transmission line or feed line for conveying the signal energy from the signal source to the antenna. The antenna feed refers to the components between the antenna and an amplifier.

An antenna counterpoise is a structure of conductive material most closely associated with ground that may be 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 structures are usually connected to the terminal of a receiver or source opposite to the antenna terminal.

An 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 reradiate into free space impinging electromagnetic radiation coming from or going to the active antenna.

An 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 waves 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 uses 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 divided by its velocity factor (the ratio of the speed of wave propagation in the wire to c0, the speed of light in a vacuum). 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

"Gain" as a parameter measures the directionality of a given antenna. An antenna with a low gain emits radiation in all directions equally, whereas a high-gain antenna will preferentially radiate in particular directions. Specifically, the Gain, Directive gain or Power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by an hypothetical isotropic antenna:

<math>G={\left({P \over S}\right)_{ant} \over \left({P \over S}\right)_{iso}}</math>

We write "hypothetical" because a perfect isotropic antenna cannot exist in reality (the electric and magnetic field would not satisfy Maxwell equations for electromagnetic fields). Gain is a dimensionless number (without units).

The gain of an antenna is a passive phenomenon - 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 greater than one gain in some directions, it must have a less than one gain in other directions since energy is conserved by the antenna. An antenna designer must take into account the application for the antenna when determining the gain. High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is inconsequential. For example, a dish antenna on a spacecraft is a high-gain device (must be pointed at the planet to be effective), while a typical WiFi antenna in a laptop computer is low-gain (as long as the base station is within range, the antenna can be in an any orientation in space).

As an example, consider an antenna that radiates an electromagnetic wave whose electrical field has an amplitude <math>\scriptstyle{E_\theta}</math> at a distance <math>\scriptstyle{r}</math>. This amplitude is given by:

<math>E_\theta= {AI \over r}</math>

where:

For a large distance <math>\scriptstyle{r}</math>. The radiated wave can be considered locally as a plane wave. The intensity of an electromagnetic plane wave is:

<math>{P\over S}={c\varepsilon_\circ\over2}E_B^2={1\over 2} {E_B^2\over Z_\circ}</math>

where <math> \scriptstyle{Z_\circ=\sqrt= 376.730313461\, \Omega}</math> is a universal constant called vacuum impedance.and

<math>\left({P\over S}\right)_{ant}={1\over 2Z_\circ} {A^2I^2\over r^2}</math>

If the resistive part of the series impedance of the antenna is <math>\scriptstyle{R_s}</math>, the power fed to the antenna is <math>\scriptstyle{{1\over 2}R_sI^2}</math>. The intensity of an isotropic antenna is the power so fed divided by the surface of the sphere of radius <math>\scriptstyle{r}</math>:

<math>\left({P \over S}\right)_{iso}=</math>

The (small) electric field of the electromagnetic wave radiated by an element of current is:

<math>dE_\theta(t+\textstyle)=\displaystyle{-d\ell j\omega \over 4\pi\varepsilon_\circ c^2} {\sin\theta \over r} e^{j\omega t}\,</math>

And for the time <math>\textstyle{t}\,</math>:

<math>dE_\theta(t)={-d\ell j\omega \over 4\pi\varepsilon_\circ c^2} {\sin\theta \over r} e^{j\left(\omega t-{\omega\over c}r\right)}\,</math>

The electric field of the electromagnetic wave radiated by an antenna formed by wires is the sum of all the electric fields radiated by all the small elements of current. This addition is complicated by the fact that the direction and phase of each of the electric fields are, in general, different.

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.

Effect of ground

At frequencies used in antennas, the ground behaves mainly as a dielectric. The conductivity of ground at these frequencies is negligible. When an electromagnetic wave arrives at the surface of an object, two waves are created: one enters the dielectric and the other is reflected. If the object is a conductor, the transmitted wave is negligible and the reflected wave has almost the same amplitude as the incident one. When the object is a dielectric, the fraction reflected depends (among others things) on the angle of incidence. When the angle of incidence is small (that is, the wave arrives almost perpendicularly) most of the energy traverses the surface and very little is reflected. When the angle of incidence is near 90° (grazing incidence) almost all the wave is reflected.

Most of the electromagnetic waves emitted by an antenna to the ground below the antenna at moderate (say < 60°) angles of incidence enter the earth and are absorbed (lost). But waves emitted to the ground at gracing angles, far from the antenna, are almost totally reflected. At grazing angles, the ground behaves as a mirror. Quality of reflection depends on the nature of the surface. When the irregularities of the surface are smaller than the wavelength reflection is good.

Antenna (radio):The wave reflected by earth can be considered as emitted by the image antenna.
The wave reflected by earth can be considered as emitted by the image antenna.

This means that the receptor "sees" the real antenna and, under the ground, the image of the antenna reflected by the ground. If the ground has irregularities, the image will appear fuzzy.

If the receiver is placed at some height above the ground, waves reflected by ground will travel a little longer distance to arrive to the receiver than direct waves. The distance will be the same only if the receiver is close to ground.

In the drawing at right, we have drawn the angle <math>\scriptstyle{\theta}</math> far bigger than in reality. Distance between the antenna and its image is <math>\scriptstyle{d}</math>.

Situation is a bit more complex because the reflection of electromagnetic waves depends on the polarization of the incident wave. As the refractive index of the ground (average value <math>\scriptstyle{\simeq 2}</math>) is bigger than the refractive index of the air (<math>\scriptstyle{\simeq 1}</math>), the direction of the component of the electric field parallel to the ground inverses at the reflection. This is equivalent to a phase shift of <math>\scriptstyle{\pi}</math> radians or 180°. The vertical component of the electric field reflects without changing direction. This sign inversion of the parallel component and the non-inversion of the perpendicular component would also happen if the ground were a good electrical conductor.

Antenna (radio): The vertical component of the current reflects without changing sign. The horizontal component reverses sign at reflection.
The vertical component of the current reflects without changing sign. The horizontal component reverses sign at reflection.

This means that a receiving antenna "sees" the image antenna with the current in the same direction if the antenna is vertical or with the current inverted if the antenna is horizontal.

For a vertical polarized emission antenna the far electric field of the electromagnetic wave produced by the direct ray plus the reflected ray is:

<math>\textstyle{\left|E_\perp\right|=2\left|E_{\theta_1}\right|\left|\cos\left({kd\over2}\sin\theta\right) \right|}</math>

The sign inversion for the parallel field case just changes a cosine to a sinus:

<math>\textstyle{\left|E_=\right|=2\left|E_{\theta_1}\right|

\left|\sin\left({kd\over2}\sin\theta\right) \right|}</math>

In these two equations:

Antenna (radio):Radiation patterns of antennas and their images reflected by the ground. At left the polarization is vertical and there is always a maximum for <math>\scriptstyle{\theta=0}</math>. If the polarization is horizontal as at right, there is always a zero for <math>\scriptstyle{\theta=0}</math>.
Radiation patterns of antennas and their images reflected by the ground. At left the polarization is vertical and there is always a maximum for <math>\scriptstyle{\theta=0}</math>. If the polarization is horizontal as at right, there is always a zero for <math>\scriptstyle{\theta=0}</math>.

For emitting and receiving antenna situated near the ground (in a building or a mast) far form each other, distances traveled by direct and reflected rays are nearly the same. There is no induced phase shift. If the emission is polarized vertically the two fields (direct and reflected) adds and there is maximum of received signal. Is the emission is polarized horizontally the two signals subtracts and the received signal is minimum. This is depicted in the image at right. In the case of vertical polarization, there is always a maximum at earth level (left pattern). For horizontal polarization, there is always a minimum at earth level. Note that in these drawings the ground is considered as a perfect mirror, even for low angles of incidence. In these drawings the distance between the antenna and its image is just a few wavelengths. For greater distances, the number of lobes increases.

Note that the situation is different – and more complex – if reflections in the ionosphere occur. This happens over very long distances (thousands of kilometers). There is not a direct ray but several reflected rays that add with different phase shifts.

This is the reason why almost all public address radio emissions have vertical polarization. As public uses to be near ground, horizontal polarized emissions would be poorly received. Observe household and automobile radio receivers. They all have vertical antennas or horizontal ferrite antennas for vertical polarized emissions. In cases where the receiving antenna must work in any position, as in mobile phones, the emitter and receivers in base stations use circular polarized electromagnetic waves.

Classical (analog) television emissions are an exception. They are almost always horizontally polarized, because the presence of buildings makes it unlikely that a good emitter antenna image will appear. However, these same buildings reflect the electromagnetic waves and can create ghost images. Using horizontal polarization, reflections are attenuated because of the low reflection of electromagnetic waves whose magnetic field is parallel to the dielectric surface near the Brewster's angle. Vertically polarized analog television has been used in some rural areas.In digital terrestrial television reflections are less annoying because of the type of modulation.

Mutual impedance and interaction between antennas

Current circulating in any antenna induces currents in all others. One can postulate a mutual impedance <math>\scriptstyle{Z_{12}}</math> between two antennas that has the same significance as the <math>\scriptstyle{j\omega M}</math> in ordinary coupled inductors. The mutual impedance <math>\scriptstyle{Z_{12}}</math> between two antennas is defined as:

<math>Z_{12}={v_2\over i_1}</math>

where <math>\textstyle{i_{1}}</math> is the current flowing in antenna 1 and <math>\textstyle{v_2}</math> is the voltage that would have to be applied to antenna 2 – with antenna 1 removed – to produce the current in the antenna 2 that was produced by antenna 1.

From this definition, the currents and voltages applied in a set of coupled antennas is:

Antenna (radio):Mutual impedance between parallel <math>\scriptstyleTemplate:\lambda \over 2</math> dipoles not staggered. Curves Re and Im are the resistive and reactive parts of the impedance.
Mutual impedance between parallel <math>\scriptstyle</math> dipoles not staggered. Curves Re and Im are the resistive and reactive parts of the impedance.


<math>\begin{matrix} v_1&=&i_1Z_{11}&+&i_2Z_{12}&+& \cdots &+& i_nZ_{1n}\\

v_2&=&i_1Z_{21}&+& i_2Z_{22}&+&\cdots&+&i_nZ_{2n} \\ \vdots & & \vdots & & \vdots & & & & \vdots \\

v_n&=&i_1Z_{n1}&+&i_2Z_{n2}&+&\cdots&+&i_nZ_{nn}\end{matrix}

</math>

where:

Note that, as is the case for mutual inductances,

<math>\scriptstyle{Z_{ij}\,= \,Z_{ji}}</math>

If some of the elements are not fed (there is a short circuit instead a feeder cable), as is the case in television antennas (Yagi-Uda antennas), the corresponding <math>\textstyle{v_i}</math> are zero. Those elements are called parasitic elements. Despite their name, parasitic elements are not useless but most useful.

In some geometrical settings, the mutual impedance between antennas can be zero. This is the case for crossed dipoles used in circular polarization antennas.

Computer external antennas for wireless connection

See also: MCX connectors are standards for extensions for external antenna.

See also

External articles and further reading

General references and footnotes
  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.
  2. ^ "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.


"Practical antennas" references
General websites
Theory and simulations
Patents and USPTO
Effect of ground references

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Radio frequency antennas | Amateur radio | Radio electronics

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