Antenna (radio)

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Short wave "curtain" antenna (Moosbrunn, Austria)

An antenna (or aerial) is a transducer that transmits or receives electromagnetic waves. In other
words, antennas convert electromagnetic radiation into electric current, or vice versa. Antennas
generally deal in the transmission and reception of radio waves, and are a necessary part of all
radio equipment. Antennas are used in systems such as radio and television broadcasting, point-
to-point radio communication, wireless LAN, cell phones, radar, and spacecraft communication.
Antennas are most commonly employed 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 one or more conductors, usually called elements in

this context. In transmission, an alternating current is created in the elements by applying a
voltage at the antenna terminals, causing the elements to radiate an electromagnetic field. In
reception, the inverse occurs: an electromagnetic field from another source induces an alternating
current in the elements and a corresponding voltage at the antenna's terminals. Some receiving
antennas (such as parabolic and horn types) incorporate shaped reflective surfaces to collect the
radio waves striking them and direct or focus them onto the actual conductive elements.

Some of the first rudimentary antennas were built in 1888 by Heinrich Hertz (1857–1894) in his
pioneering experiments to prove the existence of electromagnetic waves predicted by the theory
of James Clerk Maxwell. Hertz 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
[hide]
  1 Terminology
 2 Overview
 3 Parameters
o 3.1 Resonant frequency
o 3.2 Gain
o 3.3 Radiation pattern
o 3.4 Impedance
o 3.5 Efficiency
o 3.6 Bandwidth
o 3.7 Polarization
o 3.8 Transmission and reception
 4 Basic antenna models
 5 Practical antennas
 6 Effect of ground
 7 Mutual impedance and interaction between antennas
 8 Antenna gallery
o 8.1 Antennas and antenna arrays
o 8.2 Antennas and supporting structures
o 8.3 Diagrams as part of a system
 9 See also
 10 Notes
 11 References
o 11.1 General references
o 11.2 "Practical antenna" references
o 11.3 Theory and simulations
o 11.4 Patents and USPTO
 12 Further reading

Terminology
The words antenna (plural: antennas[1]) and aerial are used interchangeably; but usually a rigid
metallic structure is termed an antenna and a wire format is called an aerial. In the United
Kingdom and other British English speaking areas the term aerial is more common, even for
rigid types. The noun aerial is occasionally written with a diaeresis mark—aërial—in
recognition of the original spelling of the adjective aërial from which the noun is derived.

The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi.
In 1895, while testing early radio apparatuses 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 Hertzian antenna is a set of terminals that does not require the presence of a ground for its
operation (versus a Tesla antenna which is grounded[3]). 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.

In colloquial usage, the word antenna may refer broadly to an entire assembly including support
structure, enclosure (if any), etc. in addition to the purely functional components.

Parabolic antenna for

communicating with Yagi antenna used for
spacecraft, Canberra, mobile military
"Rabbit ears" dipole
Australia communications
antenna for television Cell phone base station
station, Dresden,
reception antennas
Germany, 1955
 Large Yagi antenna
used by amateur radio
hobbyist
"Turnstile" type Folded dipole antenna A vertical mast radiator,
transmitting antenna for
Chapel Hill, North
commercial radio
Carolina
broadcasting station,
Germany.

Overview
Antennas have practical uses for the transmission and reception of radio frequency signals such
as radio and television. In air, those signals travel very quickly and with a very low transmission
loss. The signals are absorbed when moving through more conductive materials, such as concrete
walls or rock. When encountering an interface, the waves are partially reflected and partially
transmitted through.

A common antenna is a vertical rod 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.

There are two fundamental types of antenna directional patterns, which, with reference to a
specific two dimensional plane (usually horizontal [parallel to the ground] or vertical
[perpendicular to the ground]), are either:

1. Omni-directional (radiates equally in all directions), such as a vertical rod (in the
horizontal plane) or
2. Directional (radiates more in one direction than in the other).

In colloquial usage "omnidirectional" usually refers to all horizontal directions with reception
above and below the antenna being reduced in favor of better reception (and thus range) near the
horizon. A "directional" antenna usually refers to one focusing a narrow beam in a single specific
direction such as a telescope or satellite dish, or, at least, focusing in a sector such as a 120°
horizontal fan pattern in the case of a panel antenna at a cell site.

All antennas radiate some energy in all directions in free space but careful construction results in
substantial transmission of energy in a preferred direction and negligible energy radiated in other
directions. By adding additional elements (such as rods, loops or plates) and carefully arranging
their length, spacing, and orientation, an antenna with desired directional properties can be
created.

An antenna array is two or more simple antennas combined to produce a specific directional
radiation pattern. In common usage an array is composed of active elements, such as a linear
array of parallel dipoles fed as a "broadside array". A slightly different feed method could cause
this same array of dipoles to radiate as an "end-fire array". Antenna arrays may be built up from
any basic antenna type, such as dipoles, loops or slots.

The directionality of the array is due to the spatial relationships and the electrical feed
relationships between individual antennas. Usually all of the elements are active (electrically fed)
as in the log-periodic dipole array which offers modest gain and broad bandwidth and is
traditionally used for television reception. Alternatively, a superficially similar dipole array, the
Yagi-Uda Antenna (often abbreviated to "Yagi"), has only one active dipole element in a chain
of parasitic dipole elements, and a very different performance with high gain over a narrow
bandwidth.

An active element is electrically connected to the antenna terminals leading to the receiver or
transmitter, as opposed to a parasitic element that modifies the antenna pattern without being
connected directly. The active element(s) couple energy between the electromagnetic wave and
the antenna terminals, thus any functioning antenna has at least one active element. A careful
arrangement of parasitic elements, such as rods or coils, can improve the radiation pattern of the
active element(s). Directors and reflectors are common parasitic elements.

An antenna lead-in is the medium, for example, a transmission line or feed line for conveying the
signal energy between the signal source or receiver and 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.

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) as that of the
velocity of the impinging wave.

A director is a parasitic element, usually a metallic conductive structure, which re-radiates into
free space impinging electromagnetic radiation coming from or going to the active antenna, the
velocity of the re-radiated wave having a component in the direction of the velocity of the
impinging wave.

A reflector is a parasitic element, usually a metallic conductive structure (e.g., screen, rod or
plate), which re-radiates back into free space impinging electromagnetic radiation coming from
or going to the active antenna. The velocity of the returned wave has a component in a direction
opposite to the direction of the velocity of the impinging wave. The reflector modifies the
radiation of the active antenna.

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.

Parameters
Main article: Antenna measurement

There are several critical parameters affecting an antenna's performance that 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. All of these
parameters can be measured through various means.

Resonant frequency

The "resonant frequency" and "electrical resonance" is related to the electrical length of an
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 that are usually centered on that resonant frequency. However, other properties of an
antenna change with frequency, in particular the radiation pattern and impedance, 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; this resonance gives much better coupling to the electromagnetic wave, and
makes the aerial act as if it were physically larger.

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

Main article: Antenna gain

Gain as a parameter measures the efficiency of a given antenna with respect to a given norm,
usually achieved by modification of its directionality. An antenna with a low gain emits radiation
with about the same power in all directions, 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
a hypothetical isotropic antenna.

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 gain greater than one in some directions, it must have a
gain less than one 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 relatively inconsequential. For example, a dish antenna on a spacecraft is a high-
gain device that must be pointed at the planet to be effective, whereas a typical Wi-Fi antenna in
a laptop computer is low-gain, and as long as the base station is within range, the antenna can be
in any orientation in space. It makes sense to improve horizontal range at the expense of
reception above or below the antenna. Thus most antennas labelled "omnidirectional" really have
some gain.[4]

In practice, the half-wave dipole is taken as a reference instead of the isotropic radiator. The gain
is then given in dBd (decibels over dipole):

NOTE: 0 dBd = 2.15 dBi. It is vital in expressing gain values that the reference point be
included. Failure to do so can lead to confusion and error.

Radiation pattern
polar plots of the horizontal cross sections of a (virtual) Yagi-Uda-antenna. Outline connects
points with 3db field power compared to an ISO emitter.

The radiation pattern of an antenna is the geometric pattern of the relative field strengths of the
field emitted by the antenna. For the ideal isotropic antenna, this would be a sphere. For a typical
dipole, this would be a toroid. The radiation pattern of an antenna is typically represented by a
three dimensional graph, or polar plots of the horizontal and vertical cross sections. The graph
should show sidelobes and backlobes, where the antenna's gain is at a minimum or maximum.

See Antenna measurement: Radiation pattern or Radiation pattern for more information.

Impedance

As an electro-magnetic wave travels through the different parts of the antenna system (radio,
feed line, antenna, free space) it may encounter differences in impedance (E/H, V/I, etc.). At
each interface, depending on the impedance match, some fraction of the wave's energy will
reflect back to the source,[5] 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.

Efficiency
Efficiency is the ratio of power actually radiated to the power put into the antenna terminals. A
dummy load may have an 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 reduces efficiency. Mathematically, efficiency is calculated
as radiation resistance divided by total resistance.

Bandwidth

The bandwidth of an antenna is the range of frequencies over which it is effective, usually
centered on 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.

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 act 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, meaning that the polarization of the radio waves varies over time. Two special cases
are linear polarization (the ellipse collapses into a line) and circular polarization (in which the
two axes of the ellipse are equal). 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.

Transmission and reception

All of the antenna 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 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. This is a concern only for transmitting antennas,
as 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 optimize 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

Typical US multiband TV antenna (aerial)

There are many variations of antennas. Below are a few basic models. More can be found in
Category:Radio frequency antenna types.

 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 a
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 ground plane antenna, the
whip, and the J-pole.
  The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements
added which are functionality similar to adding a reflector and lenses (directors) to focus
a filament light bulb.
 The random wire antenna is simply a very long (at least one quarter 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 non-linearly with frequency.
 The horn is used where high gain is needed, the wavelength is short (microwave) and
space is not an issue. Horns can be narrow band or wide band, depending on their shape.
A horn can be built for any frequency, but horns for lower frequencies are typically
impractical. Horns are also frequently used as reference antennas.
 The parabolic antenna consists of an active element at the focus of a parabolic reflector to
reflect the waves into a plane wave. Like the horn it is used for high gain, microwave
applications, such as satellite dishes.
 The patch antenna consists mainly of a square conductor mounted over a groundplane.
Another example of a planar antenna is the tapered slot antenna (TSA), as the Vivaldi-
antenna.

Practical antennas

Very common "rabbit ears" set-top antenna

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
Antennas are typically used in an environment where other objects are present that may have an
effect on their performance. Height above ground has a very significant effect on the radiation
pattern of some antenna types.

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 grazing 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.

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 far bigger than in reality. Distance between the
antenna and its image is .

The 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 ) is
bigger than the refractive index of the air ( ), 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 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.

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:

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

In these two equations:

 is the electrical field radiated by the antenna if there were no ground.

 is the wave number.
  is the wave length.
 is the distance between antenna and its image (twice the height of the center of the
antenna).

Radiation patterns of antennas and their images reflected by the ground. At left the polarization
is vertical and there is always a maximum for . If the polarization is horizontal as at right,
there is always a zero for .

For emitting and receiving antenna situated near the ground (in a building or on a mast) far from
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)
add and there is maximum of received signal. If 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 users are 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 obtrusive, due to the inherent robustness of digital
signalling and built-in error correction.

[edit] Mutual impedance and interaction between antennas

Mutual impedance between parallel dipoles not staggered. Curves Re and Im are the resistive
and reactive parts of the impedance.

Current circulating in any antenna induces currents in all others. One can postulate a mutual
impedance between two antennas that has the same significance as the in ordinary
coupled inductors. The mutual impedance between two antennas is defined as:

where is the current flowing in antenna 1 and 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 are:

where:

 is the voltage applied to the antenna i

 is the impedance of antenna i
  is the mutual impedance between antennas i and j

Note that, as is the case for mutual inductances,

This is a consequence of Lorentz reciprocity. 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 are zero. Those elements are called parasitic elements. Parasitic elements are
unpowered elements that either reflect or absorb and reradiate RF energy.

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

[edit] Antenna gallery

[edit] Antennas and antenna arrays

A multi-band rotary Rooftop TV antenna. It is

A Yagi-Uda beam directional antenna for actually three Yagi antennas. A terrestrial
antenna. amateur radio use. The longest elements are for
microwave radio
the low band, while the
antenna array.
medium and short elements are
for the high and UHF band.

Examples of US 136- Low cost LF time

Shortwave antennas
174 MHz base stationsignal receiver, Rotatable log-periodic array
in Delano,
antennas. antenna (left) and for VHF and UHF.
California.
receiver (right).
 A T2FD broadband
An old VHF-band antenna, covering the
A US multiband "aerial" TV "Rabbit ears"
Yagi-type television 5-30MHz band.
antenna. antenna
antenna.

AM loop antenna

[edit] Antennas and supporting structures

A building rooftop supporting A water tower in

numerous dish and sectored Palmerston, Northern A three-sector Telephone site
mobile telecommunications Territory with radio telephone site in concealed as a
antennas (Doncaster, Victoria, broadcasting and Mexico City. palm tree.
Australia. communications antennas.

[edit] Diagrams as part of a system

Antennas may be connected through a multiplexing Antenna network for an emergency

arrangement in some applications like this trunked two- medical services base station.
way radio example.