Antenna (radio) From Wikipedia, the free encyclopedia

Whip antenna on car

Diagram of the electric fields (blue) and magnetic fields (red) radiated by adipole antenna (black rods)during transmission.

Large parabolic antennafor communicating with spacecraft

Rooftop television antennas in Israel. Yagi-Uda antennas like these six are widely used at VHF and UHFfrequencies.

An antenna (or aerial) is an electrical device which converts electric power into radio waves, and vice versa. It

is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an

oscillating radio frequency electric current to the antenna's terminals, and the antenna radiates the energy from

the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of

an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to

be amplified.

Antennas are essential components of all equipment that uses radio. They are used in systems such as radio

broadcasting, broadcast television, two-way radio,communications receivers, radar, cell phones, and satellite

communications, as well as other devices such as garage door openers, wireless

microphones,bluetooth enabled devices, wireless computer networks, baby monitors, and RFID tags on

merchandise.

Typically an antenna consists of an arrangement of metallic conductors ("elements"), electrically connected

(often through a transmission line) to the receiver or transmitter. An oscillating current of electrons forced

through the antenna by a transmitter will create an oscillating magnetic field around the antenna elements,

while the charge of the electrons also creates an oscillating electric field along the elements. These time-

varying fields, when created in the proper proportions, radiate away from the antenna into space as a moving

transverse electromagnetic field wave. Conversely, during reception, the oscillating electric and magnetic fields

of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back

and forth, creating oscillating currents in the antenna.

Antennas may also contain reflective or directive elements or surfaces not connected to the transmitter or

receiver, such as parasitic elements, parabolic reflectorsor horns, which serve to direct the radio waves into a

beam or other desired radiation pattern. Antennas can be designed to transmit or receive radio waves in all

directions equally (omnidirectional antennas), or transmit them in a beam in a particular direction, and receive

from that one direction only (directional or high gainantennas).

The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove

the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole

antennas at the focal point of parabolic reflectors for both transmitting and receiving. He published his work

in Annalen der Physik und Chemie (vol. 36, 1889).

Contents

[hide]

1 Terminology

2 Overview

3 Reciprocity

4 Parameters

o 4.1 Resonant antennas

4.1.1 Current and voltage distribution

4.1.2 Bandwidth

o 4.2 Gain

o 4.3 Effective area or aperture

o 4.4 Radiation pattern

o 4.5 Impedance

o 4.6 Efficiency

o 4.7 Polarization

o 4.8 Impedance matching

5 Basic antenna models

6 Practical antennas

7 Effect of ground

8 Mutual impedance and interaction between antennas

9 Antenna gallery

o 9.1 Antennas and antenna arrays

o 9.2 Antennas and supporting structures

o 9.3 Diagrams as part of a system

10 See also

11 Notes

12 References

o 12.1 General references

o 12.2 "Practical antenna" references

o 12.3 Theory and simulations

o 12.4 Patents and USPTO

13 Further reading

[edit]Terminology

The words antenna (plural: antennas[1]

) and aerial are used interchangeably. Occasionally a rigid metallic

structure is called an "antenna" while the wire form is called an "aerial". However, note the important

international technical journal, the IEEE Transactions on Antennas and Propagation.[2]

In the United

Kingdom and other areas where British English is used, the term aerial is sometimes used although 'antenna'

has been universal in professional use for many years.

The origin of the word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo

Marconi. In 1895, while testing early radio apparatus in the Swiss Alps at Salvan, Switzerland in the Mont

Blanc region, Marconi experimented with long wire "aerials". He used a 2.5 meter vertical pole, with a wire

attached to the top running down to the transmitter, as a radiating and receiving aerial element. In Italian a tent

pole is known as l'antenna centrale, and the pole with the wire was simply called l'antenna. Until then wireless

radiating transmitting and receiving elements were known simply as aerials or terminals. Because of his

prominence, Marconi's use of the word antenna (Italian for pole) spread among wireless researchers, and later

to the general public.[3]

In common usage, the word antenna may refer broadly to an entire assembly including support structure,

enclosure (if any), etc. in addition to the actual functional components. Especially at microwave frequencies, a

receiving antenna may include not only the actual electrical antenna but an integrated preamplifier or mixer.

"Rabbit ears" dipole antenna for television reception

Cell phone base stationantennas

Wi-Fi WestNet Wi-Fi base station antennas inCalgary, Alberta

Parabolic antenna by Himalaya Television Nepal

Yagi antenna used for mobile military communications station, Dresden, Germany, 1955

Turnstile type transmitting antenna for VHF low band television broadcasting station, Germany.

Folded dipole antenna

Large Yagi antenna used by amateur radiohobbyists

A mast radiator antenna for an AM radio station inChapel Hill, North Carolina

[edit]Overview

Antennas of the Atacama Large Millimeter submillimeter Array.[4]

Antennas are required by any radio receiver or transmitter to couple its electrical connection to the

electromagnetic field. Radio waves areelectromagnetic waves which carry signals through the air (or through

space) at the speed of light with almost no transmission loss. Radio transmitters and receivers are used to

convey signals (information) in systems including broadcast (audio) radio, television, mobile telephones, wi-

fi (WLAN) data networks, trunk lines and point-to-point communications links (telephone, data networks),

satellite links, many remote controlled devices such asgarage door openers, and wireless remote sensors,

among many others. Radio waves are also used directly for measurements in technologies

includingRADAR, GPS, and radio astronomy. In each and every case, the transmitters and receivers involved

require antennas, although these are sometimes hidden (such as the antenna inside an AM radio or inside a

laptop computer equipped with wi-fi).

According to their applications and technology available, antennas generally fall in one of two categories:

1. Omnidirectional or only weakly directional antennas which receive or radiate more or less in all

directions. These are employed when the relative position of the other station is unknown or arbitrary.

They are also used at lower frequencies where a directional antenna would be too large, or simply to

cut costs in applications where a directional antenna isn't required.

2. Directional or beam antennas which are intended to preferentially radiate or receive in a particular

direction or directional pattern.

In common usage "omnidirectional" usually refers to all horizontal directions, typically with reduced

performance in the direction of the sky or the ground (a truly isotropic radiator is not even possible). A

"directional" antenna usually is intended to maximize its coupling to the electromagnetic field in the direction of

the other station, or sometimes to cover a particular sector such as a 120 horizontal fan pattern in the case of

a panel antenna at a cell site.

One example of omnidirectional antennas is the very common vertical antenna or whip antenna consisting of a

metal rod (often, but not always, a quarter of a wavelength long). A dipole antenna is similar but consists of two

such conductors extending in opposite directions, with a total length that is often, but not always, a half of a

wavelength long. Dipoles are typically oriented horizontally in which case they are weakly directional: signals

are reasonably well radiated toward or received from all directions with the exception of the direction along the

conductor itself; this region is called the antenna blind cone or null.

Half-wave dipole antenna

Both the vertical and dipole antennas are simple in construction and relatively inexpensive. The dipole antenna,

which is the basis for most antenna designs, is abalanced component, with equal but opposite voltages and

currents applied at its two terminals through a balanced transmission line (or to a coaxial transmission line

through a so-called balun). The vertical antenna, on the other hand, is a monopole antenna. It is typically

connected to the inner conductor of a coaxial transmission line (or a matching network); the shield of the

transmission line is connected to ground. In this way, the ground (or any large conductive surface) plays the

role of the second conductor of a dipole, thereby forming a complete circuit.[5]

Since monopole antennas rely on

a conductive ground, a so-called groundingstructure may be employed to provide a better ground contact to the

earth or which itself acts as a ground plane to perform that function regardless of (or in absence of) an actual

contact with the earth.

Antennas more complex than the dipole or vertical designs are usually intended to increase the directivity and

consequently the gain of the antenna. This can be accomplished in many different ways leading to a plethora of

antenna designs. The vast majority of designs are fed with a balanced line (unlike a monopole antenna) and

are based on the dipole antenna with additional components (or elements) which increase its directionality.

Antenna "gain" in this instance describes the concentration of radiated power into a particular solid angle of

space, as opposed to the spherically uniform radiation of the ideal radiator. The increased power in the desired

direction is at the expense of that in the undesired directions. Power is conserved, and there is no net power

increase over that delivered from the power source (the transmitter.)

For instance, a phased array consists of two or more simple antennas which are connected together through

an electrical network. This often involves a number of parallel dipole antennas with a certain spacing.

Depending on the relative phase introduced by the network, the same combination of dipole antennas can

operate as a "broadside array" (directional normal to a line connecting the elements) or as an "end-fire array"

(directional along the line connecting the elements). Antenna arrays may employ any basic (omnidirectional or

weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical.

However a log-periodic dipole array consists of a number of dipole elements of different lengths in order to

obtain a somewhat directional antenna having an extremely wide bandwidth: these are frequently used for

television reception in fringe areas. The dipole antennas composing it are all considered "active elements"

since they are all electrically connected together (and to the transmission line). On the other hand, a

superficially similar dipole array, the Yagi-Uda Antenna (or simply "Yagi"), has only one dipole element with an

electrical connection; the other so-calledparasitic elements interact with the electromagnetic field in order to

realize a fairly directional antenna but one which is limited to a rather narrow bandwidth. The Yagi antenna has

similar looking parasitic dipole elements but which act differently due to their somewhat different lengths. There

may be a number of so-called "directors" in front of the active element in the direction of propagation, and

usually a single (but possibly more) "reflector" on the opposite side of the active element.

Greater directionality can be obtained using beam-forming techniques such as a parabolic reflector or a horn.

Since the size of a directional antenna depends on it being large compared to the wavelength, very directional

antennas of this sort are mainly feasible at UHF and microwave frequencies. On the other hand, at low

frequencies (such as AM broadcast) where a practical antenna must be much smaller than a wavelength,

significant directionality isn't even possible. A vertical antenna or loop antenna small compared to the

wavelength is typically used, with the main design challenge being that of impedance matching. With a vertical

antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of

impedance; small loop antennas are tuned with parallel capacitors for this purpose.

An antenna lead-in is the transmission line (or feed line) which connects the antenna to a transmitter or

receiver. The antenna feed may refer to all components connecting the antenna to the transmitter or receiver,

such as an impedance matching network in addition to the transmission line. In a so-called aperture antenna,

such as a horn or parabolic dish, the "feed" may also refer to a basic antenna inside the entire system

(normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active

element in that antenna system. A microwave antenna may also be fed directly from a waveguide in lieu of a

(conductive) transmission line.

An antenna counterpoise or ground plane is a structure of conductive material which improves or substitutes for

the ground. It may be connected to or insulated from the natural ground. In a monopole antenna, this 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 a structure is normally connected to the return connection

of an unbalanced transmission line such as the shield of a coaxial cable.

An electromagnetic wave refractor in some aperture antennas is a component which due to its shape and

position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it.

The refractor alters the spatial characteristics of the wave on one side relative to the other side. It can, for

instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the

directivity of the antenna system. This is the radio equivalent of an optical lens.

An antenna coupling network is a passive network (generally a combination of inductive and capacitive circuit

elements) used for impedance matching in between the antenna and the transmitter or receiver. This may be

used to improve the standing wave ratio in order to minimize losses in the transmission line and to present the

transmitter or receiver with a standard resistive impedance that it expects to see for optimum operation.

[edit]Reciprocity

It is a fundamental property of antennas that the electrical characteristics of an antenna described in the next

section, such as gain, radiation pattern, impedance, bandwidth, resonant frequencyand polarization, are the

same whether the antenna is transmitting or receiving. For example, the "receiving pattern" (sensitivity as a

function of direction) of an antenna when used for reception is identical to the radiation pattern of the antenna

when it is driven and functions as a radiator. This is a consequence of the reciprocity theorem of

electromagnetics. Therefore in discussions of antenna properties no distinction is usually made between

receiving and transmitting terminology, and the antenna can be viewed as either transmitting or receiving,

whichever is more convenient.

A necessary condition for the aforementioned reciprocity property is that the materials in the antenna and

transmission medium are linear and reciprocal. Reciprocal (or bilateral) means that the material has the same

response to an electric current or magnetic field in one direction, as it has to the field or current in the opposite

direction. Most materials used in antennas meet these conditions, but some microwave antennas use[citation

needed] high-tech components such as isolators and circulators, made of nonreciprocal materials such

as ferrite or garnet. These can be used to give the antenna a different behavior on receiving than it has on

transmitting, which can be useful in applications like radar.

[edit]Parameters

Main article: Antenna measurement

Antennas are characterized by a number of performance measures which a user would be concerned with in

selecting or designing an antenna for a particular application. Chief among these relate to the directional

characteristics (as depicted in the antenna's radiation pattern) and the resulting gain. Even in omnidirectional

(or weakly directional) antennas, the gain can often be increased by concentrating more of its power in the

horizontal directions, sacrificing power radiated toward the sky and ground. The antenna's power gain (or

simply "gain") also takes into account the antenna's efficiency, and is often the primary figure of merit.

Resonant antennas are expected to be used around a particular resonant frequency; an antenna must

therefore be built or ordered to match the frequency range of the intended application. A particular antenna

design will present a particular feedpoint impedance. While this may affect the choice of an antenna, an

antenna's impedance can also be adapted to the desired impedance level of a system using a matching

network while maintaining the other characteristics (except for a possible loss of efficiency).

Although these parameters can be measured in principle, such measurements are difficult and require very

specialized equipment. Beyond tuning a transmitting antenna using an SWR meter, the typical user will depend

on theoretical predictions based on the antenna design or on claims of a vendor.

An antenna transmits and receives radio waves with a particular polarization which can be reoriented by tilting

the axis of the antenna in many (but not all) cases. The physical size of an antenna is often a practical issue,

particularly at lower frequencies (longer wavelengths). Highly directional antennas need to be significantly

larger than the wavelength. Resonant antennas use a conductor, or a pair of conductors, each of which is

about one quarter of the wavelength in length. Antennas that are required to be very small compared to the

wavelength sacrifice efficiency and cannot be very directional. Fortunately at higher frequencies (UHF,

microwaves) trading off performance to obtain a smaller physical size is usually not required.

[edit]Resonant antennas

While there are broadband designs for antennas, the vast majority of antennas are based on the half-

wave dipole which has a particular resonant frequency. At its resonant frequency, thewavelength (figured by

dividing the speed of light by the resonant frequency) is slightly over twice the length of the half-wave dipole

(thus the name). The quarter-wave vertical antenna consists of one arm of a half-wave dipole, with the other

arm replaced by a connection to ground or an equivalent ground plane (or counterpoise). A Yagi-Uda array

consists of a number of resonant dipole elements, only one of which is directly connected to the transmission

line. The quarter-wave elements of a dipole or vertical antenna imitate a series-resonant electrical element,

since if they are driven at the resonant frequency a standing wave is created with the peak current at the feed-

point and the peak voltage at the far end.

A common misconception is that the ability of a resonant antenna to transmit (or receive) fails at frequencies far

from the resonant frequency. The reason a dipole antenna needs to be used at the resonant frequency has to

do with the impedance match between the antenna and the transmitter or receiver (and its transmission line).

For instance, a dipole using a fairly thin conductor[6]

will have a purely resistive feedpoint impedance of about

63 ohms at its design frequency. Feeding that antenna with a current of 1 ampere will require 63 volts of RF,

and the antenna will radiate 63 watts (ignoring losses) of radio frequency power. If that antenna is driven with 1

ampere at a frequency 20% higher, it will still radiate as efficiently but in order to do that about 200 volts would

be required due to the change in the antenna's impedance which is now largely reactive (voltage out of phase

with the current). A typical transmitter would not find that impedance acceptable and would deliver much less

than 63 watts to it; the transmission line would be operating at a high (poor) standing wave ratio. But using an

appropriate matching network, that large reactive impedance could be converted to a resistive impedance

satisfying the transmitter and accepting the available power of the transmitter.

This principle is used to construct vertical antennas substantially shorter than the 1/4 wavelength at which the

antenna is resonant. By adding an inductance in series with the vertical antenna (a so-called loading coil) the

capacitative reactance of this antenna can be cancelled leaving a pure resistance which can then be matched

to the transmission line. Sometimes the resulting resonant frequency of such a system (antenna plus matching

network) is described using the construct of "electrical length" and the use of a shorter antenna at a lower

frequency than its resonant frequency is termed "electrical lengthening". For example, at 30 MHz (wavelength

= 10 meters) a true resonant monopole would be almost 2.5 meters (1/4 wavelength) long, and using an

antenna only 1.5 meters tall would require the addition of a loading coil. Then it may be said that the coil has

"lengthened" the antenna to achieve an "electrical length" of 2.5 meters, that is, 1/4 wavelength at 30 MHz

where the combined system now resonates. However, the resulting resistive impedance achieved will be quite

a bit lower than the impedance of a resonant monopole, likely requiring further impedance matching. In addition

to a lower radiation resistance, the reactance becomes higher as the antenna size is reduced, and the resonant

circuit formed by the antenna and the tuning coil has a Q factor that rises and eventually causes the bandwidth

of the antenna to be inadequate for the signal being transmitted. This is the major factor that sets the size of

antennas at 1 MHz and lower frequencies.

[edit]Current and voltage distribution

The antenna conductors have the lowest feed-point impedance at the resonant frequency where they are just

under 1/4 wavelength long; two such conductors in line fed differentially thus realizes the familiar "half-wave

dipole". When fed with an RF current at the resonant frequency, the quarter wave element contains a standing

wave with the voltage and current largely (but not exactly) in phase quadrature, as would be obtained using a

quarter wave stub of transmission line. The current reaches a minimum at the end of the element (where it has

nowhere to go!) and is maximum at the feed-point. The voltage, on the other hand, is the greatest at the end of

the conductor and reaches a minimum (but not zero) at the feedpoint. Making the conductor shorter or longer

than 1/4 wavelength means that the voltage pattern reaches its minimum somewhere beyond the feed-point, so

that the feed-point has a higher voltage and thus sees a higher impedance, as we have noted. Since that

voltage pattern is almost in phase quadrature with the current, the impedance seen at the feed-point is not only

much higher but mainly reactive.

It can be seen that if such an element is resonant at f0 to produce such a standing wave pattern, then feeding

that element with 3f0 (whose wavelength is 1/3 that of f0) will lead to a standing wave pattern in which the

voltage is likewise a minimum at the feed-point (and the current at a maximum there). Thus, an antenna

element is also resonant when its length is 3/4 of a wavelength (3/2 wavelength for a complete dipole). This is

true for all odd multiples of 1/4 wavelength, where the feed-point impedance is purely resistive, though larger

than the resistive impedance of the 1/4 wave element. Although such an antenna is resonant and works

perfectly well at the higher frequency, the antenna radiation pattern is also altered compared to the half-wave

dipole.

The use of a monopole or dipole at odd multiples of the fundamental resonant frequency, however,

does not extend to even multiples (thus a 1/2 wavelength monopole or 1 wavelength dipole). Now the voltage

standing wave is at its peak at the feed-point, while that of the current (which must be zero at the end of the

conductor) is at a minimum (but not exactly zero). The antenna is anti-resonant at this frequency. Although the

reactance at the feedpoint can be cancelled using such an element length, the feed-point impedance is very

high, and is highly dependent on the diameter of the conductor (which makes only a small difference at the

actual resonant frequency). Such an antenna does not match the much lower characteristic impedance of

available transmission lines, and is generally not used. However some equipment where transmission lines are

not involved which desire a high driving point impedance may take advantage of this anti-resonance.

[edit]Bandwidth

Although a resonant antenna has a purely resistive feed-point impedance at a particular frequency, many (if not

most) applications require using an antenna over a range of frequencies. An antenna's bandwidth specifies the

range of frequencies over which its performance does not suffer due to a poor impedance match. Also in the

case of a Yagi-Uda array, the use of the antenna very far away from its design frequency reduces the

antenna's directivity, thus reducing the usable bandwidth regardless of impedance matching.

Except for the latter concern, the resonant frequency of a resonant antenna can always be altered by adjusting

a suitable matching network. To do this efficiently one would require remotely adjusting a matching network at

the site of the antenna, since simply adjusting a matching network at the transmitter (or receiver) would leave

the transmission line with a poor standing wave ratio.

Instead, it is often desired to have an antenna whose impedance does not vary so greatly over a certain

bandwidth. It turns out that the amount of reactance seen at the terminals of a resonant antenna when the

frequency is shifted, say, by 5%, depends very much on the diameter of the conductor used. A long thin wire

used as a half-wave dipole (or quarter wave monopole) will have a reactance significantly greater than the

resistive impedance it has at resonance, leading to a poor match and generally unacceptable performance.

Making the element using a tube of a diameter perhaps 1/50 of its length, however, results in a reactance at

this altered frequency which is not so great, and a much less serious mismatch which will only modestly

damage the antenna's net performance. Thus rather thick tubes are typically used for the solid elements of

such antennas, including Yagi-Uda arrays.

Rather than just using a thick tube, there are similar techniques used to the same effect such as replacing thin

wire elements with cages to simulate a thicker element. This widens the bandwidth of the resonance. On the

other hand, amateur radio antennas need to operate over several bands which are widely separated from each

other. This can often be accomplished simply by connecting resonant elements for the different bands in

parallel. Most of the transmitter's power will flow into the resonant element while the others present a high

(reactive) impedance and draw little current from the same voltage. A popular solution uses so-

called traps consisting of parallel resonant circuits which are strategically placed in breaks along each antenna

element. When used at one particular frequency band the trap presents a very high impedance (parallel

resonance) effectively truncating the element at that length, making it a proper resonant antenna. At a lower

frequency the trap allows the full length of the element to be employed, albeit with a shifted resonant frequency

due to the inclusion of the trap's net reactance at that lower frequency.

The bandwidth characteristics of a resonant antenna element can be characterized according to its Q, just as

one uses to characterize the sharpness of an L-C resonant circuit. However it is often assumed that there is an

advantage in an antenna having a high Q. After all, Q is short for "quality factor" and a low Q typically signifies

excessive loss (due to unwanted resistance) in a resonantL-C circuit. However this understanding does not

apply to resonant antennas where the resistance involved is the radiation resistance, a desired quantity which

removes energy from the resonant element in order to radiate it (the purpose of an antenna, after all!). The Q is

a measure of the ratio of reactance to resistance, so with a fixed radiation resistance (an element's radiation

resistance is almost independent of its diameter) a greater reactance off-resonance corresponds to the poorer

bandwidth of a very thin conductor. The Q of such a narrowband antenna can be as high as 15. On the other

hand a thick element presents less reactance at an off-resonant frequency, and consequently a Q as low as 5.

These two antennas will perform equivalently at the resonant frequency, but the second antenna will perform

over a bandwidth 3 times as wide as the "hi-Q" antenna consisting of a thin conductor.

[edit]Gain

Main article: Antenna gain

Gain is a parameter which measures the degree of directivity of the antenna's radiation pattern. A high-gain

antenna will preferentially radiate in a particular direction. Specifically, the antenna gain, or power gain of an

antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in the direction

of its maximum output, 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. 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.[7]

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.

[edit]Effective area or aperture

Main article: Antenna effective area

The effective area or effective aperture of a receiving antenna expresses the portion of the power of a

passing electromagnetic wave which it delivers to its terminals, expressed in terms of an equivalent area.

For instance, if a radio wave passing a given location has a flux of 1 pW / m2 (10

12 watts per square

meter) and an antenna has an effective area of 12 m2, then the antenna would deliver 12 pW of RF power

to the receiver (30 microvolts rms at 75 ohms). Since the receiving antenna is not equally sensitive to

signals received from all directions, the effective area is a function of the direction to the source.

Due to reciprocity (discussed above) the gain of an antenna used for transmitting must be proportional to

its effective area when used for receiving. Consider an antenna with no loss, that is, one whose electrical

efficiency is 100%. It can be shown that its effective area averaged over all directions must be equal to

2/4, the wavelength squared divided by 4. Gain is defined such that the average gain over all directions

for an antenna with 100% electrical efficiency is equal to 1. Therefore the effective area Aeff in terms of the

gain G in a given direction is given by:

For an antenna with an efficiency of less than 100%, both the effective area and gain are reduced by

that same amount. Therefore the above relationship between gain and effective area still holds. These

are thus two different ways of expressing the same quantity. Aeff is especially convenient when

computing the power that would be received by an antenna of a specified gain, as illustrated by the

above example.

[edit]Radiation pattern

Main article: 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 a plot of the relative field strength of the radio waves emitted by

the antenna at different angles. It is typically represented by a three dimensional graph, or polar plots

of the horizontal and vertical cross sections. The pattern of an ideal isotropic antenna, which radiates

equally in all directions, would look like a sphere. Many nondirectional antennas, such

as monopoles and dipoles, emit equal power in all horizontal directions, with the power dropping off at

higher and lower angles; this is called an omnidirectional pattern and when plotted looks like a torusor

donut.

The radiation of many antennas shows a pattern of maxima or "lobes" at various angles, separated by

"nulls", angles where the radiation falls to zero. This is because the radio waves emitted by different

parts of the antenna typically interfere, causing maxima at angles where the radio waves arrive at

distant points in phase, and zero radiation at other angles where the radio waves arrive out of phase.

In a directional antenna designed to project radio waves in a particular direction, the lobe in that

direction is designed larger than the others and is called the "main lobe". The other lobes usually

represent unwanted radiation and are called "sidelobes". The axis through the main lobe is called the

"principal axis" or "boresight axis".

[edit]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,[8]

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 thestanding 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.

[edit]Efficiency

Main article: Antenna efficiency

Efficiency of a transmitting antenna is the ratio of power actually radiated (in all directions) to the

power absorbed by the antenna terminals. The power supplied to the antenna terminals which is not

radiated is converted into heat. This is usually through loss resistance in the antenna's conductors, but

can also be due to dielectric or magnetic core losses in antennas (or antenna systems) using such

components. Such loss effectively robs power from the transmitter, requiring a stronger transmitter in

order to transmit a signal of a given strength.

For instance, if a transmitter delivers 100 W into an antenna having an efficiency of 80%, then the

antenna will radiate 80 W as radio waves and produce 20 W of heat. In order to radiate 100 W of

power, one would need to use a transmitter capable of supplying 125 W to the antenna. Note that

antenna efficiency is a separate issue from impedance matching, which may also reduce the amount

of power radiated using a given transmitter. If an SWR meter reads 150 W of incident power and 50 W

of reflected power, that means that 100 W have actually been absorbed by the antenna (ignoring

transmission line losses). How much of that power has actually been radiated cannot be directly

determined through electrical measurements at (or before) the antenna terminals, but would require

(for instance) careful measurement of field strength. Fortunately the loss resistance of antenna

conductors such as aluminum rods can be calculated and the efficiency of an antenna using such

materials predicted.

However loss resistance will generally affect the feedpoint impedance, adding to its resistive (real)

component. That resistance will consist of the sum of the radiation resistance Rr and the loss

resistance Rloss. If an rms current I is delivered to the terminals of an antenna, then a power of I2Rr will

be radiated and a power of I2Rloss will be lost as heat. Therefore the efficiency of an antenna is equal

to Rr / (Rr + Rloss). Of course only the total resistance Rr + Rloss can be directly measured.

According to reciprocity, the efficiency of an antenna used as a receiving antenna is identical to the

efficiency as defined above. The power that an antenna will deliver to a receiver (with a

properimpedance match) is reduced by the same amount. In some receiving applications, the very

inefficient antennas may have little impact on performance. At low frequencies, for example,

atmospheric or man-made noise can mask antenna inefficiency. For example, CCIR Rep. 258-3

indicates man-made noise in a residential setting at 40 MHz is about 28 dB above the thermal noise

floor. Consequently, an antenna with a 20 dB loss (due to inefficiency) would have little impact on

system noise performance. The loss within the antenna will affect the intended signal and the

noise/interference identically, leading to no reduction in signal to noise ratio (SNR).

This is fortunate, since antennas at lower frequencies which are not rather large (a good fraction of a

wavelength in size) are inevitably inefficient (due to the small radiation resistance Rr of small

antennas). Most AM broadcast radios (except for car radios) take advantage of this principle by

including a small loop antenna for reception which has an extremely poor efficiency. Using such an

inefficient antenna at this low frequency (5301650 kHz) thus has little effect on the receiver's net

performance, but simply requires greater amplification by the receiver's electronics. Contrast this tiny

component to the massive and very tall towers used at AM broadcast stations for transmitting at the

very same frequency, where every percentage point of reduced antenna efficiency entails a

substantial cost.

The definition of antenna gain or power gain already includes the effect of the antenna's efficiency.

Therefore if one is trying to radiate a signal toward a receiver using a transmitter of a given power,

one need only compare the gain of various antennas rather than considering the efficiency as well.

This is likewise true for a receiving antenna at very high (especially microwave) frequencies, where

the point is to receive a signal which is strong compared to the receiver's noise temperature. However

in the case of a directional antenna used for receiving signals with the intention

ofrejecting interference from different directions, one is no longer concerned with the antenna

efficiency, as discussed above. In this case, rather than quoting the antenna gain, one would be more

concerned with the directive gain which does not include the effect of antenna (in)efficiency. The

directive gain of an antenna can be computed from the published gain divided by the antenna's

efficiency.

[edit]Polarization

Main article: Polarization (waves)

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"[citation needed]

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

iselliptical, 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[citation needed]

so that there will never be a complete

mismatch with signals from other sources.

[edit]Impedance matching

Main article: Impedance matching

Maximum power transfer requires matching the impedance of an antenna system (as seen looking

into the transmission line) to the complex conjugate of the impedance of the receiver or transmitter. In

the case of a transmitter, however, the desired matching impedance might not correspond to the

dynamic output impedance of the transmitter as analyzed as a source impedancebut rather the design

value (typically 50 ohms) required for efficient and safe operation of the transmitting circuitry. The

intended impedance is normally resistive but a transmitter (and some receivers) may have additional

adjustments to cancel a certain amount of reactance in order to "tweak" the match. When a

transmission line is used in between the antenna and the transmitter (or receiver) one generally would

like an antenna system whose impedance is resistive and near the characteristic impedance of that

transmission line in order to minimize the standing wave ratio(SWR) and the increase in transmission

line losses it entails, in addition to supplying a good match at the transmitter or receiver itself.

Antenna tuning generally refers to cancellation of any reactance seen at the antenna terminals,

leaving only a resistive impedance which might or might not be exactly the desired impedance (that of

the transmission line). Although an antenna may be designed to have a purely resistive feedpoint

impedance (such as a dipole 97% of a half wavelength long) this might not be exactly true at the

frequency that it is eventually used at. In some cases the physical length of the antenna can be

"trimmed" to obtain a pure resistance. On the other hand, the addition of a series inductance or

parallel capacitance can be used to cancel a residual capacitative or inductive reactance, respectively.

In some cases this is done in a more extreme manner, not simply to cancel a small amount of residual

reactance, but to resonate an antenna whose resonance frequency is quite different than the intended

frequency of operation. For instance, a "whip antenna" can be made significantly shorter than 1/4

wavelength long, for practical reasons, and then resonated using a so-called loading coil. This

physically large inductor at the base of the antenna has an inductive reactance which is the opposite

of the capacitative reactance that such a vertical antenna has at the desired operating frequency. The

result is a pure resistance seen at feedpoint of the loading coil; unfortunately that resistance is

somewhat lower than would be desired to match commercialcoax[citation needed]

.

So an additional problem beyond canceling the unwanted reactance is of matching the remaining

resistive impedance to the characteristic impedance of the transmission line. In principle this can

always be done with a transformer, however the turns ratio of a transformer is not adjustable. A

general matching network with at least two adjustments can be made to correct both components of

impedance. Matching networks using discrete inductors and capacitors will have losses associated

with those components, and will have power restrictions when used for transmitting. Avoiding these

difficulties, commercial antennas are generally designed with fixed matching elements or feeding

strategies to get an approximate match to standard coax, such as 50 or 75 Ohms. Antennas based on

the dipole (rather than vertical antennas) should include a balun in between the transmission line and

antenna element, which may be integrated into any such matching network.

Another extreme case of impedance matching occurs when using a small loop antenna (usually, but

not always, for receiving) at a relatively low frequency where it appears almost as a pure inductor.

Resonating such an inductor with a capacitor at the frequency of operation not only cancels the

reactance but greatly magnifies the very small radiation resistance of such a loop[citation needed]

. This is

implemented in most AM broadcast receivers, with a small ferrite loop antenna resonated by a

capacitor which is varied along with the receiver tuning in order to maintain resonance over the AM

broadcast band

[edit]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[citation needed]) 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 antenna 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.

[edit]Practical antennas

"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[citation needed]

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 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.

[edit]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 other

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, the angle has been drawn far bigger than in reality. The 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 therefractive 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 antennas 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 subtract 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 differentand more complexif 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[citation needed]

. 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. Indigital 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 Reand 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

ordinarycoupled 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 2with antenna 1 removedto 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

is the impedance of antenna

is the mutual impedance between antennas and

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.

Microstrip antenna From Wikipedia, the free encyclopedia

It has been suggested that this article or section be merged with Patch antenna.

(Discuss) Proposed since January 2012.

In telecommunication, there are several types of microstrip antennas (also known as printed antennas) the

most common of which is the microstrip patch antenna or patch antenna.

Contents

[hide]

1 Patch antenna

2 Advantages

3 Rectangular patch

4 Specifications

5 Other types

6 References

7 External links

[edit]Patch antenna

A patch antenna is a narrowband, wide-beam antenna fabricated by etching the antenna element pattern in

metal trace bonded to an insulating dielectric substrate, such as a printed circuit board, with a continuous metal

layer bonded to the opposite side of the substrate which forms a ground plane. Common microstrip antenna

shapes are square, rectangular, circular and elliptical, but any continuous shape is possible. Some patch

antennas do not use a dielectric substrate and instead made of a metal patch mounted above a ground plane

using dielectric spacers; the resulting structure is less rugged but has a wider bandwidth. Because such

antennas have a very low profile, are mechanically rugged and can be shaped to conform to the curving skin of

a vehicle, they are often mounted on the exterior of aircraft and spacecraft, or are incorporated into mobile

radio communications devices.

[edit]Advantages

Microstrip antennas are relatively inexpensive to manufacture and design because of the simple 2-dimensional

physical geometry. They are usually employed at UHF and higher frequencies because the size of the antenna

is directly tied to the wavelength at the resonant frequency. A single patch antenna provides a maximum

directive gain of around 6-9 dBi. It is relatively easy to print an array of patches on a single (large) substrate

using lithographic techniques. Patch arrays can provide much higher gains than a single patch at little

additional cost; matching and phase adjustment can be performed with printed microstrip feed structures, again

in the same operations that form the radiating patches. The ability to create high gain arrays in a low-profile

antenna is one reason that patch arrays are common on airplanes and in other military applications.

Such an array of patch antennas is an easy way to make a phased array of antennas with dynamic

beamforming ability.[1]

An advantage inherent to patch antennas is the ability to have polarization diversity. Patch antennas can easily

be designed to have vertical, horizontal, right hand circular (RHCP) or left hand circular (LHCP) polarizations,

using multiple feed points, or a single feedpoint with asymmetric patch structures. [2]

This unique property

allows patch antennas to be used in many types of communications links that may have varied requirements.

[edit]Rectangular patch

The most commonly employed microstrip antenna is a rectangular patch. The rectangular patch antenna is

approximately a one-half wavelength long section of rectangular microstrip transmission line. When air is the

antenna substrate, the length of the rectangular microstrip antenna is approximately one-half of a free-

space wavelength. As the antenna is loaded with a dielectric as its substrate, the length of the antenna

decreases as the relative dielectric constant of the substrate increases. The resonant length of the antenna is

slightly shorter because of the extended electric "fringing fields" which increase the electrical length of the

antenna slightly. An early model of the microstrip antenna is a section of microstrip transmission line with

equivalent loads on either end to represent the radiation loss.

[edit]Specifications

The dielectric loading of a microstrip antenna affects both its radiation pattern and impedance bandwidth. As

the dielectric constant of the substrate increases, the antenna bandwidth decreases which increases the Q

factor of the antenna and therefore decreases the impedance bandwidth. This relationship did not immediately

follow when using the transmission line model of the antenna, but is apparent when using the cavity model

which was introduced in the late 1970s by Lo et al.[3]

The radiation from a rectangular microstrip antenna may

be understood as a pair of equivalent slots. These slots act as an array and have the highest directivity when

the antenna has an air dielectric and decreases as the antenna is loaded by material with increasing relative

dielectric constant.

The half-wave rectangular microstrip antenna has a virtual shorting plane along its center. This may be

replaced with a physical shorting plane to create a quarter-wavelength microstrip antenna. This is sometimes

called a half-patch. The antenna only has a single radiation edge (equivalent slot) which lowers the

directivity/gain of the antenna. The impedance bandwidth is slightly lower than a half-wavelength full patch as

the coupling between radiating edges has been eliminated.

[edit]Other types

Another type of patch antenna is the Planar Inverted F Antenna (PIFA) common in cellular phones with built-in

antennas.(The Planar Inverted-F antenna (PIFA) is increasingly used in the mobile phone market. The antenna

is resonant at a quarter-wavelength (thus reducing the required space needed on the phone), and also typically

has good SAR properties. This antenna resembles an inverted F, which explains the PIFA name. The Planar

Inverted-F Antenna is popular because it has a low profile and an omnidirectional pattern. The PIFA is shown

from a side view in Figure 4.) [4]

These antennas are derived from a quarter-wave half-patch antenna. The

shorting plane of the half-patch is reduced in length which decreases the resonance frequency. Often PIFA

antennas have multiple branches to resonate at the various cellular bands. On some phones, grounded

parasitic elements are used to enhance the radiation bandwidth characteristics.

The Folded Inverted Conformal Antenna (FICA)[5]

has some advantages with respect to the PIFA, because it

allows a better volume reuse.

http://www.emtalk.com/mpacalc.php

http://www.emtalk.com/tut_1.htm

http://www.antenna-theory.com/antennas/patches/antenna.php#introduction