cwna chapter 4 Antennas
decibels dipole (dBd
) A measurement that compares the antenna gain against that of a dipole antenna.
(EIRP) is the power directed from the antenna. Antenna energy can be focused
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(IR) is the power directed to the antenna. The Equivalent Isotropically Radiated Power
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(noise) is transmit beam forming (TxBF), which uses complex antenna systems
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(perpendicular to the ground) the polarization is vertical, whereas if it is in a horizontal
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90 degrees from the orientation of the antenna. The orientation of the radio waves as
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An amplifier is a device that amplifies or increases the amplitude of an RF
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MIMO antennas, the AP receives multiple copies of the multipath signal on each of
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Rx antennas. Using these different paths improves reliability because of the unlikelhood
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Spatial diversity can increase the reliability of an RF signal by sending the same
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Standing Wave Ratio or VSWR is a measure of how well an electrical load is
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The system operating margin (SOM), or fade margin, is the difference, measured
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This type of antenna also radiates its signal vertically but provides less coverage area
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additional power or additional bandwidth. When a client device that has only one
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advantage of different signaling processing techniques in order to create high throughput.
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also can inadvertently pick up high electrical discharges from a nearby lightning
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and is not an absolute measurement of power, since it is only comparing the
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antenna focuses the energy in one direction, while a highly-directional antenna
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antenna such as an older 802.11a/b/g device) transmits to an 802.11n AP with multiple
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antenna's beamwidth indicates a transmission's width. The Fresnel zone is an elliptical
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area immediately surrounding the visual line of sight for RF transmissions. Each Fresnel zone is an ellipsoidal shape, with the zones varying in thickness depending on
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by channeling them to the ground.
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by that radio to assure that the transmission can be decoded without errors. The Voltage
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can increase performance by sending independent streams of information at the same
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copies using the maximal ratio combining (MRC) algorithm. An option for 802.11n
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coverage area and elevation charts to show the vertical coverage area. An
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device with one input and two or more outputs that divide the power in the input
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dipole antennas and employ antenna diversity to improve the reception. A semidirectional
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from antennas. The link budget is a rough calculation of all known elements of the
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gain against that of a dipole antenna.
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has no energy or power; instead, it is supplied power from the power source to which
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impedance-matched to its source.
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in decibels, between the received signal level and the signal level that is required
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it is connected. Four measurements are used with antennas. An intentional radiator
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its Rx antennas, with each signal at a different phase. The AP then processes those
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link to determine if the signal will have the proper strength when it reaches its destination.
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networks to improve the reliability of transmissions by reducing outside signal interference
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of coverage, APs should be located near the middle of the coverage area, usually
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of power that is "funneled" from an antenna compared to that of an isotropic radiator
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of power. An RF attenuator decreases the RF signal. An RF signal splitter is a small
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or masts. It is recommended that outdoor antenna installation be left to professionals.
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out horizontally in all directions equally and is used extensively in indoor WLANs.
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passive gain with an isotropic radiator. Decibels dipole (dBd) compares the antenna
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position parallel to the ground the polarization is horizontal. Many have two
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radiation chart. These charts are used to create Azimuth charts that represent the horizontal
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range but do not have to be precisely aimed at the receiver. The passive gain (increase)
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secured to the ceiling or high on a wall. Outdoor antennas are often affixed to poles
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sends a narrowly focused signal beam long distances.
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sent in all directions is measured in decibels isotropic (dBi). The dBi is a relative measurement
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signal to multiple outputs. Just as an antenna is designed to pick up RF signals, it
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signal, often necessary to "boost" the strength of a signal to compensate for its loss
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strike. A lightning arrestor limits the amplitude and disturbing interference voltages
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systems have separate radio chains connected to each antenna. MIMO can take
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that all of the paths will be degraded the same. Spatial multiplexing techniques
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the length of the signal path and signal frequency.
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they leave the antenna is known as polarization. If the antenna is in a vertical position
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through the use of high-gain antennas. By contrast, low-gain antennas have a shorter
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time over the same frequencies, resulting in a significant increase in speed without any
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to allow for different directions and beamwidths to be used.
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transmission out from different antennas that will take different paths between Tx and
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vertically than horizontally. That is because the strength of the signal flows out
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wireless LAN as well as wireless WAN communications over the past decade. MIMO
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■ A chart used to illustrate the radiation pattern of an antenna is called an antenna
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■ A technology known as multiple-input multiple-output (MIMO) has revolutionized
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■ An antenna is used to send electromagnetic waves to a receiving device. The antenna
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■ Because WLAN systems use omnidirectional antennas to provide the broadest area
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■ There are several measurements that relate to the performance of RF transmissions
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■ There are three categories of antennas. An omnidirectional antenna radiates its signal
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spatial multiplexing
A MIMO technique of sending independent streams of information at the same time over the same frequencies.
spatial diversity
A MIMO technique of sending the same transmission out from different antennas that will take different paths.
MIMO diversity
A MIMO technique of sending the same transmission out on differentpaths from different antennas.
explicit feedback
A TxBF technique in which the receiver makes a series of computations and sends them to the transmitter, which then uses them to configure how to make the best transmissions.
Azimuth chart
A chart that represents the horizontal coverage area of an antenna.
elevation chart
A chart that represents the vertical coverage area of an antenna.
antenna radiation chart
A chart used to illustrate an antenna's radiation pattern.
amplifier
A device that amplifies or increases the amplitude of an RF signal
RF attenuator
A device that decreases the RF signal and is used when the gain of an antenna did not match the power output of an AP.
RF signal splitter
A device that divides the power in the input signal to multiple outputs.
bidirectional amplifier
A device that increases the RF signal before it is injected into the device that contains or is directly connected to the antenna.
unidirectional amplifier
A device that increases the RF signal level before it is injected into the transmitting antenna.
lightning arrestor
A device that limits the amplitude and disturbing interference voltages by channeling them to the ground.
conductor
A material that allows an electrical current to flow through it.
Voltage Standing Wave Ratio (VSWR)
A measure of how well an electrical load is impedance-matched to its source.
beam width
A measurement of a transmission's width.
ground rod
A metal rod inserted in the earth to ground an antenna.
antennas
A passive conductor used to transmit electromagnetic waves through space.
radio chain
A radio with supporting infrastructure such as devices to amplify the signal or convert an analog signal into a digital signal.
link budget
A rough calculation of all known elements of a link to determine if a signal will have the proper strength when it reaches its destination.
isotropic radiator
A source of RF waves that have the exact same magnitude or properties in all directions.
Multiple-Input Multiple-Output (MIMO)
A system that uses one radio chain for each antenna so that each antenna can simultaneously transmit and receive signals.
intentional radiator (IR)
A system used to create and transmit RF signals as defined by the Federal Communications Commission (FCC).
Cyclic Shift Diversity (CSD)
A technique that sends a normal version of the signal along with a shifted version of the same signal.
RF line of sight
A theoretical straight line between a transmitter and the receiver.
single-input single-output (SISO)
A wireless communication system that uses one radio chain.
Space Time Block Coding (STBC)
An 802.11n option that sends a redundant copy of part or all of the transmited signal on an unused antenna.
variable-loss attenuator
An RF attenuator that allows the user to set the amount of loss.
fixed-loss attenuator
An RF attenuator that limits the RF power by a set amount.
dipole
An antenna consisting of a single stretched wire with connection in the middle.
sectorized antenna
An antenna that divides the coverage area into different sectors and gives each sector its own antenna.
semi directional antenna
An antenna that focuses energy in one direction.
omnidirectional antenna
An antenna that radiates its signal out horizontally in all directions equally.
highly-directional antenna
An antenna that sends a narrowly focused signal beam long distances.
Fresnel zone
An elliptical area immediately surrounding the visual line of sight of an RF transmission.
transmit beam forming (TxBF)
An option for reducing outside signal interference by using complex antenna systems to allow for different directions and beamwidths.
visual line of sight
An unobstructed straight line between objects.
high-gain antennas
Antennas that have longer ranges and higher signal quality thanlow-gain antennas yet must be aimed precisely in a particular direction.
low-gain antennas
Antennas with a shorter range than high-gain antennas that do not have to be precisely aimed at the receiver.
implicit feedback
Information that is computed by the receiver and sent back to the transmitter for use in antenna configuration.
HT (MIMO) High Throughput Multiple-Input
Multiple-Output for IEEE 802.11n WLANs that utilize a radio chain for each antenna.
return loss
The VSWR as measured in dB.
antenna diversity
The ability of an access point to examine multiple copies of a received transmission and then select the best signal.
transmit diversity
The ability of an access point to transmit on the antenna that most recently received the strongest incoming signal
plenum
The air-handling space above drop ceilings that is used to circulate and handle air in a building.
maximal ratio combining (MRC)
The algorithm a MIMO AP uses when it receives multiple copies of a signal from a non-MIMO device.
Equivalent (also called Effective) Isotropically Radiated Power (EIRP)
The amount of power that a theoretical isotropic radiator can generate.
fade margin
The difference between the received signal level and the signal level that is required by that radio to assure that the transmission can be decoded without errors.
system operating margin (SOM)
The difference between the received signal level and the signal level that is required by that radio to assure that the transmission can be decoded without errors.
active gain
The gain in which additional power was sent to the antenna from the power source.
passive gain
The gain in which no additional power is added.
polarization
The orientation of radio waves as they leave an antenna.
decibels isotropic (dBi)
The passive gain of power that is funneled from an antenna compared to that of an isotropic radiator sent in all directions.
switching
The process of choosing which antenna reception to accept.
sensitivity
The signal strength needed for a good reception.
antenna
array Multiple antennas that can be customized to send an optimal signal.
