RF Signal and Antenna

1
RF Signal and Antenna
Dr. Tahseen Al-Doori
2
Objectives

• Active and Passive Gain
• Azimuth and Elevation Chart
• Beamwidth
• Antenna Types
• Visual line of sight
• RF line of sight
• Fresnel Zone

3

• Antenna Polarization
• Antenna Diversity
• Multi-Input-Multi-Output (MIMO)

4

• The installation of antennas has the greatest
  ability to affect whether the communication is
  successful or not.
• Antenna installation can be as easy as placing an
  access point in your office or it can be as
  complex as installing an assortment of
  directional antennas.
• With the proper understanding of antennas and how
  they function, successful planning for and
  installing them in a wireless network will become
  a skillful and rewarding task.

5
Active and Passive Gain

• You can increase the signal that is radiated out
  of the antenna (EIRP), by increasing the output
  of the transmitter, which in turn increase the
  amount of power provided by the antenna
  (Intentional Radiator) and thus the amount of
  power from the antenna (EIRP).
• This increase is referred to as Active Gain.

6

• When power is focused, the amount provided to the
  antenna does not change. It is the antenna acting
  like a lens on a flashlight that increase the
  power output by concentrating the RF signal in a
  specific direction.
• Since the gain was created by shaping or
  directing the signal, and not by increasing the
  overall power, this increase is referred to as
  Passive gain.
• Passive gain is caused by focusing the existing
  power, while the active gain is caused by adding
  more power.

7
Azimuth and Elevation Chart

• There are many antenna types designed for many
  different purposes, just as there are many types
  of lights designed for many purposes.
• It is easy to see the different lights by
  lighting them on, unfortunately, in the case of
  antennas that is not the case.

8

• With the antenna measurements you need to walk
  around the antenna with an RF meter, take
  measurements and then plot it on piece of paper
  to find out the behavior of the RF Signal. This
  is a time consuming task and the results will be
  affected by the interference of many objects.
  Therefore, the manufacturers create azimuth
  charts and elevation charts. Which is known as
  radiation patterns.

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Azimuth and elevation charts
10

• Here are a few statements that will help you
  interpret the radiation charts
• In either chart, the antenna is placed at the
  middle of the chart.
• Azimuth chart H-plane top-down view
• Elevation chart E-plane side view
• The outer ring of the chart usually represents
  the strongest signal of the antenna. The chart
  does not represent distance or any level of power
  or strength. It only represents the relationship
  of power between different points on the chart.

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• One way to think of the chart is to consider the
  way a shadow behaves.
• If you were to move a flashlight closer or
  farther from your hand, the shadow of your hand
  would grow larger or smaller. The size of the
  shadow does not represent the size of the hand.
  The shadow only shows the relationship between
  the hand and the fingers.
• With an antenna, the radiation pattern will grow
  larger or smaller depending upon how much power
  the antenna receives, but the shape and the
  relationships represented by the patterns will
  always stay the same.

12
Beamwidth

• Is the measurement of how broad or narrow the
  focus of an antenna is and is measured both
  horizontally and vertically.
• It is the measurement from the center, or
  strongest point, of the antenna signal to each of
  the points along the horizontal and vertical axes
  where the signal decreases by half power (3 dB),
  as seen in the Fig. below

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• These 3 dB points are often referred to as half
  power points. The distance between the two half
  power points on the horizontal axis is measured
  in degrees, giving the horizontal beamwidth
  measurement. The distance between the two half
  power points on the vertical axis is also
  measured in degrees, giving the vertical
  beamwidth measurement.

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Antenna Types

• There are three main categories of antennas
• Omni-directional radiate RF in a fashion similar
  to the way a table or floor lamp radiates light.
  They are designed to provide general coverage in
  all directions.
• The small rubber dipole antenna, often referred
  to as a rubber duck antenna, is the classic
  example of an omni-directional antenna and is the
  default antenna of most access points.
• The closest thing to an isotropic radiator is the
  omni-directional dipole antenna.

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• An easy way to explain the radiation pattern of a
  typical omni-directional antenna is to hold your
  index finger straight up (this represents the
  antenna) and place a donut on it as if it were a
  ring (this represents the RF signal).
• If you were to slice the Donut in half
  horizontally, as if you were planning to spread
  butter on it, the cut surface of the Donut would
  represent the azimuth chart, or H-plane, of the
  omni-directional antenna. If you took another
  Donut and sliced it vertically instead,
  essentially cutting the hole that you are looking
  through in half, the cut surface of the Donut
  would now represent the elevation, or E-plane, of
  the omni-directional antenna.

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• We have learned that antennas can focus or direct
  the signal that they are transmitting.
• It is important to know that the higher the dBi
  or dBd value of an antenna, the more focused the
  signal.
• When discussing omni-directional antennas, it is
  not uncommon to initially question how it is
  possible to focus a signal that is radiated in
  all directions.
• With higher-gain omni-directional antennas, the
  vertical signal is decreased and the horizontal
  power is increased.

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• The Fig. below shows the elevation view of three
  theoretical antennas. Notice that the signal of
  the higher-gain antennas is elongated, or more
  focused horizontally. The horizontal beamwidth of
  omni-directional antennas is always 360 degrees,
  and the vertical beamwidth ranges from 7 to 80
  degrees, depending upon the particular antenna.

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• Because of the narrower vertical coverage of the
  higher-gain omni-directional antennas, it is
  important to carefully plan how they are used.
  Placing one of these higher-gain antennas on the
  first floor of a building may provide good
  coverage to the first floor, but because of the
  narrow vertical coverage, the second and third
  floors may receive minimal signal.
• In some installations you may want this in
  others you may not. Indoor installations
  typically use low-gain omni-directional antennas
  with gain of about 2.14 dBi.

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• Antennas are most effective when the length of
  the element is an even fraction (such as 1/4 or
  1/2) or a multiple of the wavelength (?).
• A 2.4 GHz half-wave dipole antenna consists of
  two elements, each 1/4? in length (about 1 inch),
  running in the opposite direction from each
  other.

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• Omni-directional antennas are typically used in
  point-to-multipoint environments.
• The omni-directional antenna is connected to a
  device (such as an access point) that is placed
  at the center of a group of client devices,
  providing central communications capabilities to
  the surrounding clients.
• High-gain omni-directional antennas can also be
  used outdoors to connect multiple buildings
  together in a point-to-multipoint configuration.

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• A central building would have an omni-directional
  antenna on its roof, and the surrounding
  buildings would have directional antennas aimed
  at the central building. In this configuration,
  it is important to make sure that the gain of the
  omni-directional antenna is high enough to
  provide the coverage necessary but not so high
  that the vertical beamwidth is too narrow to
  provide an adequate signal to the surrounding
  buildings.

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• The Fig. below shows an installation where the
  gain is too high. The building to the left will
  be able to communicate, but the building on the
  right is likely to have problems.

23

• Semi-directional radiate RF in a fashion similar
  to the way a wall sconce is designed to radiate
  light away from the wall or the way a street lamp
  is designed to shine light down on a street or a
  parking lot, providing a directional light across
  a large area.
• Semi-directional antennas are used for short- to
  medium-distance communications, with
  long-distance communications being served by
  highly-directional antennas

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• There are three types of antennas that fit into
  the semi-directional category
• Patch
• Panel
• Yagi (pronounced YAH-gee)
• Patch and panel antennas are more accurately
  classified or referred to as planar antennas.

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The exterior of a patch antenna and the internal
antenna element
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• These antennas can be used for outdoor
  point-to-point communications up to about a mile
  but are more commonly used as a central device
  for indoor point-to-multipoint communications.
• It is common for patch or panel antennas to be
  connected to access points to provide directional
  coverage within a building.
• Planar antennas can be used effectively in
  libraries, warehouses, and retail stores with
  long aisles of shelves.

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• Due to the tall, long shelves, omni-directional
  antennas often have difficulty providing RF
  coverage effectively.
• In contrast, planar antennas can be placed high
  on the side walls of the building, aiming through
  the rows of shelves.
• The antennas can be alternated between rows with
  every other antenna being placed on the opposite
  wall. Since planar antennas have a horizontal
  beamwidth of 180 degrees or less, a minimal
  amount of signal will radiate outside of the
  building. With the antenna placement alternated
  and aimed from opposite sides of the building,
  the RF signal is more likely to radiate down the
  rows, providing the necessary coverage.

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• Planar antennas are also often used to provide
  coverage for long hallways with offices on each
  side or hospital corridors with patient rooms on
  each side.
• A planar antenna can be placed at the end of the
  hall and aimed down the corridor. A single planar
  antenna can provide RF signal to some or all of
  the corridor and the rooms on each side and some
  coverage to the floors above and below.
• How much coverage will depend upon the power of
  the transmitter, the gain and beamwidth (both
  horizontal and vertical) of the antenna, and the
  attenuation properties of the building.

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• Using semi-directional antennas indoors often
  reduces reflections, thus minimizing some of the
  negative effects of multipath such as data
  corruption.

30

• Yagi antennas are not as unusual as they sound.
• The traditional television antenna that is
  attached to the roof of a house or apartment is a
  yagi antenna. The television antenna looks quite
  different because it is designed to receive
  signals of many different frequencies (different
  channels) and the length of the elements vary
  according to the wavelength of the different
  frequencies.

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• A yagi antenna that is used for 802.11
  communications is designed to support a very
  narrow range of frequencies, so the elements are
  all about the same length.
• Yagi antennas are commonly used for short- to
  medium-distance point-to-point communications of
  up to about 2 miles, although high-gain yagi
  antennas can be used for longer distances.

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The exterior of a yagi antenna and the internal
antenna element
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• Another benefit of semi-directional antennas is
  that they can be installed high on a wall and
  tilted downward toward the area to be covered.
• This cannot be done with an omni-directional
  antenna without causing the signal on the other
  side of the antenna to be tilted upward. Since
  the only RF signal that radiates from the back of
  a semi-directional antenna is incidental, the
  ability to aim it vertically is an additional
  benefit.

34

• Highly directional radiate RF in a fashion
  similar to the way a spotlight is designed to
  focus light on a flag or a sign. Each type of
  antenna is designed with a different objective in
  mind.
• Highly-directional antennas are strictly used for
  point-to-point communications, typically to
  provide network bridging between two buildings.
  They provide the most focused, narrow beamwidth
  of any of the antenna types.

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• There are two types of highly-directional
  antennas
• Parabolic dish and grid antennas.
• The parabolic dish antenna is similar in
  appearance to the small digital satellite TV
  antennas that can be seen on the roofs of many
  houses.
• The grid antenna resembles the rectangular grill
  of a barbecue, with the edges slightly curved
  inward. The spacing of the wires on a grid
  antenna is determined by the wavelength of the
  frequencies that the antenna is designed for.

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• Because of the high gain of highly-directional
  antennas, they are ideal for long-distance
  communications as far as 35 miles (58 km). Due to
  the long distances and narrow beamwidth,
  highly-directional antennas are affected more by
  antenna wind loading, which is antenna movement
  or shifting caused by wind. Even slight movement
  of a highly-directional antenna can cause the RF
  beam to be aimed away from the receiving antenna,
  interrupting the communications. In high-wind
  environments, grid antennas, due to the spacing
  between the wires, are less susceptible to wind
  load and may be a better choice.

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• Another option in high-wind environments is to
  choose an antenna with a wider beamwidth.
• In this situation, if the antenna were to shift
  slightly, due to its wider coverage area, the
  signal would still be received. No matter which
  type of antenna is installed, the quality of the
  mount and antenna will have a huge effect in
  reducing wind load.

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Phased Array

• A phased array antenna is actually an antenna
  system and is made up of multiple antennas that
  are connected to a signal processor.
• The processor feeds the individual antennas with
  signals of different relative phases, creating a
  directed beam of RF signal aimed at the client
  device.
• Because it is capable of creating narrow beams,
  it is also able to transmit multiple beams to
  multiple users simultaneously. Phased array
  antennas do not behave like other antennas since
  they can transmit multiple signals at the same
  time. Because of this unique capability, they are
  often regulated differently by the local RF
  regulatory agency.

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Sector Antennas

• Sector antennas are a special type of high-gain,
  semi-directional antennas that provide a
  pie-shaped coverage pattern.
• These antennas are typically installed in the
  middle of the area where RF coverage is desired
  and placed back to back with other sector
  antennas. Individually, each antenna services its
  own piece of the pie, but as a group, all of the
  pie pieces fit together and provide
  omni-directional coverage for the entire area.

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• Unlike other semi-directional antennas, a sector
  antenna generates very little RF signal behind
  the antenna (back lobe) and therefore does not
  interfere with the other sector antennas that it
  is working with.
• The horizontal beamwidth of a sector antenna is
  from 60 to 180 degrees, with a narrow vertical
  beamwidth of from 7 to 17 degrees.
• Sector antennas typically have a gain of at least
  10 dBi.

41

• Installing a group of sector antennas to provide
  omni-directional coverage for an area provides
  many benefits over installing a single
  omni-directional antenna.
• To begin with, sector antennas can be mounted
  high over the terrain and tilted slightly
  downward, with the tilt of each antenna at an
  angle appropriate for the terrain it is covering.
  Omni-directional antennas can also be mounted
  high over the terrain however, if an
  omni-directional antenna is tilted downward on
  one side, the other side will be tilted upward.

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• Since each antenna covers a separate area, each
  antenna can be connected to a separate
  transceiver and can transmit and receive
  independently of the other antennas.
• This would provide the capability for all of the
  antennas to be transmitting at the same time,
  providing much greater throughput.
• A single omni-directional antenna would be
  capable of transmitting to only one device at a
  time.
• The last benefit of the sector antennas over a
  single omni-directional antenna is that the gain
  of the sector antennas is much greater than the
  gain of the omni-directional antenna, providing a
  much larger coverage area.

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• Sector antennas are used extensively for cellular
  telephone communications and are starting to be
  used for 802.11 networking.
• Example As you walk or drive around your town or
  city, look for radio communications towers that
  are around your neighborhood. Many of these
  towers have what appear to be rings of antennas
  around them. These rings of antennas are sector
  antennas. If a tower has more than one grouping
  or ring around it, then there are multiple
  cellular carriers using the same tower.

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Visual Line of Sight

• When light travels from one point to another, it
  travels across what is perceived to be an
  unobstructed straight line, known as visual line
  of sight (LOS).
• For all intents and purposes, it is a straight
  line, but due to the possibility of light
  refraction, diffraction, and reflection, there is
  a slight chance that it is not. If you have been
  outside on a summer day and looked across a hot
  parking lot at a stationary object, you may have
  noticed that because of the heat rising from the
  pavement, the object that you were looking at
  seemed to be moving. This is an example of how
  visual LOS is sometimes altered slightly.
• When it comes to RF communications, visual LOS
  has no bearing on whether the RF transmission is
  successful or not.

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RF Line of Sight

• Point-to-point RF communication also needs to
  have an unobstructed line of sight between the
  two antennas.
• So the first step for installing a point-to-point
  system is to make sure that from the installation
  point of one of the antennas, you can see the
  other antenna.
• Unfortunately, for RF communications to work
  properly, this is not sufficient. An additional
  area around the visual LOS needs to remain clear
  of obstacles and obstructions.
• This area around the visual LOS is known as the
  Fresnel zone and is often referred to as RF line
  of sight.

46
Fresnel Zone

• The Fresnel zone (pronounced FRUH-nel the s is
  silent) is an imaginary Rugby ball-shaped area
  that surrounds the path of the visual LOS between
  two point-to-point antennas. The fig. shows an
  illustration of the Fresnel zones rugby
  ball-like shape.

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• Theoretically, there are an infinite number of
  Fresnel zones. The closest ellipsoid is known as
  the first Fresnel zone, the next one is the
  second Fresnel zone, and so on.
• For simplicitys sake, and since they are the
  most relevant for this section, only the first
  two Fresnel zones are displayed in the figure.
  The subsequent Fresnel zones have very little
  effect on the communications.

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• The fig. below illustrates a link that is 1 mile
  long. The top solid line is a straight line from
  the center of one antenna to the other. The
  dotted line shows 60 percent of the bottom half
  of the Fresnel zone. The bottom solid line shows
  the bottom half of the first Fresnel zone. The
  trees are potential obstructions along the path.

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• The typical obstacles that you are likely to
  encounter are trees and buildings.
• It is important to periodically visually check
  your link to make sure that trees have not grown
  into the Fresnel zone or that buildings have not
  been constructed that encroach into the Fresnel
  zone. Do not forget that the Fresnel zone exists
  below, to the sides, and above the visual LOS. If
  the Fresnel zone does become obstructed, you will
  need to either move the antenna (usually raise
  it) or remove the obstacle.

50

• To determine if an obstacle is encroaching into
  the Fresnel, you will need to learn a few
  formulas that will allow you to calculate its
  radius.
• We are only concerned with the following formula
  as it calculate the radius of the first Fresnel
  zone at the mid-point between the two antennas.
  This is the point where the Fresnel zone is the
  largest. This formula is as follows

51
N which Fresnel Zone you are calculating
(usually 1 or 2) d1 distance from one antenna
to the location of the obstacle in miles d2
distance from the obstacle to the other antenna
in miles D total distance between the antennas
in miles ( D d1 d2 ) F frequency in GHz
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Example

• Figure shows a point-to-point communications
  link that is 10 miles long. There is an obstacle
  that is 3 miles away and 40 feet tall. So the
  values and the formula to calculate the radius of
  the Fresnel zone at a point 3 miles from the
  antenna are as follows
• N 1 (for first Fresnel zone)
• d1 3 miles
• d2 7 miles
• D 10 miles
• F 2.4 GHz

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• So if the obstacle is 40 feet tall and the
  Fresnel zone at that point is 67.53 feet tall,
  then the antennas will need to be mounted at
  least 108 feet (40' 67.53' 107.53') above the
  ground to have complete clearance. If we are
  willing to allow the obstruction to encroach up
  to 40 percent into the Fresnel zone, we need to
  keep 60 percent of the Fresnel zone clear. So 60
  percent of 67.53 feet is 40.52 feet. The absolute
  minimum height of the antennas will need to be 81
  feet (40' 40.52' 80.52'). Later, you will
  learn that due to the curvature of the earth, you
  will actually need to raise the antennas even
  higher to compensate for the earths bulge.

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• When highly-directional antennas are used, the
  beamwidth of the signal is smaller, causing a
  more focused signal to be transmitted.
• Many people think that a smaller beamwidth would
  decrease the size of the Fresnel zone.
• This is not the case. The size of the Fresnel
  zone is a function of the frequency being used
  and the distance of the link.
• Since the only variables in the formula are
  frequency and distance, the size of the Fresnel
  zone will be the same regardless of the antenna
  type or beamwidth.

55

• The first Fresnel zone is technically the area
  around the point source, where the waves are in
  phase with the point source signal.
• The second Fresnel zone is then the area beyond
  the first Fresnel zone, where the waves are out
  of phase with the point source signal.
• All of the odd-numbered Fresnel zones are in
  phase with the point source signal, and all of
  the even-numbered Fresnel zones are out of phase.

56
Earth Bulge

• When you are installing long distance
  point-to-point RF communications, another
  variable that must be considered is the curvature
  of the earth, also known as the earth bulge.
• Since the landscape varies throughout the world,
  it is impossible to specify an exact distance for
  when the curvature of the earth will affect a
  communications link.
• The recommendation is that if the antennas are
  more than seven miles away from each other, you
  should take into consideration the earth bulge,
  since after seven miles, the earth itself begins
  to impede upon the Fresnel zone.

57

• The following formula can be used to calculate
  the additional height that the antennas will need
  to be raised to compensate for the earth bulge
• H D2 8
• H height of the earth bulge in feet
• D distance between the antennas in miles

58

• You now have all of the pieces to estimate how
  high the antennas need to be installed. Remember,
  this is an estimate that is being calculated
  since it is assumed that the terrain between the
  two antennas does not vary. You need to know or
  calculate the following three things
• The 60 percent radius of the first Fresnel zone
• The height of the earth bulge
• The height of any obstacles that may encroach
  into the Fresnel zone, and the distance of those
  obstacles from the antenna

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• Taking these three pieces and adding them
  together gives you the following formula, which
  can be used to calculate the antenna height
• H obstacle height earth bulge Fresnel zone
• HOB (D2/8)(72.2x((Nxd1xd2)/(FxD))1/2

60
Example

• Fig. below shows a point-to-point link that spans
  a distance of 12 miles. In the middle of this
  link is an office building that is 30 feet tall.
  A 2.4 GHz signal is being used to communicate
  between the two towers. Can you calculate the
  height of the antenna.

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Antenna Polarization

• Another consideration when installing antennas is
  antenna polarization.
• Although it is a lesser-known concern, it is
  extremely important for successful
  communications. Proper polarization alignment is
  vital when installing any type of antennas.
  Whether the antennas are installed with
  horizontal or vertical polarization is
  irrelevant, as long as both antennas are aligned
  with the same polarization.

62

• Polarization is not as important for indoor
  communications because the polarization of the RF
  signal often changes when it is reflected, which
  is a common occurrence indoors.
• Most access points use low-gain omni-directional
  antennas and they should be polarized vertically
  when mounted from the ceiling. Laptop
  manufacturers build diversity antennas into the
  sides of the monitor. When the laptop monitor is
  in the upright position, the internal antennas
  are vertically polarized as well.

63
Antenna Diversity

• Wireless networks, especially indoor networks,
  are prone to multipath signals.
• To help compensate for the effects of multipath,
  antenna diversity, also called space diversity,
  is commonly implemented in wireless networking
  equipment such as access points (APs).
• Antenna diversity is when an access point has two
  antennas and receivers functioning together to
  minimize the negative effects of multipath.

64

• The Figure shows a picture of an access point
  that uses antenna diversity.

65

• Since the wavelengths of 802.11 wireless networks
  are less than 5 inches long, the antennas can be
  placed very near each other and be effective.
  When the access point senses an RF signal, it
  compares the signal that it is receiving on both
  antennas and uses whichever antenna has the
  higher signal strength to receive the frame of
  data. This sampling is performed on a
  frame-by-frame basis, choosing whichever antenna
  has the higher signal strength.

66

• The access point has to handle transmitting data
  differently than receiving data.
• When the access point needs to transmit data back
  to the client, it has no way of determining which
  antenna the client would receive from the best.
  The way the access point can handle transmitting
  data is to transmit using the antenna that it
  used most recently to receive data. This is often
  referred to as transmission diversity. Not all
  access points are equipped with this capability.

67

• There are many different kinds of antenna
  diversity.
• The most common implementation of antenna
  diversity utilizes one radio card, two
  connectors, and two antennas.
• The question often gets asked why client cards
  seem to have only one antenna.

68

• In reality, PCMCIA client cards typically have
  two diversity antennas encased inside the card.
• Laptops with internal cards have diversity
  antennas mounted inside the laptop monitor.
  Remember that due to the half-duplex nature of
  the RF medium, when antenna diversity is used,
  only one antenna is operational at any given
  time. In other words, a radio card transmitting a
  frame with one antenna cannot be receiving a
  frame with the other antenna at the same time.

69
Multiple Input Multiple Output (MIMO)

• Multiple input multiple output (MIMO, pronounced
  MY-moh) is another, more sophisticated form of
  antenna diversity.
• Unlike conventional antenna systems, where
  multipath propagation is an impairment, MIMO
  systems take advantage of multipath. There is
  much research and development currently happening
  with this technology and thus much disagreement
  about MIMO. There currently are no official or de
  facto standards for the technology.
• MIMO can safely be described as any RF
  communications system that has multiple antennas
  at both ends of the communications link being
  used concurrently..

70

• How the antennas are to be used has not yet been
  standardized. There are multiple vendors
  providing different current and proposed
  solutions. Complex signal processing techniques
  known as Space Time Coding (STC) are often
  associated with MIMO. These techniques send data
  using multiple simultaneous RF signals and the
  receiver then reconstructs the data from those
  signals. The proposed 802.11n standard will
  include MIMO technology

71
Conclusion

• Don't ignore the importance of the antenna.
  Choosing the right antenna and matching its
  characteristics to the best propagation path are
  the two most important factors in setting up a
  communications circuit.
• The weakest link in the communications circuit is
  the wrong propagation path.
• The best transmitter, antenna, and receiver are
  of little use if the propagation path is improper.

72

• The role of a wireless antenna is to direct radio
  frequency (RF) power from a radio into the
  coverage area.
• Different antennas produce different coverage
  patterns, however, and need to be selected and
  placed according to site coverage requirements.

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• Finally, remember that the choices made during
  the antenna selection process can make or break a
  WLAN system, just like the choice of speakers can
  make or break a stereo system.
• For example, a good antenna properly deployed can
  reduce stray RF radiation, by making the signal
  up to 100 times lower outside of the work area,
  and thus much harder to surreptitiously
  intercept.

74
Supplements

• Types of WiFi Antenna Designs
• Radio Frequency and Antenna Behaviors and
  Characteristics