ITNW 1351
Fundamentals of Wireless
LANs
Chapter 2
Antennas
Antennas…


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Antennas are most often used to
increase the range of a wireless LAN
system
Proper antenna selection can also
enhance security of a wireless LAN
…are most sensitive to RF signals
whose wavelength is an even multiple
of the antenna’s length (including
fractional multiples – such as ½ or ¼)
Basic Antenna Principles
1. Antennae convert electrical energy
to RF waves in the case of
transmitting antennae
OR
…RF waves into electrical energy in
the case of receiving antennae
2. The physical dimensions (especially
length) of an antenna are directly
related to the frequency at which
the antenna can propagate or
receive waves
Basic Antenna Principles
3. The physical structure of an
antenna is directly related to the
shape of the area in which it
concentrates most of its radiated RF
energy
Generic Categories of RF
antennas…
Omni directional
 Semi-directional
 Highly-directional


Each category has multiple types of
antennas, each having different RF
characteristics and appropriate uses
Omni directional (Dipole)
Antennas…

Most common wireless LAN antenna
is a dipole
• Standard equipment on most access
points
• Radiates energy equally in all directions
around its axis
Omni directional (Dipole)
Antennas…

Radiates in a 360-dgree horizontal
beam
• Sphere = isotropic radiator – sun =
example – only theoretical
• Radiates in all directions around axis,
but does not radiate along the length of
the wire itself
Looks like a donut
 The higher the gain, the more the donut is
squeezed until it looks like a pancake

Omni directional (Dipole)
Antennas…
Dipole Side-View
Coverage Area – Top View
Omni directional (Dipole)
Antennas…
Coverage Area – High-gain
Side View
Coverage Area – Top View
Omni directional (Dipole)
Antennas…

If placed in center of a single floor of
a multistory building, most energy
will be radiated along the length of
that floor, with some sent to the
floors above and below
• High-gain omnis offer more horizontal
coverage area, but vertical coverage is
reduced
• Important consideration when mounting
on high indoor ceiling
Omni directional (Dipole)
Antennas…

…used when coverage in all
directions around the horizontal is
required
• most effective where large coverage
areas are needed around a central point
• commonly used for point-to-multipoint
designs with star topology
• outdoors – should be placed on top of
structure in the middle of the coverage
area
Passive Gain…


Antennas use passive gain
• Total amount of energy emitted by
antenna doesn’t increase – only the
distribution of energy around the
antenna
• Antenna is designed to focus more
energy in a specific direction
Passive gain is always a function of the
antenna (i.e. independent of the
components leading up to the antenna
Passive Gain…



Advantage…
• Does not require external power
Disadvantage…
• As the gain increases, its coverage
becomes more focused
• Highest-gain antennas can’t be used
for mobile users because of their
tight beam
Active gain involves an amplifier
Omni directional Antenna Usage…
Used when coverage is required in all
directions around the horizontal axis
 Most effective when large coverage
areas are needed around a central
point
 Commonly used for point-tomultipoint designs

Omni directional Antenna Usage…


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…2 to 5 dBi
• treat as an isotropic radiator
• Signal above and below center line will
be weaker
…5 to 8 dBi
• Appropriate for mounting above users
• Increase in gain means decreased
vertical tolerance
…8 to 10 dBi
• Very flat
Semi-directional Antennas…
Direct energy from the transmitter
significantly more in one particular
direction
 Often radiate in a hemispherical or
cylindrical coverage pattern
 Have back and side lobes that, if
used effectively, may further reduce
the need for additional access points

Semi-directional Antennas…
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Frequently used types:
• Patch & Panel– flat, designed for wall
mounting

Focus coverage in horizontal arc of 180° or
less
• Yagi – elongated, ribbed, and usually
housed in an enclosure for moisture
protection
Common vertical & horizontal beamwidths
of 90° or less
 30° or less average

Semi-directional Antennas…
Directional Patch
Antenna
Directional
Yagi Antenna
Beamwidth…

Calculated by measuring the number
of degrees off-axis where beam
drops to ½ (3 dB) of strength at the
0° position…
-3 dB
Beamwidth
(degrees)
-3 dB
Beamwidth…
Two vectors must be considered when discussing
an antenna’s beamwidths…
• Horizontal = parallel to Earth
• Vertical = perpendicular to Earth
Vertical
Beamwidth

Horizontal
Beamwidth
Beamwidth…
Antenna
Type
Omni
Patch/Panel
Yagi
Parabolic
Dish
Horizontal
Beamwidth
Vertical
Beamwidth
360°
7° to 80°
30° to 180°
6° to 90°
30° to 78°
14° to 64°
4° to 25°
4° to 21°
Azimuth & Elevation Charts…
Provide a more accurate picture of
antenna’s beamwidth
 Standard way of representing
coverage pattern
 Azimuth = top-down view
 Elevation = side-view

See Text…
Semi-directional Antenna Usage…

Ideally suited for short and medium-range
bridging
• In some cases, semidirectional antennas
provide such long-range coverage that they
eliminate the need for multiple access points in
a building
• Patch or panel typically used on short range
building-to-building & in-building directional
links
• Yagis most often used on short to medium
length building-to-building bridging up to 2
miles (3.3 km)
Highly-Directional Antennas…

High-gain antennas that emit the most
narrow signal beam of any antenna type
• Greatest gain of any of the types

Typically concave, disk-shaped devices
(similar to satellite TV antenna)
• Parabolic dishes
• Some are grids (grid antennas) – provides
good resistance to wind loading

Ideal for long distance, point-to-point
wireless links
Highly-Directional Antenna
Radiation Pattern…
Highly-directional antennas are never
appropriate for mobile users
Highly-Directional Antennas…

These are not for clients usage…
• Used for point-to-point communication
links
Have a very narrow beamwidth and must be
accurately aimed at each other
 May be aimed directly at each other within a
building in order to “blast” through an RF
signal absorbing obstruction

• Can transmit at distances up to 58 km
(about 35 miles)
Basic Antenna Principles…

Line of Sight (LOS) = the apparently
straight line from the transmitter to
the receiver
• Why “apparently” straight?

Remember refraction, diffraction, &
reflection?
• Can be affected by blockage of the
Fresnel Zone
Basic Antenna Principles…

Fresnel* Zone = an area centered on the
visible LOS between the transmitting and
receiving antenna
• It defines an area around the LOS that
can introduce RF signal interference if
blocked
• As an obstacle obstructs the zone,
energy is absorbed and prevented from
getting to the receiver
*frā-něl'
Basic Antenna Principles…

Another way of defining the Fresnel
Zone is a series of concentric
ellipsoid-shaped areas around the
LOS path…
TX
Fresnel
Zone
LOS
RX
Fresnel Zone…
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20 to 40 % blockage introduces little to
no interference into the link
• Best to allow no more that 20 %
blockage
• >40 % means link will be unreliable
Usually not encroached indoors unless
signal is partially or fully blocked
Constantly changes in mobile
environment
• Users dismiss it to simply bad
coverage
Fresnel Zone…
Formula to calculate the 60 % unobstructed
(minimum clear) radius around the visual LOS…
r = 43.3* X√d/4f
r = radius in feet
d = link distance in miles
f = frequency in GHz
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The radius is 60 % smaller than the radius of the
entire zone.
*Substitute 72.2 to calculate the radius of the
Fresnel Zone itself
The beamwidth is NOT a factor in calculating
Basic Antenna Principles…

Antenna Gain = results from focusing
the RF radiation into a tighter beam
which creates a seemingly “brighter”
beam
• Example: radiating at 30 degrees rather
that 360 degrees at the same power –
the beam will radiate farther
Basic Antenna Principles…

Intentional Radiator (IR) = an RF device
specifically designed to generate and
radiate RF signals
• Includes:
 RF device
 All cabling
 All connectors up to but not including
the antenna
• Any reference to power output of the IR
refers to power at the last connector
before the antenna