Antennas – Part 1
Antenna Characteristics and
Line of Sight Paths
Cisco Fundamentals of Wireless LANs version 1.1
Rick Graziani
Cabrillo College
Acknowledgements
• Thanks Jack Unger and his
•
•
•
book Deploying License-Free
Wireless Wide-Area Networks
Published by Cisco Press
ISBN: 1587050692
Published: Feb 26, 2003
Rick Graziani graziani@cabrillo.edu
2
Antenna Directivity
• Antennas radiate wireless power
– Accept wireless signal energy from the transmission line connected
to a transmitter
– Launch that wireless energy into free-space
Rick Graziani graziani@cabrillo.edu
3
Antenna Directivity
• Antennas focus wireless energy like a flashlight reflector
•
(focusing element) focuses light from a flashlight bulb.
Without the focusing element, the bulb radiates light
energy in all direction.
– No direction receives more light than any other
direction.
Rick Graziani graziani@cabrillo.edu
4
Antenna Directivity
Theoretical Isotropic Antenna
• Light energy from an unfocused flashlight bulb is similar to the wireless
•
energy radiated from a theoretical isotropic antenna.
Like a light bulb, an isotropic antenna radiates wireless energy
equally in all directions and does not focus the energy in any single
direction.
Rick Graziani graziani@cabrillo.edu
5
Antenna Directivity
• A flashlight focuses the light into a beam that comes out the front of the
•
•
flashlight.
The flashlight (reflector) does not amplify the power or total amount of
light from the bulb.
The flashlight simply focuses the light so all of it travels in the same
direction.
Rick Graziani graziani@cabrillo.edu
6
Antenna Directivity
• By focusing the light, the flashlight provides more directivity (beam
•
•
•
focusing power).
An antenna provides directivity for the wireless energy that it focuses.
Depending upon the design of the antenna, antennas focus and
radiate their energy more strongly in on favored direction.
When receiving, antennas focus and gather energy from their
favored direction and ignore most of the energy arriving from all other
directions.
Rick Graziani graziani@cabrillo.edu
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Antenna Radiated Patterns
Top
View
Main Lobe
Front
Back
Null
Side Lobes
• Antennas exhibit directivity by radiating most of their power in one
direction.
– Major or Main Lobe – Main direction of the power from the
antenna
– Minor or Side Lobes – Small amount of power in other directions
– Nulls – Where no power is radiated
Rick Graziani graziani@cabrillo.edu
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Antenna Radiated Patterns
Top
View
Main Lobe
Front
Back
Null
Side Lobes
• Antennas provide the same directivity for transmitting and receiving.
– Antennas radiate transmitter power in the favored direction(s) when
transmitting.
– Antennas gather signals coming in from the favored directions(s)
when receiving.
Rick Graziani graziani@cabrillo.edu
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Antenna Radiated Patterns
Patch Antenna
(Directional Antenna)
• When selecting antennas, remember:
– When receiving, antenna directivity not only gathers incoming
signals from the favored direction, but also reduces noise,
interference, and unwanted signals coming in from other directions.
Rick Graziani graziani@cabrillo.edu
10
Antenna Radiated
Patterns
Top View (H)
Side View (V)
Dipole Antenna
(Omnidirectional Antenna)
• An omnidirectional antenna radiates equally well in all horizontal
•
directions around the main lobe, surrounding the antenna like a donut.
More later…
Rick Graziani graziani@cabrillo.edu
11
Antenna Gain
Like a flashlight, there is always a tradeoff
between gain, which is comparable to
brightness in a particular direction, and
beamwidth, which is comparable to the
narrowness of the beam. (coming)
• Antenna gain – Measurement of the power in the main lobe of an
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•
•
antenna and comparing that power to the power in the main lobe of a
reference antenna.
Gain - This refers to the amount of increase in energy that an antenna
appears to add to an RF signal.
Measure in dBi or dBd
– dBd – “d” is the gain measured relative to the gain of a dipole
reference antenna.
– dBi – “i” is the gain measure relative to the gain of a theoretical
isotropic antenna.
More later…
Rick Graziani graziani@cabrillo.edu
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Antenna Gain
+21 dBi or about 100
times the signal
strength when
comparing it to an
isotropic antenna
Top View
•
The dBi is a unit measuring how much better the antenna is compared to an
isotropic radiator.
– An isotropic radiator is an antenna which sends signals equally in all
directions (including up and down).
– An antenna which does this has an 0dBi gain.
– The higher the decibel figure the higher the gain.
– For instance, a 6dBi gain antenna will receive a signal better than a 3dBi
antenna.
Rick Graziani graziani@cabrillo.edu
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Antenna Gain
Dipole antenna
• A dBd unit is a measurement of how much better an antenna performs
•
•
against a dipole antenna.
– As a result a dipole antenna has a 0dBd gain.
Note: Wireless power never stops exactly on a sharp line like the lobe
drawings show, but tapers off.
More later…
Rick Graziani graziani@cabrillo.edu
14
Antenna Beamwidth
• Beamwidth – The width of the main beam (main lobe) of an antenna.
– Measures the directivity of an antenna
– The smaller the beamwidth in degrees, the more the antenna
focuses power into its main lobe.
– The more power of the main lobe, the further the antenna can
communicate.
Rick Graziani graziani@cabrillo.edu
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Antenna Beamwidth
15 dBi
-3 dBi
12 dBi
15 dBi
• Beamwidth is a measurement used to describe directional antennas.
• Beamwidth is sometimes called half-power beamwidth.
• Half-power beamwidth is the total width in degrees of the main
radiation lobe, at the angle where the radiated power has fallen below
that on the centerline of the lobe, by -3 dB (half-power).
Rick Graziani graziani@cabrillo.edu
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• Remember, wireless power does not stop and start exactly along a
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•
straight line, but declines gradually with distance.
The smooth outlines of the main lobes show the approximate
intensity of the wireless power at various distances away from the
antenna.
The dotted lines pass through the half-power points – the points on
each side of the center of the main lobe where the wireless power is
one-half as strong as it is at the center of the lobe.
Rick Graziani graziani@cabrillo.edu
17
Line-of-Sight (LOS)
Line of Sight
Attenuated Signal
Diffracted Signal
• When a wireless signal encounters an obstruction, the signal is always
•
•
attenuated and often reflected or diffracted.
It is important to try and obtain a wireless line-of-sight whenever
possible, especially in a wireless WAN environment (outdoor
connections between building or different parts of a campus).
A wireless LOS typically requires visual LOS plus additional path
clearance to account for the spreading of the wireless signal (Fresnel
Zone – coming).
Rick Graziani graziani@cabrillo.edu
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Visual LOS
“I see you!”
“And, I see you!”
1 Mile
1 Mile
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There is a difference between visual LOS and wireless LOS.
This is because of the difference in wavelengths.
The wavelength of visual light is very small.
For example, the wavelength of a green light is only about 1/50,000th
of an inch
• Remember, the wavelength of a 2.4 GHz WLAN signal is about 4.8
Rick inches.
Graziani graziani@cabrillo.edu
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LOS
1 Mile
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•
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•
1 Mile
A lightwave and a wireless wave are similar.
Both are forms of electromagnetic radiation.
Both must obey the same laws of physics as they propagate.
Wireless signals are like lightwaves that you cannot see.
Rick Graziani graziani@cabrillo.edu
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LOS
1 Mile
1 Mile
• The shorter the wavelength of an electromagnetic wave, the less
clearance it needs form objects that it passes as it travels between two
points.
• The less clearance it needs, the closer it can pass to an obstruction
without experience additional loss of signal strength.
•Rick The
distance is known as the Fresnel Zone.
Graziani clearance
graziani@cabrillo.edu
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LOS
1 Mile
1 Mile
• The green light has a shorter wavelength so only needs a fraction of an
•
inch to avoid additional attenuation.
A 2.4 GHz (802.11b/g) wireless signal has a larger Fresnel zone and
needs to clear the building by quite a few feet (about 10 feet in this
example).
Rick Graziani graziani@cabrillo.edu
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Fresnel Zone
• Fresnel zone (pronounced “frA-nel”; the “s” is silent).
• Provides a method for calculating the amount of clearance that a
wireless wave (or light wave) needs from an obstacle to avoid
additional attenuation of the signal.
Rick Graziani graziani@cabrillo.edu
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Fresnel Zone
•
Fresnel Zone = 72.1 * SqrRoot (dist1Mi * dist2Mi / FreqGHz * DistanceMi)
• At least 60% of the calculated Fresnel Zone must clear to avoid
significant signal attenuation.
Rick Graziani graziani@cabrillo.edu
25
19.7
feet
1 Mile
1 Mile
Example:
Diameter = 72.1 * [ SquareRoot (D1 * D2) / FreqGhZ * (D1 + D2) ]
= 72.1 * [ SquareRoot (1 * 1) / 2.4 * (1 + 1) ]
= 72.1 * [ SquareRoot 1 / 2.4 * (2) ]
= 72.1 * [ SquareRoot 1 / 4.8 ]
= 72.1 * [ SquareRoot .208 ]
= 72.1 * .456
= 32.9 feet
60% of FZ = 0.6 (32.9) ft. = 19.7 feet
Rick Graziani graziani@cabrillo.edu
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9.85 feet
60% of FZ = 0.6 (32.9) ft. = 19.7 feet
So the wireless wave must clear the building by one-half of the 19.7
ft. diameter or or 9.85 feet
Rick Graziani graziani@cabrillo.edu
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Fresnel Zone Calculators
• http://www.wisp-router.com/calculators/fresnel.php
• http://www.tuanistechnology.com/education/calculators/fzc.htm
• http://www.firstmilewireless.com/calc_fresnel.html
Rick Graziani graziani@cabrillo.edu
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Antennas – Part 1
Antenna Characteristics and
Line of Sight Paths
Cisco Fundamentals of Wireless LANs version 1.1
Rick Graziani
Cabrillo College