Cisco WLAN - Chapter. 07 : Antennas

Ch. 7 - Antennas
Cisco Fundamentals of Wireless LANs version 1.1
Rick Graziani
Cabrillo College

Rick Graziani graziani@cabrillo.edu 2
Overview
• Everything about antenna choice involves a tradeoff.
• If maximum range is desired, coverage must be traded.
• With a directional antenna, the same amount of power reaches the
antenna, but the antenna design can reflect and direct the RF energy
in tighter and stronger waves, or wider and less intense waves, just as
with a flashlight.

Introduction
• Antennas generally fall into two categories:
– Directional
– Omnidirectional
Radiate RF energy predominantly in
one direction.
Radiate RF energy equally in all
horizontal directions.

Introduction
• The antennas used for WLANs have two functions:
– Receive:
• This is the sink or terminator of a signal on a transmission
medium.
• In communications, it is a device that receives Information,
control, or other signals from a source.
– Transmit:
• This is the source or generator of a signal on a transmission
medium.

Introduction
• Two way radio communications can take place with:
– FDD (Frequency Division Duplex)
• Full duplex
• A different frequency is used in each direction
• Must allocate two spectrum in two bands, one for each
direction.
– TDD (Time Division Duplex)
• Half duplex
• Uses same channel or frequency, but with alternating periods of
transmitting and listening.

Variables
• Antenna maximum distances are usually expressed in kilometers or
meters.
• The maximum link distance is not easy to solve and is governed by all
of the following:
– Maximum available transmit power
– Receiver sensitivity
– Availability of an unobstructed path for the radio signal
– Maximum available gain, for the antenna(s)
– System losses (such as loss through coax cable runs, connectors,
and so on)
– Desired reliability level (availability) of the link

Variables
More later…

Ranges
• Vendor ranges are usually optimized for best conditions.
• A link distance can exceed standard distances, if consistently higher
error rates are acceptable.

Antenna Bandwidth: Frequency Range
• (There are various definitions of antenna bandwidth.)
• The bandwidth of an antenna is the band of frequencies, over which
it is considered to perform acceptably.
• The wider the range of frequencies a band encompasses, the wider the
bandwidth of the antenna.
• Antennas are ordered pre-tuned by the manufacturer, for use in a
specified band segment.
• The trade-off in designing an antenna for a wider bandwidth is that it
would generally not have as good of performance in comparison to a
similar antenna that is optimized for a narrower bandwidth.

Beamwidth
15 dBi
12 dBi
3 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).
15 dBi

Gain – It’s all relative!
• The gain of any antenna is essentially a measurement of how well that
antenna focuses radiated RF energy, in a particular direction.
• There are different methods for measuring this.
• Cisco is standardizing on dBi to specify gain measurements.
• This method of measuring gain uses a theoretical isotropic antenna
as a reference point.
• Some antennas are rated in dBd, which uses a half -wave dipole type
antenna.
• To convert any number from dBd to dBi, simply add 2.14 to the dBd
number.
dBi = dBd + 2.14
theoretical
isotropic
antenna
Half-wave
dipole antenna

Decibel references
(Review)
• Example:
– 1 mW = .001 Watts
– Using 1 mW as our reference we start at: 0 dB
– Using the dB formula, doubling the milliwatts to 2 mW or .002 Watts
we get +3 dBm
– +10 dBm is 10 times the original 1 mW value or 10 mW
– +20 dBm is 100 times the original 1 mW value or 100 mW

Path-loss (Review)
• Every time the distance from the transmitter to the receiver is
doubled, the signal level is lowered (or increased) by 6 dB 1/4th or
4 times).
– 6 dBm = 4 times or ¼
– 3 dB + 3dB = 2 times + 2 times = 4 times
– -3dB + -3dB = ½ + ½ = ¼
• This is also know as the inverse square law.
• “Signal strength does not fade in a linear manner, but inversely as the
square of the distance. This means that if you are a particular distance
from an access point and you move measure the signal level, and then
move twice a far away, the signal level will decrease by a factor of four.
You move 2x and the signal decreases by 1/4x; hence the inverse
square law. (Move 4x, signal decreases by 1/16x.) In any case, the
fact that exponential measurements are involved in signal strength
measurement is one reason why the use of logarithmic scale of
measurement was developed as an alternative way of representing RF
power.” WildPackets White Paper

Half-wave dipole antenna (FYI)
• Half-wave dipole
– The length from end to end is equal to half the wavelenth at that
frequency.
– “At any angle, the distance of the "surface" from the origin indicates
the intensity of radiated power in that particular direction. The
surface is shaped something like a "bagel", such that zero power is
transmitted along the line of the axis, and the maximum power is
radiated along the "equator" (i.e. the plane orthogonal to the axis).
In the "equatorial plane", however, the antenna is omnidirectional -
that is to say it radiates energy uniformly in all directions.”
http://www.kingston.ac.uk/~ku12881/bcomms/lect3.htm
Half-wave
dipole antenna

Other decibel references besides mW
• dB dipole (dBd) - This refers to the gain an antenna has, as
compared to a dipole antenna at the same frequency.
– A dipole antenna is the smallest, least gain practical antenna that
can be made.
• dB isotropic (dBi) - This refers to the gain a given antenna has, as
compared to a theoretical isotropic, or point source, antenna.
– Unfortunately, an isotropic antenna cannot exist in the real world,
but it is useful for calculating theoretical coverage and fade areas.
– A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna.
– For example, a simple dipole antenna has a gain of 2.14 dBi or 0
dBd.
From Ch. 3

EIRP - Effective
Isotropic Radiated
Power
• EIRP – Effective Isotropic Radiated Power
– The actual power transmitted by a radio connected to an antenna.
– EIRP takes the gain of an antenna in units of dBi, relative to an
isotropic antenna, plus the net power offered by the transmitter to
the antenna.
– Measured in dBi
• ERP (Effective Radiated Power)
– Same as EIRP but with gain expressed relative to a dipole antenna.
– Measured in dBd

Other decibel references besides mW
• Effective Isotropic Radiated Power (EIRP) - EIRP is defined as the
effective power found in the main lobe of a transmitter antenna.
– It is equal to the sum of the antenna gain, in dBi, plus the power
level, in dBm, into that antenna.
• Gain - This refers to the amount of increase in energy that an
antenna appears to add to an RF signal.
– There are different methods for measuring this, depending on the
chosen reference point.
– Cisco Aironet wireless is standardized on dBi to specify gain
measurements.
– Some antennas are rated in dBd.
– To convert any number from dBd to dBi, simply add 2.14 to the dBd
number.
From Ch. 3

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.

Gain
• Antennas have gain in particular directions
• Direction other than the main intended radiation pattern,
are typically related to the main lobe gain

Cisco Aironet 802.11b Antennas
• FCC requires that ALL antennas sold by spread spectrum vendor be
certified with the radio they are to be sold with
• All Cisco Aironet 802.11b supplied cables, RF devices and antennas
have reverse polarity TNC (RP-TNC) connectors
• Cisco Aironet supplied antennas meet all FCC rules Wide variety of
802.11b antennas for most applications

Cisco Aironet 802.11a Antennas
• FCC requires that all radios utilizing the UNII-1 Band (5.15 GHz – 5.25 GHz)
must have non-removable or integrated antennas
• FCC allows radios utilizing the UNII-2 Band (5.25 GHz – 5.35 GHz) to have
external or removable antennas
• The Cisco Aironet 802.11a radios utilize both UNII-1 and UNII-2 bands,
therefore cannot have external or removable antennas
• Cisco 802.11a antennas are integrated into the radio module
• Cisco 1400 radios utilize UNII-3 bands, therefore have external or
removable antennas

Polarization
• Polarization is the physical orientation of the element on
the antenna that actually emits the RF energy.
• An omnidirectional antenna is usually a vertically
polarized antenna.
• All Cisco antennas are set for vertical polarization.

Using different Antennas
• The antennas for both ends of a link do not need to be the same size
or type.
• In some cases, the antenna mounting arrangements at one end of a
link may only be able to physically support a relatively small antenna.
• The link may require a larger antenna at the other end to provide the
needed antenna gain for the path length.
• On the other hand, a high-gain, narrow-pattern antenna may be
needed at one end in order to avert an interference problem, which
may not be a concern at the other end.
• If two antennas have different gains, it does not matter which antenna
is at which end, except in consideration of mounting or interference
issues.
• Remember that even though the two antennas for a link may look very
different from each other, they must have the same polarization for the
link to work properly.

Radiation patterns
• Imagine pressing in the top and bottom of a balloon.
• This causes the balloon to expand in an outward direction, which
covers more area in the horizontal pattern.
• It also reduces the coverage area above and below the balloon.
• This yields a higher gain, as the balloon, which represents the antenna,
appears to extend to a larger horizontal coverage area.

Space Diversity
• With space diversity, the receiver of a microwave radio accepts
signals from two or more antennas that are spaced apart by many
wavelengths.
• The signal from each antenna is received and then simultaneously
connected to a diversity combiner.
• Depending upon the design, the function of the combiner is either to
select the best signal from its inputs or to add the signals together.

Frequency Diversity
• With frequency diversity, the information signal is simultaneously
transmitted by two transmitters operating at two different frequencies
• If the separation in frequencies of the two transmitters is large, the
frequency selective fading will have low probability of affecting both
paths to the same extent.
• This will improve the system performance
• Access points can have two antennas attached to them.
– These two antennas are for diversity in signal reception, not to
increase coverage.

Omni-directional Antennas
• An omni-directional antenna is designed to provide a 360 degree
radiation pattern.
• This type of antenna is used when coverage in all directions from the
antenna is required.
• Omni-directional antennas come in many different styles and shapes.
• Most operated in the 2.4 GHz ranges, whereas a few operate in the 5
GHz range.
• Omni-directional antennas include dipoles, mast mount, pillar, and
patch antennas.
• The standard 2.14 dBi "Rubber Duck" is the most commonly used
omni-directional antenna.

Dipole Antenna Radiation Pattern
Top View
(H)
Side View
(E)
• The radiation patterns will be shown
as a horizontal, looking down (H-
plane) radiation pattern, an
Elevation, looking across (E-
plane), or Vertical radiation pattern,
or both.
Side View
(E)
Side View (E)

2.2 dBi Dipole “rubber duck” antenna(s)
(AIR-ANT4941)
• Indoor diversity dipole
antennas with a base are
designed to extend the range
of Aironet LMC client adapters
and has two MMCX (2)
connectors instead of the RP-
TNC connector.

Cisco 2.2 dBi ceiling mount diversity patch
antenna
Side View (E Plane)
Vertical Radiation

Cisco 5.2 dBi ceiling mount omni-
directional antenna
Side View (E Plane)
Vertical Radiation

5.2 dBi Mast Mount Vertical
Omnidirectional indoor/outdoor antenna
Side View (E Plane)
Vertical Radiation

12 dBi Omnidirectional antenna (outdoor
only)
Side View (E Plane)
Vertical Radiation

5 GHz outdoor wireless bridge 9-dBi
omnidirectional antenna
Side View (E Plane)
Vertical Radiation
Top View
(H)
Used with
1400 Bridge

5.14 dBi Pillar Mount Diversity Omni
Side View (E Plane)
Vertical Radiation
Designed to be
mounted to the side
of a pillar

Integrated antennas - 1100

Integrated Antennas - 1200

Directional Antennas
• Directional antennas do not offer any added power to the signal, and
instead simply redirects the energy it received from the transmitter.
• By redirecting this energy, it has the effect of providing more energy in
one direction, and less energy in all other directions.
• As the gain of a directional antenna increases, the angle of radiation
usually decreases, providing a greater coverage distance, but with a
reduced coverage angle.
• Directional antennas include Yagis, patch antennas, and parabolic
dishes.
• Parabolic dishes have a very narrow RF energy path and the installer
must be accurate in aiming these at each other.

Cisco 6 dBi patch antenna
Side View (E Plane)
Vertical Radiation

6 dBi diversity patch antenna
Indoor/outdoor
antenna with two RP-
TNC connectors.
It is similar to the
above patch, but
providing diversity
antennas in the same
package for areas
where multipath
problems exist
Side View (E Plane)
Vertical Radiation

Cisco 8.5 dBi antenna
Side View (E Plane)
Vertical Radiation

13.4 dBi Yagi
(outdoor/indoor)
• The Yagi is constructed of at least three elements, which are metal
bars that supplement the wave energy transmitted.
• In a Yagi antenna, there is at least one driven element, one reflector
element, and usually one or more director elements.
• The Yagi antenna is also known as a linear end-fire antenna or a Yagi-
Uda array, has a linear array of parallel dipoles.
• Yagi antennas are directional and designed for long distance
communication.
Linear array of parallel dipoles

13.4 dBi Yagi (outdoor/indoor)
• The Cisco Yagi provides 13.5 dBi of gain and features a
range of up to 10 km (6.5 miles) at 2 Mbps, and 3.2 km (2
miles) at 11 Mbps.
• Most Yagi antennas are mounted with U-bolts, to a sturdy
mast.
Top View (H Plane)
Horizontal Radiation

21 dBi Parabolic Dish
• Distances of up to 40 km (25 miles) may be possible.
• It is important to evaluate how well the dish will withstand icy conditions
and high winds.
• Equally important is the sturdiness of the mast and tower the antenna
will be mounted on.
• The Cisco high gain parabolic dish is designed to be used as a bridge
antenna between two networks or for point-to-point communications
Side View (E Plane)
Vertical Radiation

5 GHz 28-dBi dish antenna antenna
• Operates in the UNII-3 band (5725 to 5825 MHz)
• Can be extended up to 12.9 miles (20.7 kilometers) at 54 Mbps.
Side View (E Plane)
Vertical Radiation
Top View (H Plane)

9.5-dBi sector antenna
• Used with the Cisco Aironet 1400 Series Outdoor Wireless Bridge
• The antenna is not compatible with other Cisco Aironet radio products
operating in the 5-GHz frequency band.
Side View (E Plane)
Vertical Radiation
Top View (H Plane)

Antenna Cables
• It might be possible to use existing coaxial cable. This determination
will depend on the quality of the cable and whether it meets the
following three specifications:
– Impedance must be 50 ohms.
– Total loss at 400 MHz, for the entire run length, must be 12 dB or
less.
– The cable center conductor size must be #14 AWG, or larger.

Cable loss
• The amount of energy lost in the cable is called cable loss.
• The use of coaxial cable to carry RF energy, always results in some loss of
signal strength.
• The amount of loss depends on the four factors below:
– Length - Long cables lose more power than short cables.
– Thickness - Thin cables lose more power than thick cables.
– Frequency - Lower frequencies of 2.4 GHz lose less power than higher
frequencies of 5 GHz, as shown in Figure .
– Cable materials - Flexible cables lose more power than rigid cables.
• Cable loss does not depend upon which direction the signal travels.
Transmitted signals lose the same percentage of strength as received signals.
• Lost energy is wasted as heat.
• Interestingly, the low power levels of WLANs make cable heating almost
undetectable.

Cable connectors and splitters
• Connector
– Cisco antennas use the Reverse-polarity TNC (RP-TNC) connector.
• Splitters
– A splitter allows a signal to be used with two antennas at once.
– Using two antennas with a splitter may provide more coverage.
– Using a splitter adds approximately 4 dB of loss.

Amplifiers
• FCC has laws that limit the use of amplifiers with a WLAN.
• An amplifier may only be used, if it is sold as part of a system.
• This means that the AP, amplifier, extension cable, and antenna are all
sold as a system.
• These laws help to ensure that amplifiers are tested with certain
products and legally marketed and sold.
• Be aware of the local laws and of other systems in the area, which may
be affected by an amplifier.

Lightning arrestor
• A lightning arrestor is designed to protect WLAN devices from static
electricity and lightning surges.
• It is similar in function to a safety valve on a steam boiler.
• A lightning arrestor prevents energy surges from reaching the
equipment by shunting the current to the ground.

Link Engineering and RF Path
Planning

Path Considerations
• Radio line of sight
• Earth bulge
• Fresnel zone
• Antenna and cabling
• Data rate

Line of Sight
• The following obstructions might obscure a visual link:
– Topographic features, such as mountains
– Curvature of the Earth
– Buildings and other man-made objects
– Trees
Line of sight!

Tools
• The following tools can be helpful in making an accurate alignment:
– Balloon - The tether should be marked at three meter (ten feet)
intervals, so a height can be established. This value will help
determine the overall height of the tower or mast needed.
– Binoculars or a telescope - These are needed for the more
distant links. Remember that the balloon must be visible from the
remote site.
– GPS - For very distant radio links, this tool allows the installer to
aim the antennas in the correct direction.
– Strobe light - This can be used instead of the balloon. Use this at
night to determine where to align the antenna and at what height.

Rick Graziani graziani@cabrillo.edu
Longer Distances
• Line of Sight disappears at 6 miles (9.7 Km) due
to the earth curve

Fresnel
Zone
• The Fresnel zone is an elliptical area immediately surrounding the
visual path.
• It varies, depending on the length of the signal path and the frequency
of the signal.
• The Fresnel zone can be calculated, and it must be taken into account
when designing a wireless link.

Fresnel Zone
• Obstructions that can interfere with visual line of sight can also
interfere with radio line of sight.
• But one must also consider the Fresnel effect.
• If a hard object, such as a mountain ridge or building, is too close to
the signal path, it can damage the radio signal or reduce its strength.
• This happens even though the obstacle does not obscure the direct,
visual line of sight.
• The Fresnel zone for a radio beam is an elliptical area immediately
surrounding the visual path.
• It varies in thickness depending on the length of the signal path and the
frequency of the signal.
• The necessary clearance for the Fresnel zone can be calculated, and it
must be taken into account when designing a wireless links.

Fresnel Zone
• As shown in the picture above, when a hard object protrudes into the
signal path within the Fresnel zone, knife-edge diffraction can deflect
part of the signal and cause it to reach the receiving antenna slightly
later than the direct signal.
• Since these deflected signals are out of phase with the direct signal,
they can reduce its power or cancel it out altogether.
• If trees or other 'soft' objects protrude into the Fresnel zone, they
can attenuate (reduced the strength of) a passing signal.
• In short, the fact that you can see a location does not mean that you
can establish a quality radio link to that location.

Improving Fresnel Effect
Raise the antenna
New structure
Existing structure
Different mounting point
Remove trees

Total Distance
Fresnel @ 60% (Value “F”)
Earth Curvature (Value “C”)
Antenna
Height
(Value “H”)
Site to Site Fresnel Zone
• Antenna Height
– Fresnel zone consideration
– Line-of-Sight over 25 miles (40 Km) hard to implement
• Earth curvature becomes a concern for links longer than 11 km (7 miles).
• Line of sight disappears at 25 km (16 miles).
• Therefore, the curvature of the Earth must be considered when determining the
antenna mounting height.

Site survey and path profiling

Alignment and
interference
• When aligning antennas, be sure that the two antennas for the link are
not cross-polarized.
• Next, ensure that each antenna is pointed or aligned to maximize the
received signal level.
• A signal strength tool is provided, which gives a reading of the received
signal level.
• At one end of the link at a time, the antenna pointing direction is
carefully adjusted to maximize or peak the reading on the signal-
indicator tool.
• After this is done for both ends, it is very important to obtain the actual
received signal level, in dBm.

Antenna Mounting

Mounting (Cont.)

Interference
• Carrier Detect Test (Spectrum Analyzer)
– Built into Bridge
– Run from Console Menu

Antenna Installation
Towers and antennas
may require permits and
must meet local
regulations

Antenna Installation (cont.)
• Antenna Alignment Tool
Id Name Address Signal Strength Signal Quality
18 Cisco Bridge #1 00409644fd35 100% -10 dBm 100%

Antenna Installation (cont.)
• Aironet Client Utility
• Site Survey Utility for
antenna alignment

Examples from EtherDesigns.com

Ladder Safety

Summary

Non-Cisco Antenna

Other Vendors getting in the market

It’s important to keep your identity secret

Distance Calculations for Bridges

Cisco WLAN - Chapter. 07 : Antennas

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