Chapter 02 Radio Frequency

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Chapter 02 Radio Frequency Antenna
Fundamentals(part 2)

• Faculty of Computer Sciense and Engineering

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Basic RF Math
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Units of Power
Watt (W) is a basic unit of power. One watt is
equal to one ampere of current flowing at one
volt. Milliwatt (mW) 1W 1000 mW The reason to
be concerned with milliwatts is because most of
the 802.11 equipment that transmits at power
levels between 1 and 100 mW. For indoor use, it
is recommended that the output power is less than
100 mW. For outdoor WLANs may use more power if
they are providing site-to-site links or are
providing coverage to a large outdoor area. The
FCC limits the total output power from the
antenna to 4 W for point-to-multipoint
applications in the 2.4 GHz ISM band.
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Units of Power
Decibel (dB) The decibel is a comparative
measurement value. It is a measurement of the
difference between two power levels. 1 bel is a
ratio of 101 between two power levels.
Therefore, a power ratio of 20020 is 1 bel
(101) and 20040 is .5 bels (51) and 20010 is
2 bels (201). bels log(P1/P2) decibels
10log(P1/P2) 10xlog(Pout/Pin) The decibel is
relative where the milliwatt is absolute. The
decibel is logarithmic where the milliwatt is
linear.
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dB
The differences between values can become
extremely large or small and more difficult to
deal with. It is easier to say that a 100 mW
signal decreased by 70 decibels than to say that
it decreased to .00001 milliwatts. 10log(100/0.00
001) 70dB
Comparison of Milliwatts and Decibel Change
(relative to 1 mW)
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10s and 3s Rules of RF Math
1. A gain of 3 dB magnifies the output power by
two. 2. A loss of 3 dB equals one half of the
output power. 3. A gain of 10 dB magnifies the
output power by 10. 4. A loss of 10 dB equals
one-tenth of the output power. 5. dB gains and
losses are cumulative.
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10s and 3s Rules of RF Math
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Quiz 1
An RF signal of 2 Watts is applied to a 100-foot
antenna cable, however, only 1 Watt of transmit
power is actually developed at the input of the
transmitting antenna. What is the resulting cable
loss, measured in dB? A. 0.5 dB B. 1 dB C. 3
dB D. 5 dB
Ans C
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Quiz 2
A loss of -10dB yields a power ratio of? A.
13 B. 110 C. 21 D. 101
Ans B
-10dB 10log(P1/P2) ? log(P1/P2) -1 P1/P2
10-1 110
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Quiz 3
A gain of 3dB yields a power ratio of? A. 21 B.
31 C. 101 D. 110
Ans A
3dB 10log(P1/P2) ? log(P1/P2) 3/10 P1/P2
103/10 1.995262 ? 21
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Quiz 4
You have an access point that is transmitting at
50 mW. The signal loss between the access point
and the antenna is 1 dB, and the access point is
using a 5 dBi antenna. Calculate mW output by
antenna. A. 100 mW B. 125 mW C. 150 mW D.
200 mW
Ans B
dB mW 50 10 500 -3 250 -3 125
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Combination of 10s and 3s
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dBm
The dBm represents an absolute measurement of
power where the m stands for milliwatts. dBm
references decibels relative to 1 milliwatt such
that 0 dBm equals 1 milliwatt. The formula to
get dBm from milliwatts is dBm 10xlog(PowermW)
The benefits of working with dBm values instead
of milliwatts is the ability to easily add and
subtract simple decibels instead of multiplying
and dividing often huge and tiny numbers.
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Quiz
In terms of RF power, 1 Watt ______ dBm. A.
3 B. 10 C. 20 D. 30
Ans D
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dBi
The dBi (the i stands for isotropic) represents a
measurement of power gain used for RF antennas.
It is a comparison of the gain of the antenna
and the output of a theoretical isotropic
radiator. An isotropic radiator is an ideal
antenna that we cannot create with any known
technology. This is an antenna that radiates
power equally in all directions. The dBi value
must be calculated against the input power
provided to the antenna to determine the actual
output power in the direction in which the
antenna propagates RF signals.
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dBd
dBi is a calculation of directional gain compared
to an isotropic radiator, dBd is a calculation of
directional gain compared to a dipole
antenna. dBd is a value calculated against the
input power to determine the directional output
power of the antenna. The difference is that a
dBd value is compared with a dipole antenna,
which itself has a gain of 2.14 over an isotropic
radiator. An antenna with a gain of 7 dBd has a
gain of 9.14 dBi. To convert from dBd to dBi,
just add 2.14. To convert from dBi to dBd, just
subtract 2.14. 0 dBd 2.14 dBi.
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SNR
Background RF noise, which can be caused by all
the various systems and natural phenomena that
generate energy in the electromagnetic spectrum,
is known as the noise floor. The power level of
the RF signal relative to the power level of the
noise floor is known as the signal-to-noise ratio
or SNR.
The higher the ratio, the less obtrusive the
background noise is.
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RSSI
The received signal strength indicator is an
arbitrary measurement of received signal strength
defined in the IEEE 802.11 standards. Cisco uses
a range of 0100 in their devices, and most
Atheros-based chipsets use a range of 060. The
RSSI rating is also arbitrarily used to determine
when to reassociate and when to transmit. In
other words, vendors will decide what the lowest
RSSI rating should be before attempting to
reassociate to a basic service set with a
stronger beacon signal
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Link Budget
Link budget is an accounting of all components of
power, gain, loss, receiver sensitivity, and fade
margin. This includes the cables and connectors
leading up the antenna, as well as the antennas
themselves. It also includes free space path
loss. A link budget is used to predict
performance before the link is established.
- Show in advance if it will be acceptable
- Show if one option is better than
another - Provide a criterion to
evaluate actual performance
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Receive Sensitivity
The minimum signal strength needed at the
receiver. The receive sensitivity is not a single
dBm rating it is a series of dBm ratings
required to communicate at varying data
rates. Ex The lowest number in dBm, which is
-94dBm is the weakest signal the radio can
tolerate.
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System Operating Margin
The SOM is the amount of received signal strength
relative to the client devices receive
sensitivity. The SOM link budget, is the
calculation of the amount of RF signal that is
received minus the amount of signal required by
the receiver. Ex If we have a client device with
a receive sensitivity of -94dBm and the card is
picking up the wireless signal at -65dBm, the SOM
is the difference between -94 and -65. Therefore,
the link budget is SOM RS - S where S is the
signal strength at the wireless client device and
RS is the receive sensitivity of the client
device. The resulting SOM is 29dBm. This means
that the signal strength can be weakened by 29dBm
and the link can be maintained.
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Link budget calculation
The receive sensitivity of both bridges is
-94dBm. The calculations are as follows Link
budget calculation 1 100mW 20dBm Link budget
calculation 2 20dBm-3dB7dBi-83dB -59dBm Link
budget calculation 3 (-94dBm) - (-59dBm) 35
dBm SOM 35 dBm
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Fade Margin
By including a few extra dB of strength in the
required link budget, we can provide a link that
will endure longer. This extra signal strength
is fade margin. We do not add to the link
budget/SOM dBm value, but instead we take away
from the receive sensitivity. For example, we
may decide to work off of an absolute receive
sensitivity of -80dBm instead of the -94dBm
supported by the Cisco Aironet card mentioned
earlier. This would provide a fade margin of
14dBm. SOM in this case is 21 dBm.
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Intentional Radiator
The intentional radiator is the point at which
the antenna is connected. The signal originates
at a transmitter and may pass through connectors,
amplifiers, attenuators, and cables before
reaching the antenna. These components amplify or
attenuate the signal, resulting in the output
power at the intentional radiator before entering
the antenna. The FCC sets the rules regarding
the power that can be delivered to the antenna
and radiated by the antenna. The FCC allows 1
watt of output power from the intentional
radiator and 4 watts of antenna output power in a
point-to-multipoint link in the 2.4 GHz ISM band.
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EIRP
The equivalent isotropically radiated power
(EIRP) is the output power from the intentional
radiator plus the directional gain provided by
the antenna. Power radiated out of antenna of a
wireless system
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Example 1
We have a wireless bridge that generates a 100 mW
signal. The bridge is connected to an antenna
using cable that creates 3 dB of signal loss. The
antenna provides 10 dBi of signal gain. In this
example, calculate the IR and EIRP values.
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Solution
10xlog100mW 20 dBm IR 20 dBm - 3 dB 17
dBm EIRP 17 dBm 10 dB 27 dBm
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Quiz 1
An access point is emitting a 100 mW signal that
is connected to a length of cable with a 3dB
loss. If the cable is then connected to a 9dBi
antenna, what is the EIRP from the antenna in
dBm? A. 20 dBm B. 23 dBm C. 26 dBm D. 29 dBm
Ans C
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Quiz 2
An access point that emits a 1000 mW signal is
connected to a cable and its connectors with 10dB
loss. The cable is then connected to a 3dB gain
antenna. What is the resulting in mW from the
antenna? A. 100 mW B. 200 mW C. 300 mW D. 500 mW
Ans B
dB mW 1000 -10 100 3 200
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RF Signal and Antenna Concepts
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Visual LOS - RF LOS
If we can physically see something, it is said to
be in our visual line of sight. This LOS is
actually the transmission path of the light waves
from the object we are viewing (transmitter) to
our eyes (receiver). RF LOS is more sensitive
than visible LOS to interference near the path
between the transmitter and the receiver. More
space is needed for the RF waves to be seen by
each end of the connection. This extra space is
called the Fresnel zone.
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The Fresnel Zone
The Fresnel zones are a theoretically infinite
number of ellipsoidal areas around the LOS in an
RF link.
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Beamwidths
Beamwidth is the measurement of how broad or
narrow the focus of the RF energy is as it
propagates from the antenna along the main lobe.
The main lobe is the primary RF energy coming
from the antenna.
Beamwidth is measured both vertically and
horizontally The beamwidth is a measurement taken
from the center of the RF signal to the points on
the vertical and horizontal axes where the signal
decreases by 3 dB or half power.
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Beamwidths
Beamwidth measurements give the propagation
pattern of an antenna, they are less than perfect
in illustrating the actual areas that are covered
by the antenna. For more useful visual
representations reference Azimuth and Elevation
charts.
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Azimuth and Elevation
Azimuth and Elevation charts provide a
visualization of the antennas propagation
patterns.
The Azimuth chart shows a top-down view of the
propagation path The Elevation chart shows a side
view of the propagation path
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Isotropic Radiator
The isotropic radiator is a fictional device or
concept that cannot be developed using todays
technology. We cannot currently create an
antenna that propagates RF energy equally in all
directions. dBi is a measurement of the gain of
an antenna in a particular direction over the
power level that would exist in that direction if
the RF energy were propagated by an isotropic
radiator. In other words, dBi is a measurement of
the difference between the power levels at a
point in space generated by a real antenna versus
the theoretical isotropic radiator. The Sun is
often used as an analogy of an isotropic radiator.
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Polarization
Antenna polarization refers to the physical
orientation of the antenna in a horizontal or
vertical position. The electric field forms what
is known as the E-plane. The magnetic field forms
what is known as the H-plane. The E-plane
determines the polarization of the antenna, since
it is parallel to the antenna. Therefore, if the
antenna is in a vertical position, it is said to
be vertically polarized. If the antenna is in a
horizontal position, it is said to be
horizontally polarized. Vertical polarization
means that most of the signal is being propagated
horizontally, and horizontal polarization means
that most of the signal is being propagated
vertically.
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Antenna Diversity
Antenna diversity is a feature that allows the
device to receive signals using two antennas and
one receiver. The reason to use antenna
diversity is that a device transmit a signal and
it may arrive at a receiving device from multiple
angles with multiple signal strengths. The
device supporting antenna diversity will look at
the signal that comes into each antenna during
the frame preamble of a single frame and choose
the signal that is best on a frame-by-frame
basis. The best frame preamble will determine
which antenna is used to receive the rest of the
current frame.
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Antenna Diversity
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Antennas Antenna Systems
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Omnidirectional/Dipole Antennas
Omnidirectional antennas, the most popular type
being the dipole antenna, are antennas with a
360-degree horizontal propagation pattern.
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Omnidirectional Antenna Usage
Omnidirectional antennas provide coverage on a
horizontal plane with some coverage vertically
and outward from the antenna. This means they may
provide some coverage to floors above and
below. To reach people farther away horizontally
use higher gain To reach people farther up or
down vertically use lower gain
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Semidirectional Antennas
Semidirectional antennas are antennas that focus
most of their energy in a particular direction.
Patch, Panel, and Yagi are semidirectional
antennas. Patch and panel antennas usually
focus their energy in a horizontal arc of 180
degrees or less, whereas Yagi antennas usually
have a coverage pattern of 90 degrees or less.
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Semidirectional Antennas
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Coverage area of a semi-directional antenna
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Usage of semi-directional antenna
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Highly Directional Antennas
Highly directional antennas are antennas that
transmit with a very narrow beam. These types of
antennas often look like the satellite dish. They
are generally called parabolic dish or grid
antennas. They are mostly used for PtP or PtMP
links.
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Highly Directional Antennas
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Sectorized and Phased-Array Antennas
A sectorized antenna is a high-gain antenna that
works back-to-back with other sectorized
antennas. A phased-array antenna is a special
antenna system that is actually composed of
multiple antennas connected to a single
processor. The antennas are used to transmit
different phases that result in a directed beam
of RF energy aimed at client devices.
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MIMO Antenna Systems
Multiple-Input Multiple-Output (MIMO) can be
described as any RF communications system that
has multiple antennas at both ends of the
communications link being used concurrently. The
proposed 802.11n standard will include MIMO
technology.