Lesson 2
Radio Frequency Fundamentals
Dr. Tahseen Al-Doori
Objectives
 Define a Radio Frequency Signal
 Define and Describe the Following RF
Characteristics
– Polarity
– Wavelength
– Frequency
– Amplitude
– Phase
Dr. Tahseen Al-Doori
Objectives (Cont.)
 Define and Describe the Following RF
Behaviors
–
–
–
–
–
–
–
–
–
–
Wave Propagation
Absorption
Reflection
Scattering
Refraction
Diffraction
Loss (Attenuation)
Free Space Path Loss
Multipath
Gain (Amplification)
Dr. Tahseen Al-Doori
 To properly design, deploy, and administer an
802.11 wireless network, in addition to
understanding the OSI model and basic
networking concepts, you must broaden your
understanding of many other networking
technologies.
 For instance, when administering an Ethernet
network, you typically need a comprehension of
TCP/IP, bridging, switching, and routing. The skills
to manage an Ethernet network will also aid you
as a Wi-Fi administer because most 802.11
wireless networks act as “portals” into wired
networks. The IEEE only defines the 802.11
technologies at the Physical layer and the MAC
sublayer of the Data-Link layer.
Dr. Tahseen Al-Doori
 In order to fully understand the 802.11
technology, it is necessary to have a clear
concept of how wireless works at the first
layer of the OSI model, and at the heart of
the Physical layer is radio frequency (RF)
communications.
Dr. Tahseen Al-Doori
 if you have a good grasp of the RF characteristics
and behaviors, your skills as a wireless network
administrator will be ahead of the curve.
 Why does a wireless network perform differently in
an auditorium full of people than it does inside an
empty auditorium? Why does the performance of a
wireless LAN seem to degrade in a storage area
with metal racks? Why does the range of a 5 GHz
radio transmitter seem shorter than the range of a
2.4 GHz radio card? These are the type of
questions that can be answered with some basic
knowledge of how RF signals work and perform.
Dr. Tahseen Al-Doori
What Is an RF (Radio Frequency)
Signal?
 An RF signal radiates in a continuous pattern that is
governed by certain properties such as wavelength,
frequency, amplitude, phase, and polarity. Additionally,
electromagnetic signals can travel through mediums of
different materials or travel in a perfect vacuum.
 When an RF signal travels through a vacuum, it moves at
the speed of light, which is approximately 300,000,000
meters per second, or 186,000 miles per second.
 RF signals travel using a variety or combination of
movement behaviors. These movement behaviors are
referred to as propagation behaviors. We will discuss some
of these propagation behaviors later, including absorption,
reflection, scattering, refraction, diffraction, amplification,
and attenuation.
Dr. Tahseen Al-Doori
Identifying Radio Frequency
Characteristics
In every RF signal exists characteristic that are
defined by the laws of physics:
 Polarity
 Wavelength
 Frequency
 Amplitude
 Phase
We will look at each of these in more detail in the
following sections.
Dr. Tahseen Al-Doori
Polarity
 When the movement of the electron flow
changes direction in an antenna,
electromagnetic waves that change and
move away from the antenna are also
produced.
 The waves consist of two component fields:
the electrical (E-field) and the H-field, which
is magnetic.
Dr. Tahseen Al-Doori
 Think of a wave as a physical disturbance that
transfers energy back and forth between these two
fields. These fields are at right angles to each
other, and the transfer of energy between these
fields is known as oscillation.
 Polarization is the vertical or horizontal positioning
of an antenna. The orientation of the antenna
affects the polarity of the signal. The electric field
always resides parallel in the same orientation
(plane) of the antenna element. As shown in
Figure 1, the parallel plane is called the E-plane
and the plane that is perpendicular to the antenna
element is known as the H-plane.
Dr. Tahseen Al-Doori
Fig. 1
Polarity,
E-Plane and
H-plane
Dr. Tahseen Al-Doori
Wavelength
 A wavelength is the distance between the
two successive crests (peaks) or two
successive troughs (valleys) of a wave
pattern. In simpler words, a wavelength is
the distance that a single cycle of an RF
signal actually travels.
Dr. Tahseen Al-Doori
It is very important to understand the following
statement:
 The higher the frequency, the less distance the
propagated wave will travel. AM radio stations
operate at much lower frequencies than wireless
LAN radios. For instance, WSB-AM in Atlanta
broadcasts at 750 KHz and has a wavelength of
1,312 feet, or 400 meters. That is quite a distance
for one single cycle of an RF signal to travel.
 In contrast, some radio navigation satellites
operate at a very high frequency, near 252 GHz,
and a single cycle of the satellite’s signal has a
wavelength of less than .05 inches, or 1.2
millimeters. Figure 2 displays a comparison of
these two extremely different types of RF signals.
Dr. Tahseen Al-Doori
Fig. 2
750 KHz wavelength and 252 GHz wavelength
Dr. Tahseen Al-Doori
 The majority of wireless LAN (WLAN) radio
cards operate in either the 2.4 GHz
frequency range or the 5 GHz range. In
Figure 3, you see a comparison of a single
cycle of the two different frequency WLAN
radio cards.
Fig.3
Dr. Tahseen Al-Doori
 As you can see by these illustrations, the
wavelengths of the different frequency
signals are different because, although each
signal only cycles one time, the waves travel
dissimilar distances.
Wavelength=C/Freq.
Where C=3x10^8
 Because the wavelength property is shorter
in the 5 GHz frequency range, Wi-Fi
equipment using 5 GHz radio cards will
have shorter range and coverage area than
Wi-Fi equipment using 2.4 GHz radio cards.
Dr. Tahseen Al-Doori
Frequency
 Frequency is the number of times a
specified event occurs within a specified
time interval.
 The measurement unit for frequency is Hz.
Dr. Tahseen Al-Doori
Amplitude
 In Fig. 4, you can see that (λ) represents
wavelength and (y) represents the
amplitude.
 The first signal’s crests and troughs have
more magnitude, thus it has more
amplitude.
 The second signal’s crests and troughs
have decreased, and therefore the signal
has less amplitude.
Dr. Tahseen Al-Doori
Amplitude
Dr. Tahseen Al-Doori
 Note that although the signal strength
(amplitude) is different, the frequency of the
signal remains constant.
 A variety of factors can cause an RF signal
to lose amplitude, otherwise known as
attenuation, which we will discuss later in
this chapter in the section “Loss
(Attenuation).”
Dr. Tahseen Al-Doori
Phase
 Phase is not a property of just one RF signal but
instead involves the relationship between two or
more signals that share the same frequency. The
phase involves the relationship between the
position of the amplitude crests and troughs of two
waveforms.
 Phase can be measured in distance, time, or
degrees. If the peaks of two signals with the same
frequency are in exact alignment at the same time,
they are said to be in phase.
Dr. Tahseen Al-Doori
 What is important to understand is the effect that
phase has on amplitude when radio cards receive
multiple signals.
 Signals that have 0 (zero) degrees phase
separation (in phase) actually combine their
amplitude, which results in a received signal of
much greater signal strength, or twice the
amplitude.
 If two RF signals are 180 degrees out of phase
(the peak of one signal is in exact alignment with
the trough of the second signal), they cancel each
other out and the effective received signal strength
is null.
 Depending on the amount of phase separation of
two signals, the received signal strength may be
either cumulative or diminished.
Dr. Tahseen Al-Doori
Identifying RF Behaviors
 As an RF signal travels through the air and
other different mediums, it can move and
behave in different manners.
 RF propagation behaviors include
absorption, reflection, scattering, refraction,
diffraction, loss, free space path loss, multipath, attenuation, and gain.
Dr. Tahseen Al-Doori
Wave Propagation
 Now that you have learned about some of the
various characteristics of an RF signal, it is
important to have an understanding of the way an
RF signal behaves as it moves away from an
antenna.
 The way in which the RF waves move—known as
wave propagation—can vary drastically
depending on the materials in the signal’s path.
Drywall will have a much different effect on an RF
signal than metal.
Dr. Tahseen Al-Doori
 What happens to an RF signal between two
locations is a direct result of how the signal
propagates.
 When we use the term propagate, try to envision
an RF signal broadening or spreading as it travels
farther away from the antenna. An excellent
analogy is shown in Figure 5, which depicts an
earthquake. Note the concentric seismic rings that
propagate away from the epicenter of the
earthquake.
 RF waves behave in much the same fashion. The
manner in which a wireless signal moves is often
referred to as propagation behavior.
Dr. Tahseen Al-Doori
Earth quick
As a WLAN engineer, it is important to have an understanding of RF
propagation behaviors for making sure that access points are deployed in the
proper location, for making sure the proper type of antenna is chosen, and for
monitoring the health of the wireless network.
Dr. Tahseen Al-Doori
Absorption
 The most common RF behavior is
absorption. If the signal does not bounce
off an object, move around an object, or
pass through an object, then 100 percent
absorption has occurred.
Dr. Tahseen Al-Doori
Scenario
 Mr. Sabir performs a wireless site survey at a
campus lecture hall. He determined how many
access points are required and their proper
placement so that he will have the necessary RF
coverage. Ten days later, Professor Banks gives a
heavily attended lecture on business economics.
During this lecture, the signal strength and quality
of the wireless LAN was less than desirable. What
happened?
Dr. Tahseen Al-Doori
Reflection
 One of the most important RF propagation
behaviors to be aware of is reflection. When
a wave hits a smooth object that is larger
than the wave itself, depending upon the
media, the wave may bounce in another
direction.
 This behavior is categorized as reflection.
Dr. Tahseen Al-Doori
There are two major types of reflections:
 sky wave reflection and microwave
reflection.
 Sky wave reflection can occur in frequencies
below 1 GHz where the signal has a very
large wavelength. The signal bounces off
the surface of the charged particles of the
ionosphere in the earth’s atmosphere. This
is why you can be in Dubai, UAE, and listen
to Iran Station on a clear night.
Dr. Tahseen Al-Doori
 Microwave signals, however, exist between 1 GHz
and 300 GHz. Because they are higher-frequency
signals, they have much smaller wavelengths,
thus the term microwave.
 Microwaves can bounce off of smaller objects like
a metal door.
 Microwave reflection is what we are concerned
about in wireless LAN environments. In an outdoor
environment, microwaves can reflect off of large
objects and smooth surfaces such as buildings,
roads, bodies of water, and even the earth’s
surface. In an indoor environment, microwaves
reflect off of smooth surfaces such as doors, walls,
and file cabinets. Anything made of metal will
absolutely cause reflection. Other materials such
as glass and concrete may cause reflection as
well.
Dr. Tahseen Al-Doori
 Reflection can be the cause of serious
performance problems in a wireless LAN.
 As a wave radiates from an antenna, it
broadens and disperses. If portions of this
wave are reflected, new wave fronts will
appear from the reflection points. If these
multiple waves all reach the receiver, the
multiple reflected signals cause an effect
called multipath.
 Multipath can degrade the strength and
quality of the received signal or even cause
data corruption or cancelled signals.
Dr. Tahseen Al-Doori
 Although reflection and multipath can be
your number one enemy, new antenna
technologies such as Multiple Input Multiple
Output (MIMO) may become commonplace
in the future to actually take advantage of
reflected signals.
 MIMO is very much WiMax technology.
Dr. Tahseen Al-Doori
Scattering
 Did you know that the color of the sky is blue
because the wavelength of light is smaller than the
molecules of the atmosphere? This blue sky
phenomenon is known as Rayleigh scattering.
 The shorter blue wavelength light is absorbed by
the gases in the atmosphere and radiated in all
directions.
 This is another example of an RF propagation
behavior called scattering, sometimes called
scatter.
Dr. Tahseen Al-Doori
 Scattering can most easily be described as
multiple reflections. These multiple reflections
occur when the electromagnetic signal’s
wavelength is larger than pieces of whatever
medium the signal is passing through.
Scattering can happen in two different ways.
 The first type of scatter is on a smaller level and
has a lesser effect on the signal quality and
strength. This type of scatter may manifest itself
when the RF signal moves through a substance
and the individual electromagnetic waves are
reflected off the minute particles within the
medium. Smog in our atmosphere and sandstorms
in the desert can cause this type of scattering.
Dr. Tahseen Al-Doori
 The second type of scattering occurs when
an RF signal encounters some type of
uneven surface and is reflected into multiple
directions. Chain link fences, tree foliage,
and rocky terrain commonly cause this type
of scattering.
 When striking the uneven surface, the main
signal dissipates into multiple reflected
signals, which can cause substantial signal
downgrade and may even cause a loss of
the received signal.
Dr. Tahseen Al-Doori
Scattering
Dr. Tahseen Al-Doori
Refraction
 In addition to RF signals being absorbed or
bounced (via reflection or scattering), if
certain conditions exist, an RF signal can be
bent in a behavior known as refraction.
 A straightforward definition of refraction is
the bending of an RF signal as it passes
through a medium with a different density,
thus causing the direction of the wave to
change. RF refraction most commonly
occurs as a result of atmospheric conditions.
Dr. Tahseen Al-Doori
The three most common causes of refraction
are:
 water vapor, changes in air temperature,
and changes in air pressure.
Dr. Tahseen Al-Doori
Diffraction
 Not to be confused with refraction, another
RF propagation behavior exists that also
bends the signal; it’s called diffraction.
 Diffraction is the bending of an RF signal
around an object (whereas refraction, as
you recall, is the bending of a signal as it
passes through a medium).
Dr. Tahseen Al-Doori
 Diffraction is the bending and the spreading
of an RF signal when it encounters an
obstruction. The conditions that must be met
for diffraction to occur depend entirely on
the shape, size, and material of the
obstructing object as well as the exact
characteristics of the RF signal, such as
polarization, phase, and amplitude.
Dr. Tahseen Al-Doori
 Typically, diffraction is caused by some sort
of partial blockage of the RF signal, such as
a small hill or a building that sits between a
transmitting radio and a receiver. The waves
that encounter the obstruction slow down in
speed, which causes them to bend around
the object. The waves that did not encounter
the object maintain their original speed and
do not bend.
 Example is a rock in a river.
Dr. Tahseen Al-Doori
Loss (Attenuation)
 Loss, also known as attenuation, is best
described as the decrease of amplitude or
signal strength.
 Try the EMANIM software to view
Attenuation effect of materials due to
absorption.
Dr. Tahseen Al-Doori
 Both loss and gain can be gauged in a
relative measurement of change in power
called decibels (dB), which will be discussed
extensively in Lesson 3.
 It is important to understand that an RF
signal will also lose amplitude merely as a
function of distance in what is known as free
space path loss. Also, reflection propagation
behaviors can produce the negative effects
of multipath and as a result cause
attenuation in signal strength.
Dr. Tahseen Al-Doori
Free Space Path Loss
 Due to the laws of physics, an electromagnetic
signal will attenuate as it travels despite the lack of
attenuation caused by obstructions, absorption,
reflections, diffractions, and so on.
 Free space path loss is the loss of signal strength
caused by the natural broadening of the waves,
often referred to beam divergence.
 RF signal energy spreads over larger areas as the
signal travels farther away from an antenna, and
as a result, the strength of the signal attenuates
Dr. Tahseen Al-Doori
 One way to illustrate free space path loss is to use
a balloon analogy.
 Before a balloon is filled with helium, it remains
small but with a dense rubber thickness. After the
balloon is inflated and has grown and spread in
size, the rubber becomes very thin.
 RF signals will lose strength in much the same
manner. Luckily, this loss in signal strength is
logarithmic and not linear, thus the amplitude does
not decrease as much in a second segment of
equal length as it decreases in the first segment. A
2.4 GHz signal will change in power by about 80
dB after 100 meters but will only lessen another 6
dB in the next 100 meters.
Dr. Tahseen Al-Doori
Here are the formulas to calculate free space
path loss:
 LP = 36.6 + (20log10F) + (20log10D)
– LP = path loss in dB
– F = frequency in MHz
– D = distance in miles between antennas
 LP = 32.4 + (20log10F) + (20log10D)
– LP = path loss in dB
– F = frequency in MHz
– D = distance in kilometers between antennas
Dr. Tahseen Al-Doori
 The dB calculations will be covered in the
next subject which is “RF components and
measurements and mathematics.”
Dr. Tahseen Al-Doori
Multipath
 Multipath is a propagation phenomenon that
results in two or more paths of a signal
arriving at a receiving antenna at the same
time or within nanoseconds of each other.
Due to the natural broadening of the waves,
the propagation behaviors of reflection,
scattering, diffraction, and refraction will
occur. A signal may reflect off an object or
scatter, refract, or diffract.
Dr. Tahseen Al-Doori
Multipath
Dr. Tahseen Al-Doori
Scenario
Why Is Free Space Path Loss Important?
 All radio cards have what is known as a receiver sensitivity
level. A radio card can properly interpret and receive a
signal down to a certain fixed amplitude threshold. If a
radio card receives a signal above its amplitude threshold,
the card can differentiate between the signal and other RF
noise that is in the background. The background noise is
typically referred to as the noise floor.
 Once the amplitude of a received signal falls below the
radio card’s threshold, the card can no longer make the
distinction between the signal and the background noise.
The concept of free space path loss also applies to road
trips in your car. When you are in a car listening to AM
radio, eventually you will drive out of range and will no
longer be able to hear the music above the static noise.
Dr. Tahseen Al-Doori
 When designing both indoor wireless LANS
and outdoor wireless bridge links, you must
make sure that the RF signal will not
attenuate below the receiver sensitivity level
of your wireless radio card simply due to
free space path loss.
 You achieve this goal indoors during a site
survey. An outdoor bridge link requires a
series of calculations called a link budget.
(Site surveys and link budgets will be
covered later)
Dr. Tahseen Al-Doori
 The time differential between these multiple paths
is known as the delay spread. You will learn later
that certain spread spectrum technologies are
more tolerant than others of delay spread.
 So what exactly happens when mutipath presents
itself?
 In television signal transmissions, multipath
causes a ghost effect with a faded duplicate image
to the right of the main image.
 With RF signals, the effects of multipath can be
either constructive or destructive. Quite often they
are very destructive. Due to the differences in
phase of the multiple paths, the combined signal
will often attenuate, amplify, or become corrupted.
These effects are sometimes called Rayleigh
fading
Dr. Tahseen Al-Doori
The four results of multipath are as
follows:
 Downfade This is decreased signal
strength. When the multiple RF signal paths
arrive at the receiver at the same time and
are out of phase with the primary wave, the
result is a decrease in signal strength
(amplitude). Phase differences of between
121 and 179 degrees will cause downfade.
Dr. Tahseen Al-Doori
 Upfade This is increased signal strength. When
the multiple RF signal paths arrive at the receiver
at the same time and are in phase or partially out
of phase with the primary wave, the result is an
increase in signal strength (amplitude). Smaller
phase differences of between 0 and 120 degrees
will cause upfade.
 Please understand, however, that the final
received signal can never be stronger than the
original transmitted signal due to free space path
loss.
Dr. Tahseen Al-Doori
 Nulling This is signal cancellation. When
the multiple RF signal paths arrive at the
receiver at the same time and are 180
degrees out of phase with the primary wave,
the result can be a complete cancellation of
the RF signal.
Dr. Tahseen Al-Doori
 Data Corruption Intersymbol interference can
cause data corruption. Because of the difference
in time between the primary signal and the
reflected signals known as the delay spread, along
with the fact that there may be multiple reflected
signals, the receiver can have problems
demodulating the RF signal’s information.
 The delay spread time differential can cause bits
to overlap with each other and the end result is
corrupted data, as seen in the next slide.
 This type of multipath interference is often known
as intersymbol interference (ISI).
Dr. Tahseen Al-Doori
Dr. Tahseen Al-Doori
 So how is a WLAN engineer supposed to
deal with all these multipath issues?
 The use of unidirectional antennas will often
reduce the amount of reflections, and
antenna diversity can also be used to
compensate for the negative effects of
multipath.
Dr. Tahseen Al-Doori
Exercise
Create the following situations using EMANIM:
 Two identical, vertically polarized waves are superposed
(you might not see both of them because they cover each
other). The result is a wave having double the amplitude of
the component waves.
 Two identical, 70 degrees out of phase waves are
superposed. The result is a wave with an increased
amplitude over the component waves.
 Two identical, 140 degree out of phase waves are
superposed. The result is a wave with a decreased
amplitude over the component waves.
 Two identical, vertically polarized waves are superposed.
The result is a cancellation of the two waves.
Dr. Tahseen Al-Doori
Gain (Amplification)
 also known as amplification , can best be
described as the increase of amplitude or
signal strength.
 There are two types of gain known as active
gain and passive gain.
 Active Gain is usually caused by the use of
an amplifier on the wire that connects the
transceiver to the antenna.
 Passive Gain is accomplished by focusing
the RF signal with the use of an antenna.
Dr. Tahseen Al-Doori
 Despite the usual negative effects of
multipath, it should be reiterated that when
multiple RF signals arrive at the receiver at
the same time and are in phase or partially
out of phase with the primary wave, the
result can be an increase or gain in
amplitude.
Dr. Tahseen Al-Doori