Radio Frequency Fundamentals

1
Lesson 2

• Radio Frequency Fundamentals

2
Objectives

• Define a Radio Frequency Signal
• Define and Describe the Following RF
  Characteristics
• Polarity
• Wavelength
• Frequency
• Amplitude
• Phase

3
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)

4

• 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.

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• 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.

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• 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.

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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.

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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.

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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.

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• 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.

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Fig. 1Polarity, E-Plane and H-plane
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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.

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• 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 satellites 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.

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Fig. 2750 KHz wavelength and 252 GHz wavelength
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• 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

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• 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.
• WavelengthC/Freq.
• Where C3x108
• 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.

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Frequency

• Frequency is the number of times a specified
  event occurs within a specified time interval.
• The measurement unit for frequency is Hz.

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Amplitude

• In Fig. 4, you can see that (?) represents
  wavelength and (y) represents the amplitude.
• The first signals crests and troughs have more
  magnitude, thus it has more amplitude.
• The second signals crests and troughs have
  decreased, and therefore the signal has less
  amplitude.

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Amplitude
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• 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).

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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.

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• 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.

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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, multi-path,
  attenuation, and gain.

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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 moveknown as wave
  propagationcan vary drastically depending on the
  materials in the signals path. Drywall will have
  a much different effect on an RF signal than
  metal.

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• 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.

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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.
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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.

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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?

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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.

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• 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
  earths atmosphere. This is why you can be in
  Dubai, UAE, and listen to Iran Station on a clear
  night.

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• 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 earths
  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.

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• 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.

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• 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.

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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.

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• Scattering can most easily be described as
  multiple reflections. These multiple reflections
  occur when the electromagnetic signals
  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.

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• 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.

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Scattering
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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.

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• The three most common causes of refraction are
• water vapor, changes in air temperature, and
  changes in air pressure.

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Diffraction

• Not to be confused with refraction, another RF
  propagation behavior exists that also bends the
  signal its 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).

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• 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.

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• 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.

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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.

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• 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.

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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

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• 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.

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• 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

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• The dB calculations will be covered in the next
  subject which is RF components and measurements
  and mathematics.

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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.

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Multipath
51
ScenarioWhy 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 cards 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.

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• 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)

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• 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

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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.

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• 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.

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• 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.

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• 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 signals 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).

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• 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.

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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.

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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.

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• 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.