3. Electromagnetic WavesElectromagnetic Waves
AnAn electromagnetic wave is a fluctuation of energy consistingis a fluctuation of energy consisting
of two fields: electric and magnetic.of two fields: electric and magnetic.
These fields oscillate or move back and forth at right anglesThese fields oscillate or move back and forth at right angles
to each other, and the wave moves out from the propagatingto each other, and the wave moves out from the propagating
antenna in a direction related to the shape of the antenna.antenna in a direction related to the shape of the antenna.
4. Electromagnetic WavesElectromagnetic Waves
AnAn electromagnetic wave is a propagating combination of electric andis a propagating combination of electric and
magnetic fields.magnetic fields.
The alternating current (AC)The alternating current (AC) in the antenna generates a magnetic fieldin the antenna generates a magnetic field
around the antenna that generates and electric field.around the antenna that generates and electric field.
The electric and magnetic fieldsThe electric and magnetic fields are oscillating perpendicular to eachare oscillating perpendicular to each
other, and they are both perpendicular to the direction of propagationother, and they are both perpendicular to the direction of propagation
5. Electromagnetic WavesElectromagnetic Waves
A very specific form of these
electromagnetic waves is used
to communicate wirelessly in
IEEE 802.11 networks.
This form of wave is a radio
An RF-based system is a
system that relies on the
theory to provide data and
6. RF Characteristics
All RF waves have characteristics that vary to define the wave.
Some of these properties can be modified to modulate
information onto the wave. These properties are wavelength,
frequency, amplitude, and phase.
refers to the number
of wave cycles that
occur in a second.
The impact of frequency usage on WLANs is tremendous. By
using different frequencies, you can enable distinct connections
or RF links in a given coverage area or cell. For example, an
IEEE 802.11g network using channel 1 can exist in the same
cell as an IEEE 802.11g network using channel 11. This is
because these channels use different frequencies that do not
cancel or interfere with each other.
An RF wave with greater
amplitude is easier to detect than
an RF wave with lesser
amplitude. Realize that RF
waves travel, theoretically,
forever. This being the case, the
detectability of the wave is
greater at certain distances when
the wave starts with a greater
amplitude. A wave with a lesser
amplitude may not be detectable
due to the noise floor. The noise
floor can be defined as a
measure of the level of
Phase is not a characteristic
of a single RF wave but is a
comparison between two RF
When the waves are in phase, they strengthen each other, and when
the waves are out of phase, they sometimes strengthen and
sometimes cancel each other.
In specific out-of-phase cases, they only cancel each other.
Three types of modulations enable carrier signals to carryThree types of modulations enable carrier signals to carry
Height of signal (amplitude)Height of signal (amplitude)
Frequency of signal (frequency)Frequency of signal (frequency)
Relative starting point (phase)Relative starting point (phase)
Modulation can be done on analog or digitalModulation can be done on analog or digital
13. Analog ModulationAnalog Modulation
Amplitude: Height of carrier waveAmplitude: Height of carrier wave
Amplitude modulation (AM): Changes amplitude so that highest peaksAmplitude modulation (AM): Changes amplitude so that highest peaks
of carrier wave represent 1 bit while lower waves represent 0 bitof carrier wave represent 1 bit while lower waves represent 0 bit
Frequency modulation (FM):Frequency modulation (FM): Changes number of waves representingChanges number of waves representing
one cycleone cycle
Number of waves to represent 1 bit more than number ofNumber of waves to represent 1 bit more than number of
waves to represent 0 bitwaves to represent 0 bit
Phase modulation (PM):Phase modulation (PM): Changes starting point of cycleChanges starting point of cycle
When bits change from 1 to 0 bit or vice versaWhen bits change from 1 to 0 bit or vice versa
14. Digital ModulationDigital Modulation
Advantages over analog modulation:Advantages over analog modulation:
- Better use of bandwidth- Better use of bandwidth
- Requires less power- Requires less power
- Better handling of interference from other signals- Better handling of interference from other signals
- Error-correcting techniques more compatible with- Error-correcting techniques more compatible with
other digital systemsother digital systems
Unlike analog modulation, changes occur in discreteUnlike analog modulation, changes occur in discrete
steps using binary signalssteps using binary signals
Uses same three basic types of modulation as analogUses same three basic types of modulation as analog
15. High FrequencyHigh Frequency
Passed along a conductor and then radiated intoPassed along a conductor and then radiated into
the air via an antennathe air via an antenna
Converts an electrical signal to aConverts an electrical signal to a
wirelessly radiated signal (Transmit)wirelessly radiated signal (Transmit)
Converts a wirelessly radiated signal intoConverts a wirelessly radiated signal into
an electrical signal (Receive)an electrical signal (Receive)
What is RF?What is RF?
16. RF BehaviorsRF Behaviors
Radio waves move away from the antennaRadio waves move away from the antenna
in a straight line in all directionsin a straight line in all directions
17. RF BehaviorsRF Behaviors
RF waves that have been
modulated to contain
information are called
RF signals. These RF
signals have behaviors
that can be predicted and
■ Return Loss
■ Amplification and Attenuation
■ Wave Propagation
■ Free Space Path Loss
■ Delay Spread
Gain is defined as the positive relative amplitude difference
between two RF wave signals.
Amplification is an active process used to increase an RF signal’s
amplitude. = gain.
There are two basic types of gain: active and passive.
Active gain is achieved by placing an amplifier in-line between
the RF signal generator and the propagating antenna.
Passive gain is an increase in the amplitude of the signal, in a
favored direction, by focusing or directing the output power.
Passive gain can be either intentional or unintentional.
Loss is defined as the negative relative amplitude difference
between two RF signals. Loss can be either intentional or
Intentional loss may be necessary to decrease signal strength
to comply with standards or to prevent interference by RF
attenuator or by mismatch electrical cable .
Unintentional loss can be cause by many factors such as
Objects in path, Reflection, Scatter
Loss = Attenuation
Diffraction is a change in the direction of a wave as it passes by
the edge of an obstacle.
the wave bends around the object
The effect of waves turning, or bending around an obstacle
Scattering happens when an RF signal strikes an uneven surface
causing the signal to be scattered. The resulting signals are less
significant than the original signal.
Can occur when a wave strikes an uneven surface and is
reflected in many directions simultaneously
Yields many small amplitude reflections and destroys the
Scattering = Multiple Reflections
Absorption is the conversion of the RF signal energy into heat.
Many materials absorb RF signals in the 2.4 GHz ISM
spectrum. These include water, drywall, wood, and even
Voltage standing wave ratio is a measurement of mismatched
impedance in an RF system and is stated as an X:1 ratio.
Cables, connectors, and devices have some level of inherent loss.
If all cables, connectors, and devices in the chain from the RF
signal generator to the antenna do not have the same impedance
rating, there is said to be an impedance mismatch.
Maximum power output and transfer can only be achieved when
the impedance of all devices is exactly the same.
28. Return LossReturn Loss
This energy that is reflected back toward the RF generator or
transmitter results in return loss.
Return loss is a measurement, usually expressed in decibels, of
the ratio between the forward current (incident wave) and the
reflected current (reflected wave).
To minimize VSWR and return loss, we must avoid impedance
29. Amplification - AttenuationAmplification - Attenuation
Amplification is an increase of the amplitude of an RF signal.
Amplification is achieved through active gain and is
accomplished with an amplifier.
Attenuation is the process of reducing an RF signal’s amplitude.
This is occasionally done intentionally with attenuators to reduce
a signal’s strength to fall within a regulatory domain’s imposed
Loss is the result of attenuation.
Gain is the result of amplification.
30. Wave PropagationWave Propagation
The way RF waves move through an environment is known as
Attenuation occurs as RF signals propagate through an
The signal cannot be detected after a certain distance, and this
becomes the usable range of the signal.
Some of the signal strength is lost through absorption by
materials encountered by the RF signal. This is due to a
phenomenon known as free space path loss.
34. Ground Wave PropagationGround Wave Propagation
Follows contour of the earthFollows contour of the earth
Can Propagate considerable distancesCan Propagate considerable distances
Frequencies up to 2 MHzFrequencies up to 2 MHz
AM radioAM radio
36. Sky Wave PropagationSky Wave Propagation
Signal reflected from ionized layer of atmosphere back downSignal reflected from ionized layer of atmosphere back down
to earthto earth
Signal can travel a number of hops, back and forth betweenSignal can travel a number of hops, back and forth between
ionosphere and earth’s surfaceionosphere and earth’s surface
Reflection effect caused by refractionReflection effect caused by refraction
Can travel thousands of kilometersCan travel thousands of kilometers
Frequency: 2-30MHzFrequency: 2-30MHz
Amateur radioAmateur radio
CB radioCB radio
38. Line-of-Sight PropagationLine-of-Sight Propagation
Transmitting and receiving antennas must be within line ofTransmitting and receiving antennas must be within line of
Satellite communication – signal above 30 MHz notSatellite communication – signal above 30 MHz not
reflected by ionospherereflected by ionosphere
Ground communication – antennas withinGround communication – antennas within effectiveeffective line ofline of
site due to refractionsite due to refraction
Refraction – bending of microwaves by the atmosphereRefraction – bending of microwaves by the atmosphere
Velocity of electromagnetic wave is a function of theVelocity of electromagnetic wave is a function of the
density of the mediumdensity of the medium
When wave changes medium, speed changesWhen wave changes medium, speed changes
Wave bends at the boundary between mediumsWave bends at the boundary between mediums
39. Line-of-Sight EquationsLine-of-Sight Equations
Optical line of sightOptical line of sight
Effective, or radio, line of sightEffective, or radio, line of sight
dd = distance between antenna and horizon (km)= distance between antenna and horizon (km)
hh = antenna height (m)= antenna height (m)
K = adjustment factor to account for refraction,K = adjustment factor to account for refraction,
rule of thumb K = 4/3rule of thumb K = 4/3
hd Κ= 57.3
40. Line-of-Sight EquationsLine-of-Sight Equations
Maximum distance between two antennas for LOSMaximum distance between two antennas for LOS
hh11 = height of antenna one= height of antenna one
hh22 = height of antenna two= height of antenna two
( )2157.3 hh Κ+Κ
41. LOS Wireless TransmissionLOS Wireless Transmission
Attenuation and attenuation distortionAttenuation and attenuation distortion
Free space lossFree space loss
Atmospheric absorptionAtmospheric absorption
Thermal noiseThermal noise
Strength of signal falls off with distance over transmissionStrength of signal falls off with distance over transmission
Attenuation factors for unguided media:Attenuation factors for unguided media:
Received signal must have sufficient strength so thatReceived signal must have sufficient strength so that
circuitry in the receiver can interpret the signalcircuitry in the receiver can interpret the signal
Signal must maintain a level sufficiently higher than noiseSignal must maintain a level sufficiently higher than noise
to be received without errorto be received without error
Attenuation is greater at higher frequencies, causingAttenuation is greater at higher frequencies, causing
43. Free Space Path LossFree Space Path Loss
Free space path loss is a weakening of the RF signal due to a
broadening of the wave front.
A 2.4 GHz signal, such as that used by many IEEE devices, will
attenuate by approximately 80 dB in the first 100 meters and then
by another 6 dB in the second 100 meters.
44. Multipath and Delay SpreadMultipath and Delay Spread
When signals bounce around in an environment through
reflection, refraction, diffraction, and scattering, they create an
effect known as multipath.
Multipath occurs when multiple paths of the signal arrive at the
receiving antenna at the same time or within a small fraction of
a second (nanoseconds) of each other.
The difference in time between the first and second signals
arriving at the receiver in a multipath occurrence is known as
the delay spread.
45. Multipath and Delay SpreadMultipath and Delay Spread
When the delay spread is greater, so that the signals arrive out of
phase, the signal will either be downfaded, corrupted, or
46. The Effects of Multipath PropagationThe Effects of Multipath Propagation
Multiple copies of a signal may arrive at differentMultiple copies of a signal may arrive at different
If phases add destructively, the signal level relativeIf phases add destructively, the signal level relative
to noise declines, making detection more difficultto noise declines, making detection more difficult
Intersymbol interference (ISI)Intersymbol interference (ISI)
One or more delayed copies of a pulse may arriveOne or more delayed copies of a pulse may arrive
at the same time as the primary pulse for aat the same time as the primary pulse for a
subsequent bitsubsequent bit
49. Omnidirectional Antenna UsageOmnidirectional 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
50. Semidirectional AntennasSemidirectional Antennas
Semidirectional antennas are
antennas that focus most of their
energy in a particular direction.
Patch, Panel, and Yagi are
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.
52. Highly Directional AntennasHighly 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.
53. Sectorized and Phased-Array AntennasSectorized and Phased-Array Antennas
A sectorized antenna is a high-gain
antenna that works back-to-back with other
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
54. MIMO Antenna SystemsMIMO Antenna Systems
Output (MIMO) can be
described as any RF
that has multiple antennas
at both ends of the
communications link being
The proposed 802.11n
standard include MIMO