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8/13/2019 Radio Frequency Fndamentals
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Radio
Frequency Fundamentals
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What is Radio Wave
Radio wave is a little part of Electro Magnetic
spectrum.
Electro Magnetic waves are made of two parts.
The first part is an electric field
The second part is a magnetic field
The two fields are at right angles to each other
High frequency AC current in copper cangenerate radio waves or radio waves can
generate AC currents
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Electro magnetic Spectrum
Radio Waves
Microwaves
Infra Red
The visible spectrum
Ultra Violet
Gamma Rays
X-Rays
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Approximate RF range assigned to major
categories
1,850 MHz - 1,990 MHzCell phones PCS
1,227 MHz - 1,575 MHzGPS
960 MHz - 1,215 MHzAir Traffic Control
869 MHz - 894 MHzCell phones GSM
824 MHz - 849 MHzCell phones CDMA
174 MHz - 220 MHzTV stations 7-13
88 MHz - 108 MHzFM radio
54 MHz - 88 MHzTV stations 2-6
26.96 MHz - 27.41 MHzCitizen's Band (CB)
5.9 MHz - 26.9 MHzShortwave radio
535 KHz - 1,700 KHzAM radio
9 KHz - 535 KHzAeronautical/Maritime
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Basic concepts of RF behavior
Gain (Amplification)
Loss (Attenuation)
Reflection Refraction
Diffraction
Scattering
VSWR
Return Loss Absorption
Wave Propagation
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Wireless Propagation
Mental picture
Wave is not a spot or a line, but a moving wave.
Like dropping a rock into a pond.
Wireless waves spread out from the antenna.
Wireless waves pass through air, space,
people, objects,
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RF Properties - Amplitude
Amplitude of an RF Signal is analogous to a
voltage level in an electrical signal.
Higher the transmit power, higher the amplitude
(signal strength)
Higher the amplitude farther the wave can travel
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RF Properties Amplitude (Cont)
Example power levels
FM Radio Stations 6000 to 100,000 watts
Microwave ovens 700 to 1000 watts
Cellular phones tenths of a watt to 1 watt
802.11 NICs 1 to 200 milliwatts (milliwatt =
1/1000 of a watt)
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Frequency
The number of times per second that the
signals amplitude peaks is the frequency of the
signal
Frequency is measured in Hertz (Hz)
The slight modulations around the central
frequency are interpreted as ones and zeros.
Signals at different frequencies do not interfere
each other
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Wavelength
The distance between one peak or crest of a wave of
light, heat, or other energy and the next
corresponding peak or crest.
Wavelength (m) = 300,000,000 m/s /frequency (Hz)
An 802.11 signal with a frequency of 2.4 GHz has a
wavelength of :
Wavelength = 300,000,000 m/s / 2,400,000,000 Hz
Wavelength = 0.125 m
Wavelength = 12.5 cm
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Wavelength and Antennae
Antennae are most receptive to signals that
have a wavelength equal to the length of the
antennas element
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Phase
Phase is a method of expressing the
relationship between the amplitudes of two RF
signals that have the same frequency.
If two signals reach their peak at exact same
time, they are said to be in phase
Two signals in phase add their energytogether, resulting in stronger signal
If one signal reaches its peak the same time the
other reaches its trough, they are said to becompletely out of phase.
Two signals completely out of phase cancels
each other, resulting in null
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Polarization
Polarization is the physical orientation of the
antenna in a horizontal or vertical position.
If the antenna is horizontal, then the
polarization is horizontal
If the antenna is vertical, then the polarization
is vertical
Radio wave is made up of two fields
Electric
Magnetic
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Polarization (Cont)
These two fields are plane perpendicular to
each other
The sum of the two fields is called the
electromagnetic field
Plane parallel with antenna element is referred
as E-Plane
Plane perpendicular to antenna element isreferred as H-Plane
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Polarization (Cont)
Horizontal polarization the electric field is
parallel to ground
Vertical polarization the electric field is
perpendicular to the ground
Transmitting and receiving antennas must be
using same polarization to effectively receive
signals. Polarization has little or no effect in enclosed
areas
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Gain
Antenna gain is used to describe an increase
in an RF signals amplitude.
Gain can be active or passive
Active gain when a powered amplifier is used
to add energy to a signal before transmission
Passive gain when a high gain antenna is
used to focus the energy of a signal duringtransmission to increase the power within the
beam
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Loss
Loss describe decrease in signal strength
Loss can occur while the signal still in the
cable or in the air.
Impedance mismatches in the cables and
connectors can cause loss
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Reflection
Reflection is the light bouncing back in the
general direction from which it came. Consider a smooth metallic surface as an
interface.
As waves hit this surface, much of their energywill be bounced or reflected.
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Reflected Waves
Reflection The shorter the wavelength of the
signal relative to the size of the obstruction, the
more likely it is that some of the signal will be
reflected off the obstruction.
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Microwave Reflections
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Microwave Reflections (Cont)
Microwave signals:
Frequencies between 1 GHz 30 GHz (this
can vary among experts). Wavelength between 12 inches down to less
than 1 inch.
Microwave signals reflect off objects that arelarger than their wavelength, such as buildings,
cars, flat stretches of ground, and bodes of
water. Each time the signal is reflected, the amplitude
is reduced.
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Reflections
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Microwave Reflections (Cont)
Multipath Reflection
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Microwave Reflections (Cont)
Advantage: Can use reflection to go around
obstruction.
Disadvantage: Multipath reflection occurs
when reflections cause more than one copy of
the same transmission to arrive at the receiver
at slightly different times.
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Multipath Reflection
Reflected signals 1 and 2 take slightly longer paths than direct signal, arriving
slightly later.
These reflected signals sometimes cause problems at the receiver by partially
canceling the direct signal, effectively reducing the amplitude.
The link throughput slows down because the receiver needs more time to
either separate the real signal from the reflected echoes or to wait for missed
frames to be retransmitted.
Solution discussed later.
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Refraction
Refraction (or bending) of signals is due to temperature, pressure, and water
vapor content in the atmosphere.
Amount of refractivity depends on the height above ground.
Refractivity is usually largest at low elevations.
The refractivity gradient (k-factor) usually causes microwave signals to curve
slightly downward toward the earth, making the radio horizon father away
than the visual horizon.
This can increase the microwave path by about 15%,
NormalRefraction
Refraction (straight line)
Sub-Refraction
Earth
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Refraction (Cont)
Radio waves also bend when entering different materials.
This can be very important when analyzing propagation in the atmosphere.
It is not very significant in WLANs, but it is included here, as part of a general background
for the behavior of electromagnetic waves.
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Diffraction
Diffraction of a wireless signal occurs when the signal is partially blocked or
obstructed by a large object in the signals path. A diffracted signal is usually attenuated so much it is too weak to provide a
reliable microwave connection.
Do not plan to use a diffracted signal, and always try to obtain an
unobstructed path between microwave antennas.
Diffracted
Signal
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Attenuation
Attenuation is the loss in amplitude that occurs whenever a signal travels
through wire, free space, or an obstruction.
At times, after colliding with an object the signal strength remaining is too
small to make a reliable wireless link.
Same wavelength
(frequency), less amplitude.
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Scattering
Scattered waves are produced by rough surfaces, small
objects or by other irregularities in the signal path
including heavy dust content.
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Absorption
Absorption is the phenomenon where RF signal strikes an
object and is absorbed into the material in such a manner
that it does not pass through, reflect off, or bend around the
object
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Voltage Standing Wave Ratio (VSWR)
VSWR is a result of mismatched impedance
between RF equipments ( radiator, cables,
connectors etc.)
VSWR causes return loss
VSWR is measured as ratio of forwarded and
reflected power
A VSWR measurement of 1:1 would denote a
perfect impedance match, hence no VSWR in the
signal path
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Effect of VSWR
Excessive VSWR can reduce the amplitude of
transmitted signal
It can burn the radio circuitry if not protected.
VSWR can be measured by SWR meters (not
part of CWNA)
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Intentional Radiator
According to FCC an intentional radiator is an
RF device specifically designed to generate and
radiate RF signals.
In terms of hardware, intentional radiator will
include the RF device and all cabling andconnectors up to but not including the antenna.
Power out of intentional radiator means poweroutput at the end of the last cable or connector
before the antenna
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Equivalent Isotropically Radiated Power (EIRP)
EIRP is the power actually radiated by the
antenna element EIRP takes into account the gain of the antenna
plus power fed by the intentional radiator.
EIRP = power of intentional radiator + gain of
antenna