Transcript Radio Frequency Fundamentals

Radio Frequency
Fundamentals
Wireless Networking Unit
Radio Frequency Signals
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Radio Frequency (RF)
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RF signals are high frequency alternating
current (AC) signals composed of
electromagnetic energy.
Imagine dropping a rock into a still pond
and watching the concentric ripples flow
away from the point where the rock hit the
water. This is how RF waves exit an
antenna.
RF Properties
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All radio frequency signals have the
following properties:
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Amplitude,
Frequency,
Wavelength,
Phase, and
Polarity,
RF Properties
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Amplitude
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RF energy is analogous to sound waves.
Sound waves are changes in the pressure
of the air. The cone of a loudspeaker
creates high and low sound waves by
moving the air back and forth.
With RF waves, electrons vibrating in an
antenna cause waves of high and low
pressure.
RF Properties
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Amplitude
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In the same way that we can measure the
change in air pressure from a passing
sound wave, we can measure the change
in RF energy caused by a passing RF wave,
The change in RF energy is know as
“Amplitude” of a signal,
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Higher amplitude signals are more likely to
show a higher signal strength,
RF Properties
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Amplitude
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Examples
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FM Radio Stations transmit @ 6000 to 100,000
watts,
Microwave Ovens @ 700- 1000 watts,
Cell phones @ tenths of a watt to 1 watt,
Wireless 802.11 networks @1-200 milliwatts
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1 milliwatt = 1/1000 of a watt
RF Properties
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Amplitude
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Amplitude is the most basic quality of an
RF signal
The higher the amplitude of an RF signal
the further it will travel before becoming
weakened to the point of being unreceivable,
RF Properties
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Frequency
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Alternating Current (AC) signal
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Since amplitude is the most basic quality of an
RF signal, information is conveyed by changing
the amplitude of the RF signal over time.
Direct Current (DC) signal
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A signal whose amplitude doesn’t change at all
over time is referred to as a direct current
signal,
RF Properties
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Frequency is measured in “Hertz”,
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Transmission and reception are easier when the
signal oscillates with a more or less regular
rhythm.
The time between one peak in the signal’s
amplitude and the next peak is constant from
peak to peak.
The number of times per second that the signal’s
amplitude peaks is the frequency of the signal.
802.11 transmissions operate at frequencies of
around 2.4 to 5.8 GHz
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1 GHz – 1,000,000,000 Hz or 1 billion cycles per second
RF properties
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Modulation
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In reality an 802.11 signal is not perfectly
fixed at one particular frequency, but
modulates slightly around a central
frequency,
Since a change in the signal is required to
convey information the slight modulations
around the central frequency are
interpreted as ones and zeros,
RF Properties
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Frequency bands
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802.11 is constrained by the FCC’s limits on
what frequencies can be used:
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2.4 GHz band is used for:
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802.11
802.11b
802.11g
802.11n
5 GHz band is used for:
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802.11a
RF Properties
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Wavelength
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The wavelength of an RF signal is a
function of the signal’s frequency and it’s
speed through space. If a signal’s wave
front it traveling through space at a certain
speed, and we know the amount of time
between each peak, then we can calculate
how far the signal will have traveled from
one peak to the next. That distance is the
signal’s wavelength.
RF Properties
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Wavelength
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RF energy travels at the speed of light,
approximately 300,000,000 meters per
second,
A signal traveling through an Ethernet
cable will travel at about two-thirds the
speed of light,
RF Properties
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If we assume that an RF signal is traveling
at the speed of light, then its wavelength
and frequency can be calculated:
Wavelength (m) = 300,000,000 m/s
Frequency (Hz)
Wavelength=300,000,000ms/2,400,000,000Hz
Wavelength = 0.125 meter
Wavelength = 12.5 centimeter
RF Properties
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By rearranging the formula, we can
calculate frequency from wavelength:
Frequency (Hz) = 300,000,000 m/s, or
the speed of light,
RF Properties
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Wavelength
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Practical Use
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The most direct way that we interact with
wavelength is through the antennae on most
802.11b access points (AP)
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Antennae are most receptive to signals that have a
wavelength equal to the length of the antenna’s
element.
Antenna elements of one-half and one-quarter
wavelength are the next best choice.
RF Properties
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Phase
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Phase is a method of expressing the relationship
between the amplitudes of two RF signals that
have the same frequency.
Phase is measured in degrees (like the degrees of
a compass)
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If two signals are aligned so that they both reach their
peak at the exact same time, we say that they have zero
degrees of phase separation. They are completely in
phase.
If the signals are aligned so that on reaches its peak at
the exact same time that the other reaches its trough
(lowest amplitude) we say that they have 180 degrees of
phase separation.
RF Properties
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To the Wireless Lan Engineer, phase is
important because two signals that are
in phase add their energy together,
resulting in a stronger signal.
Two signals that are 180 degrees out of
phase, completely cancel each other
out.
RF Properties
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Polarization
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A radio wave is actually made of up of two
fields:
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One electric,
One magnetic,
The sum of these two field is called the
“electromagnetic field”,
When energy is transferred back and forth
from one field to the other it is called
“Oscillation”
RF Properties
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E-Plane
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The plane that is parallel with the antenna
element is referred to as the “E-Plane”,
H-Plane
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The plane that is perpendicular to the
antenna element if referred to as the “Hplane”,
RF Properties
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Gain
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Is the term used to describe an increase in
an RF signal’s amplitude.
Loss
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Loss describes a decrease in signal
amplitude.
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Cable resistance can cause loss of signal, since
resistance coverts electrical signals to heat,
RF Properties
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Reflection
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Reflection occurs when a propagating
electromagnetic wave strikes an object that
has very large dimension in comparison to
the wavelength of the propagating wave.
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If the surface is smooth, the reflected signal
may remain intact, though there may be some
loss due to absorption.
RF Properties
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Refraction
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Refraction describes the bending of a radio
wave as it passes through a medium of
different density.
Incoming RF signal
Reflected RF
Refracted RF
RF Properties
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Diffraction
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Diffraction occurs when the radio path between
the transmitter and receiver is obstructed by a
surface that has sharp irregularities or an other
wise rough surface
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Diffraction is commonly confused with refraction.
Diffraction describes a wave bending around an obstacle,
whereas refraction describes a wave bending as it ravels
from a medium of one density to a medium of another
density (fog).
RF Properties
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Absorption
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Absorption occurs when the 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.