Radio Frequency Fundamentals

Radio Frequency
Fundamentals
~by John Ombagi
jnyabuti@strathmore.edu

Agenda
– Definition of radio frequency signal
– Radio frequency characteristics
– Radio frequency behaviors

Introduction
● To fully understand the 802.11 technology, you need to have
a clear concept of how wireless works at the first layer of the
OSI model.
● At the heart of the Physical layer is radio frequency (RF)
communications.
● Although the laws of physics apply, RF signals move through
the air in a sometimes unpredictable manner.
● RF signals are not saddled inside an Ethernet wire, you
should always try to envision a wireless LAN as an “ever
changing” network.

Context
● Must you be an RF engineer to deal with Wireless technology?
● Of course not, but if you have a good grasp of the RF
characteristics and behaviors, it’ll build a solid and deeper
knowledge of Wireless Tech and answer such questions :
– Why does a wireless network perform differently in
an auditorium full of people than it does inside an
empty auditorium?
– Why does the range of a 5 GHz radio transmitter
seem shorter than the range of a 2.4 GHz radio
transmitter?

What Is a RF Signal?
● It’s any frequency within the electromagnetic spectrum
associated with radio wave propagation.
● The electromagnetic (EM) spectrum, which is usually simply
referred to as spectrum, is the range of all possible EM
radiation.
● This radiation exists as self-propagating EM waves that can
move through matter or space.
● Examples of EM waves include :
– gamma rays, X-rays, visible light, and radio waves.

RF 101
● An RF signal starts out as an electrical alternating current (AC)
signal that is originally generated by a transmitter.
● This AC signal is sent through a copper conductor (typically a
coaxial cable) and radiated out of an antenna element in the
form of an electromagnetic wave.
● This electromagnetic wave is the wireless signal. Changes of
electron flow in an antenna, otherwise known as current,
produce changes in the electromagnetic fields around the
antenna.

A/C
● An alternating current is an electrical current with a magnitude
and direction that varies cyclically.
● The shape and form of the AC signal (waveform) is what is
known as a sine wave, as shown below:

RF Propagation
● An RF electromagnetic signal radiates away from the antenna
in a continuous pattern that is governed by certain properties
such as
– wavelength, frequency, amplitude, and Phase.
● EM 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 299,792,458 meters per second, or
186,000 miles per second.
● RF electromagnetic signals travel using a variety or combination
of movement behaviors. These movement behaviors are
referred to as propagation behaviors.

RF Characteristics
● These characteristics, defined by the laws of physics, exist
in every RF signal:
– Wavelength
– Frequency
– Amplitude
– Phase

Wavelength
● A wavelength is the distance between the two successive
crests (peaks) or two successive troughs (valleys) of a wave
pattern, as pictured below. In simpler words, it’s the distance
that a single cycle of an RF signal actually travels.

Wavelength contd...
● It is very important to understand that there is an inverse
relationship between wavelength and frequency.
● The higher the frequency of an RF signal, the smaller the
wavelength of that signal.
● The larger the wavelength of an RF signal, the lower the
frequency of that signal.
● AM radio stations operate at much lower frequencies than
WLAN 802.11 radios, while satellite radio transmissions occur
at much higher frequencies than WLAN radios.
● λ = c/f and f = c/λ
– Where λ is wavelength, measured in meters, or m.
– C, a constant value of 300,000,000 m/sec
– F, frequency, measured in hertz, or Hz

Wavelength contd...
● As RF signals travel through space and matter, they lose signal
strength (attenuate).
● It is often thought that a higher frequency electromagnetic
signal with a smaller wavelength will attenuate faster than a
lower frequency signal with a larger wavelength.
● In reality, the frequency and wavelength properties of an RF
signal do not cause attenuation. Distance is the main cause of
attenuation.
● All antennas have an effective area for receiving power known
as the aperture. The amount of RF energy that can be captured
by the aperture of an antenna is smaller with higher frequency
antennas.
● Although wavelength and frequency do not cause attenuation,
the perception is that higher frequency signals with smaller
wavelengths attenuate faster than signals with a larger
wavelength.

Wavelength contd...
● Wavelength formulas
– Wavelength (inches) = 11.811/frequency (GHz)
– Wavelength (centimeters) = 30/frequency (GHz)
● Example :
– 2.4 GHz
● 11.8.11/2.4 = 4.9 inches
● 30/2.4 = 12.5 centimeters

Frequency
● Frequency is the number of times a specified event occurs
within a specified time interval.
● A standard measurement of frequency is hertz (Hz), which
was named after the German physicist Heinrich Rudolf
Hertz.
● An event that occurs once in 1 second has a frequency of 1
Hz. An event that occurs 325 times in 1 second is measured
as 325 Hz.
– 1 hertz (Hz) = 1 cycle per sec
– 1 kilohertz (KHz) = 1,000 cycles per sec
– 1 megahertz (MHz) = one million cycles per sec
– 1 gigahertz (GHz) = one billion cycles per sec

So when we are talking about 2.4 GHz WLAN radios, the RF signal is oscillating 2.4
billion times per second!

Amplitude
● Amplitude can be defined as the maximum displacement of
a continuous wave.
● It can be characterized simply as the signal’s strength, or
power. (how loud or strong the signal is)
● With RF signals, the amplitude corresponds to the electrical
field of the wave.
● When discussing signal strength in a WLAN, amplitude is
usually referred to as either transmit amplitude or received
amplitude. Transmit amplitude is typically defined as the
amount of initial amplitude that leaves the radio transmitter.

The first signal’s crests and troughs
have more magnitude; thus the signal
has more amplitude.
The second signal’s crests and
troughs have decreased
magnitude, and therefore the
signal has less amplitude.
Different types of RF technologies require varying degrees of transmit amplitude. AM
radio stations may transmit narrow band signals with as much power as 50,000 watts.
The radios used in most indoor 802.11 access points have a transmit power range
between 1 mW and 100 mW. You will learn later that Wi-Fi radios can receive signals
with amplitudes as low as billionths of a milliwatt.

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.
● Conversely, if the peaks of two signals with the same
frequency are not in exact alignment at the same time, they
are said to be out of phase .

Phase contd...
● What is important to understand is the effect that phase has
on amplitude when a radio receives multiple signals.
● Signals that have 0 (zero) degree phase separation actually
combine their amplitude, which results in a received signal of
much greater signal strength, potentially as much as twice
the amplitude.
● If two RF signals are 180 degrees out of phase, they cancel
each other out and the effective received signal strength is
null.
● Phase separation has a cumulative effect. Depending on the
amount of phase separation of two signals,the received
signal strength may be either increased or diminished.

Radio Frequency Behaviors
As an RF signal travels through the air and other mediums, it
can move and behave in different manners.
– Wave Propagation
– Absorption
– Reflection
– Scattering
– Refraction
– Diffraction
– Loss (Attenuation)
– Free Space Path Loss
– Multipath
– Gain (Amplification)

Wave Propagation
● This is the way in which the RF waves move. It can vary
drastically depending on the materials in the signal’s path.
● 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.
● The manner in which a wireless signal moves is often
referred to as propagation behavior.

Absorption
● If a signal does not bounce off an object, move around an
object, or pass through an object, then 100 percent
absorption has occurred.
● Most materials will absorb some amount of an RF signal to
varying degrees. Brick and concrete walls will absorb a
signal significantly than a drywall will. Water is another
example of a medium that can absorb a signal to a large
extent.
● A 2.4 GHz signal will be 1/16 the original power after
propagating through a brick wall. That same signal will only
lose 1/2 the original power after passing though drywall
material.
● Absorption is a leading cause of attenuation (loss). The
amplitude of an RF signal is directly affected by how much
RF energy is absorbed.

Reflection
Reflection happens when a
wave hits a smooth object
that is larger than the wave
itself, depending on the
media the wave may
bounce in another
direction.
There are two major types of reflections:
a) sky wave reflection
b) microwave reflection

Reflection contd...
● 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.
● 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 smaller objects like a metal door.
Microwave reflection is what we are concerned about in WiFi
environments.
● Anything made of metal will absolutely cause reflection.

Scattering
● Why is the Sky Blue? molecules of the atmosphere are
smaller than the wavelength of light. The shorter blue
wavelength light is absorbed by the gases in the atmosphere
and radiated in all directions.
● Scattering can most easily be described as multiple
reflections.
There are 2 types of Scattering:
● First type : is on a lower level and has a lesser effect on the
signal quality and strength. Electromagnetic waves are
reflected off the minute particles within the medium.
● Second type : occurs when an RF signal encounters some
type of uneven surface and is reflected into multiple
directions.

Scattering analogy
Note how the main signal beam is
completely displaced into multiple
reflected beams with less amplitude and
into many different directions.

Refraction
● This 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.
● The three most common causes of refraction are water
vapor, changes in air temperature, and changes in air
pressure.
● In an outdoor environment, RF signals typically refract
slightly back down toward the earth’s surface; However,
changes in the atmosphere may cause the signal to bend
away from the earth.
● In long-distance outdoor wireless bridge links, refraction can
be an issue. An RF signal may also refract through certain
types of glass and other materials that are found in an indoor
environment.

Diffraction
● Diffraction is the bending of an RF signal around an object.
It’s 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.
● 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 bend around the
object, taking a longer and different path. The waves that did
not encounter the object do not bend and maintain the
shorter and original path.

Diffraction analogy
Sitting directly behind the obstruction is an area known as the RF shadow . Depending
on the change in direction of the diffracted signals, the area of the RF shadow can
become a dead zone of coverage or still possibly receive degraded signals.

Loss (Attenuation)
● It’s the decrease of amplitude or signal strength.
● Wireless network designers have moved away from planning
for coverage and have moved toward planning for capacity.
● RF engineer may add a hardware attenuator device on the
wired side of an RF system to introduce attenuation to
remain compliant with power regulations or for capacity
design purposes.
● Both loss and gain can be gauged in a relative measurement
of change in power called decibels (dB).
● RF signal will also lose amplitude merely as a function of
distance due to free space path loss.

Free Space Path Loss
● Free space path loss (FSPL) is the loss of signal strength
caused by the natural broadening of the waves, often
referred to as 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.
● 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 lessen only another 6 dB in the next 100
meters.

Formulas to calculate FSPL
● FSPL = 36.6 + (20 log 10
(f)) + (20 log 10
(D))
FSPL = path loss in dB
f = frequency in MHz
D = distance in miles between antennas
● FSPL = 32.44+ (20log 10 (f)) + (20log 10 (D))
FSPL = path loss in dB
f = frequency in MHz
D = distance in kilometers between antennas

Multipath
● It’s 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 natural broadening of the waves, the propagation
behaviors of reflection, scattering, diffraction, and refraction
will occur differently in dissimilar environments.
● These propagation behaviors can all result in multiple paths
of the same signal.
● It usually takes a bit longer for reflected signals to arrive at
the receiving antenna.
● The time differential between these signals can be
measured in billionths of a second (nanoseconds).
● The time differential between these multiple paths is known
as the delay spread.

With RF signals, the effects of
multipath can be either constructive or
destructive.
Because of 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

Four possible results of multipath
● Upfade - This is increased signal strength. 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.
● Downfade - This is decreased signal strength. Multiple RF
signal paths arrive at the receiver at the same time and are
out of phase with the primary wave.
● Nulling - This is signal cancellation. Multiple RF signal paths
arrive at the receiver at the same time and are 180 degrees
out of phase with the primary wave.
● Data Corruption - problems demodulating the RF signal’s
information because of the difference in time between the
primary signal and delay spread.

Gain (Amplification)
● Gain, also known as amplification. It’s the the increase of
amplitude, or signal strength.
● A signal’s amplitude can be boosted by the use of external
devices.
● Active gain is usually caused by the transceiver or 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. Antennas are passive devices
that do not require an external power source.

Summary
● We’ve covered the basics, of radio frequency signals. It’s
essential to have a thorough understanding of the following
principles of RF properties and RF behaviors:
● Electromagnetic waves and how they are generated
● The relationship between wavelength, frequency, and the
speed of light
● Signal strength and the various ways in which a signal can
either attenuate or amplify
● The importance of the relationship between two or more
signals
● How a signal moves by bending, bouncing, or absorbing in
some manner

Key Takeaways
● Understand wavelength, frequency, amplitude, and phase.
● Remember all the RF propagation behaviors.
● Understand what causes attenuation.
● Define free space path loss.
● Remember the four possible results of multipath and their
relationship to phase.
● Know the results of intersymbol interference and delay
spread.
● Explain the difference between active and passive gain.
● Explain the difference between transmit and received
amplitude.

Radio Frequency Fundamentals

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