Aircraft Communication

1. AAVVIIOONNIICCSS
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Modulation
In a communication transceiver, the
carrier wave is a device that carries
the information from the transmitter
to the receiver.
The carrier has a frequency high
enough to produce electromagnetic
waves that radiate from the
antenna.
 This frequency is accurately
controlled so that a sensitive
receiver can select the carrier
from a specific transmitter and
reject the carriers from all other
transmitters.
The carrier itself serves no function
other than to carry the signal from
the transmitter to the receiver, and
the carrier is routed to ground after
the intelligence is removed from it.
The process of placing intelligence
on a carrier is called modulation,
and there are several ways to do it.
Three ways most often used in
aviation communication equipment
are amplitude modulation (AM),
frequency modulation (FM), and
single-sideband (SSB).
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2. AAVVIIOONNIICCSS
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Modulation (cont’d)
When the Italian inventor
Guglielmo Marconi was
developing the first practical radio
system in 1896, there was no way
to modulate the radio wave in order
to transmit voice
 so he used a method of switching
the transmitter on and off to
transmit Morse code signals.
This simplest form of modulation is
called CW or radio telegraphy
since it borrowed the Morse code
from the telegraph industry.
Within ten years, new inventions
permitted voice and music to be
transmitted by radio using improved
types of modulation.
The simplest form of transmitting
data with radio waves is with Morse
code dots and dashes or CW:
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3. AAVVIIOONNIICCSS
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Amplitude Modulation (AM)
Amplitude modulation, or AM, is a
method of modulation in which the
voltage of the carrier is changed by
the audio signal.
The voltage of the resulting carrier
varies with the voltage of the
modulating audio frequency.
Audio frequencies (AF) are those of
20,000 Hz or less.
 They are called audio because
these are approximately the
frequencies of sound waves that
can be heard by the human ear.
Amplitude modulation
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4. AAVVIIOONNIICCSS
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Amplitude Modulation (cont’d)
An example of the use of amplitude modulation (AM)
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5. AAVVIIOONNIICCSS
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Frequency Modulation
Man-made interference cause
amplitude-modulate all radio signals
in their vicinity.
 Man-made interference caused
by electric motors and ignition
systems, and natural interference
(caused by lightning in the
atmosphere).
Frequency modulation (FM) is used
to obtain interference-free
communication.
The voltage variations of the audio
frequency signal produced by a
microphone are used to change the
frequency of the carrier.
The frequency of the carrier wave is
changed when frequency
modulation is used.
In FM:
 as the voltage of the AF rises in a
positive direction, the frequency
of the carrier increases
 as it goes negative, the
frequency of the carrier
decreases.
One of the advantages of FM is that
it is less affected by atmospheric
noise from thunderstorms and other
disturbances.
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6. AAVVIIOONNIICCSS
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Frequency Modulation (cont’d)
The amplitude of an FM carrier is
held constant by limiter circuits, and
any interference, which amplitude-modulates
the carrier, is clipped off
so it does not appear in the output.
When an FM signal is received, the
deviations in frequency are changed
into amplitude variations in an
audio-frequency voltage that is
amplified and used to drive the
speaker.
Frequency modulation (FM)
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7. AAVVIIOONNIICCSS
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Single-Side Band (SSB)
Both AM and FM are limited in that
they require a wide band of
frequencies for their transmission.
 If a 25-MHz carrier is modulated
with an AF signal that contains
frequencies up to 5,000 hertz,
the transmitted signal occupies a
band of frequencies from 24.995
to 25.005 megahertz.
 This band includes:
 the carrier,
 the lower sideband, which is
the carrier frequency minus
the modulating frequency;
 and the upper sideband, which
is the carrier frequency plus
the modulating frequency.
Advantage SSB over AM
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8. AAVVIIOONNIICCSS
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Single-Side Band (cont’d)
The advantages of SSB over AM:
 The upper illustration shows the
bandwidth required for an AM
signal
 The lower illustration shows the
bandwidth required for an SSB
signal.
 The carrier and the upper
sideband have been removed.
All the information needed is carried
in either one of the sidebands, and
it is inefficient use of energy to
transmit the carrier and both the
upper and lower sidebands.
Advantage SSB over AM
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9. AAVVIIOONNIICCSS
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Single-Side Band (cont’d)
Removing the carrier and one of the
sidebands and using all of the
available energy for transmitting
the other sideband give the
transmitter a much greater range.
Radio in the United States typically
uses the lower sideband, but the
upper sideband is used overseas.
At present, SSB is the primary type
of transmission for communication
in the high-frequency (HF) band.
Advantage SSB over AM
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10. AAVVIIOONNIICCSS
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Radio Wave Propagation
When a radio wave is transmitted
from the antenna it moves out
along three paths, depending
primarily upon its frequency.
These paths are surface waves,
sky waves, and space waves.
The lower frequencies such as VLF,
LF, and MF normally follow the
curvature of the earth in surface
waves.
 These waves travel great
distances and are used for very
long-distance communication and
navigation.
Commercial broadcast signals follow
this path in the daytime.
Radio waves propagation
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11. AAVVIIOONNIICCSS
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Radio Wave Propagation (cont’d)
Radio waves at frequencies below
the HF band (below 3 MHz) are also
called as ground waves because
they will follow the curvature of the
earth and bend.
HF (band from 3 MHz to 30 MHz)
communication and commercial
broadcast at night are carried
primarily by sky waves.
 They tend to travel in straight
lines and will not follow the
curvature of the earth.
The propagation characteristics of ground waves,
sky waves and space waves
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12. AAVVIIOONNIICCSS
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Radio Wave Propagation (cont’d)
This energy tries to radiate into
space, but it bounces off the
ionosphere and returns to the earth
at a distance from the transmitter.
 It is called “skip distance“.
The “skip distance" varies and is
responsible for the fading of many
signals heard from a long distance.
Frequencies in the VHF and higher
bands follow a straight line from the
transmitting antenna to the
receiving antenna and are said to
travel by space waves.
The propagation characteristics of ground waves,
sky waves and space waves
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13. AAVVIIOONNIICCSS
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Radio Wave Propagation (cont’d)
RF propagation characteristics are
complex because the earth appears
different to radio waves at various
frequencies.
The ground acts as a dielectric at
frequencies above 5 MHz and as a
conductor below 5 MHz.
At low and medium frequencies, the
wave formation follows the
curvature of the earth, as a ground
wave.
The propagation characteristics of ground waves,
sky waves and space waves
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14. AAVVIIOONNIICCSS
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Radio Wave Propagation (cont’d)
Another illustration of radio waves propagation characteristic is shown in Figure
below:
Radio waves propagation characteristics
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15. AAVVIIOONNIICCSS
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Radio Wave Propagation (cont’d)
VHF and above radio waves allow only line‑of‑sight communication.
 At these higher frequencies, the radio wave is not reflected by the ionosphere,
but passes right through it.
VHF communications are ideal in that they minimize interference with distant
unrelated stations operating on the same frequency.
A VHF communication system requires a much smaller antenna than an HF
system.
 As the frequency increases, the wavelength decreases
 The antenna length is usually sized to an even fraction of the operating
wavelength.
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16. AAVVIIOONNIICCSS
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Radio Wave Propagation (cont’d)
There is a definite relationship between the length of the wave and its
frequency.
 The higher the frequency, the shorter the distance between the ends of the
wave.
The speed of electromagnetic wave propagation is also 300,000,000 meters per
second.
Above 3,000 MHz, coaxial cable is replaced with waveguides and tuned circuits
take the form of resonant cavities.
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