THE ELECTROMAGNETIC SPECTRUM
A great many avionics systems use electromagnetic
(EM) waves to perform their functions
It is therefore useful to have a basic understanding of
some of their characteristics
THE ELECTROMAGNETIC SPECTRUM
Important Characteristics of EM waves
• As the name implies, an EM wave has an electric
(E) field component and a magnetic (H) field
component
• These are always at 90 degrees to each other
• The wave propagates at 90 degrees to both.
E
Direction
Of
Propagation
H
THE ELECTROMAGNETIC SPECTRUM
Important Characteristics of EM waves
• The speed of propagation, c, is approx 3 x 108 m/s
• The wavelength λ is related to the frequency f by
the equation λ=c/f
• The polarity is defined as the direction of the E
field vector.
THE ELECTROMAGNETIC SPECTRUM
Typical Polarities are:
• Vertical
• Horizontal
• Circular
In circular polarization, the vectors rotate around
the axis of propagation in a corkscrew fashion.
They rotate together so that they are always 90º
to each other
• Each polarity has characteristics which makes it
useful for particular purposes
THE ELECTROMAGNETIC SPECTRUM
•Classification of the EM Spectrum
Band
f
λ
VLF (Very Low Frequency)
30kHz – 300kHz
10km – 1 km
LF (Low Frequency)
300kHz – 3MHz
1km – 100m
HF (High Frequency)
3MHz – 30MHz
100m – 10m
VHF (Very High Frequency)
30MHz – 300 MHz
10m – 1m
UHF (Ultra High Frequency)
300MHz – 3 GHz
1m – 10cm
SHF (Super High Frequency)
3Ghz – 30GHz
10 cm – 1 cm
THE ELECTROMAGNETIC SPECTRUM
Q: All of these bands are used in aviation – Why?
A:
1. Propagation properties
2. Bandwidth usage
THE ELECTROMAGNETIC SPECTRUM
Propagation – Ionosphere
The upper levels of the atmosphere are constantly
ultraviolet radiation originating in the sun.
This radiation ionizes the atmospheric gases which
results in several layers of electron plasma
For communication, the most important is the topmost or
F layer which is situated at altitudes of 120km to
400km.
THE ELECTROMAGNETIC SPECTRUM
Propagation – Ionosphere
Within an ionospheric layer, the density of
electrons varies from a minimum at the top and
bottom, to a maximum in the middle.
Since the speed of an electromagnetic wave
increases with electron density, the wave is
refracted.
THE ELECTROMAGNETIC SPECTRUM
Propagation – Ionosphere
The amount of refraction is dependent on the
frequency of the wave – higher frequency, less
refraction.
If the frequency is low enough, the wave can be
refracted back to earth
THE ELECTROMAGNETIC SPECTRUM
This phenomenon is used to transmit signals around the earth’s curvature
THE ELECTROMAGNETIC SPECTRUM
Problems:
•The height and electron density of any layer is highly
variable, depending on the time of day, time of year,
sunspot activity etc.
•The ionosphere allows noise from thunderstorms in
the equatorial region to be received at long distances
thus increasing the noise level of communications.
•Use of ionospheric reflection is limited to the HF
band (3MHz to 30 MHz)
THE ELECTROMAGNETIC SPECTRUM
Bandwidth:
•The frequencies being discussed are what is called
carrier frequencies.
•A single frequency does not transmit any information
•To transmit information, the carrier must be modified
in some way.
•This is called modulation
BANDWIDTH
One way to modify the carrier is to alter its amplitude
with the information to be transmitted.
e.g.
BANDWIDTH
The spectrum of this signal is:
Thus, two extra signal are produced. One fm Hz above the
carrier and one fm Hz below the carrier.
Thus the bandwidth, or amount of spectrum occupied, is 2 x fm
The main disadvantage of AM is susceptibility to interference
BANDWIDTH
Another way to modify the carrier is to change its frequency in
accordance with the information being transmitted. This is
called (surprisingly) frequency modulation or FM.
The maximum amplitude of the modulating signal is
represented by a maximum frequency deviation fDMAX and its
frequency by fmod.
Thus the frequency of a carrier fC FM modulated by a 1000Hz
tone will vary sinusoidally from fC + fDMAX to fC - fDMAX at a
frequency of 1000Hz
BANDWIDTH
The quality of the signal is a function of the ratio fDMAX / fmod.
The bandwidth required is 2 x (fDMAX + fmod )
Reasonably good quality is achieved with fDMAX = fmod so the
bandwidth required is 4 x fmod .
Thus it can be seen that the bandwidth required for a given
modulation signal is roughly 2 – 4 times its frequency
BANDWIDTH
A third type of modulation varies the phase of the carrier and
hence is called phase modulation (PM).
Since phase is the integral of frequency, its characteristics are
similar to those of FM.
It is used primarily for digital data transmission
BANDWIDTH
The rate that information must be transmitted determines the
modulating frequency
i.e. to transmit 1 Mbyte/s would require a modulating
frequency of at least 1 MHz and thus would use about 4 MHz
of bandwidth.
THE ELECTROMAGNETIC SPECTRUM
Spectrum Management
Because radio signals do not stop at national boundaries and
signals occupying the same part of the spectrum will interfere
with each other, the allotment of carrier frequencies and the
type of modulation allowed on them, are controlled by
international agreement through the ITU (International
Telecommunications Union) which is part of the UN.
This group meets every two or three years to modify the
spectrum allotments to accommodate changes in technology or
requirements.
THE ELECTROMAGNETIC SPECTRUM
The Aeronautical Spectrum
Band
Usage
System
Frequencies
VLF
Navigation
Omega
(discontinued)
10kHz
LF
Navigation
LORAN C
1MHz
LF
Navigation
Non Directional
Beacons
500-1600kHz
HF
Communications Oceanic/Polar
VHF
Navigation
ILS (Instrument 108-112MHz
Landing System)
VHF
Navigation
VOR
3-30MHz
108-118MHz
THE ELECTROMAGNETIC SPECTRUM
The Aeronautical Spectrum
Band
Usage
VHF
System
Frequencies
VHF Comm
118-136MHz
UHF
Navigation
DME (Distance
Measuring Equipment)
960-1215MHz
UHF
Navigation
TACAN
960-1215MHz
UHF
Navigation
GPS
1575.42MHz
UHF
Communications
UHF Comm (Military)
225-359MHz
UHF
Radar
ATC Radar
1030 and 1090
MHz
THE ELECTROMAGNETIC SPECTRUM
The Aeronautical Spectrum
Band
Usage
System
Frequencies
MLS (Microwave Landing
System
5.031-5.1907 GHz
SHF
Navigation
SHF
Communications Satellite Communications
various
SHF
Radar
Airborne Weather Radar
10GHz
SHF
Radar
Radar Altimeter
4.2-4.4GHz
Above
30MHz
Radar
Synthetic Vision
35GHz
Above
30MHz
Radar
Passive infrared imaging
THE ELECTROMAGNETIC SPECTRUM
Antennas
Purpose:
Provides the link between the electromagnetic wave and either
the receiver or transmitter
ANTENNAS
Definitions
Antenna Pattern
The antenna pattern describes the directional characteristics of
an antenna.
i.e. the variation of sensitivity with direction for receiving
antennas or the variation of power density with direction for
transmitting antennas
ANTENNAS
Antenna Pattern
An antenna which radiates
or is equally sensitive
in all directions os
called isotropic.
This is impossible to
achieve in practice but
it gives us a reference
ANTENNAS
Antenna Pattern
If an antenna is not isotropic, it is directional. i.e. it is more
sensitive (or radiates more power) in some directions than
in others.
ANTENNAS
Antenna Pattern
Directional antennas have what is called Gain which is the
difference between the sensitivity (or power density) of the
antenna compared to an isotropic antenna.
ANTENNAS
Polarity
• Not surprisingly, antennas exhibit polarity the same way
that EM waves do.
• i.e. a vertically polarized waves are produced by a
vertically polarized antenna
• The polarity of receiving antennas must match the polarity
of the incoming wave to achieve maximum efficiency
• E.g. When a vertically polarized wave hits a horizontally
polarized antenna, the antenna output is zero
ANTENNAS
Polarity
• If a horizontally or vertically polarized EM wave impinges
on a circularly polarized antenna, the antenna output is
50% less than if the EM wave were circularly polarized.
• VLF, LF and HF antennas are vertically polarized
• Satellite antennas are circularly polarized
ANTENNAS
Polarity
• VLF, LF and HF antennas are vertically polarized
– This is because, at these frequencies, the EM waves
propagate as GROUND WAVES
– i.e.they use the fact that earth (and especially sea) are
reasonably good conductors at low frequencies
– Also, the electric field must be perpendicular to any
conducting surface
ANTENNAS
Polarity
• Satellite antennas are circularly polarized
– This is because the ionosphere rotates the polarity of
the EM wave by a random amount (Faraday Effect)
– A linearly polarized wave would not likely match the
polarity of the receiving antenna
– Circularly polarized signals simply experience a phase
shift which does not affect the effectiveness of the
receiving antenna
ANTENNAS
Types of Antennas
• Half-Wave Dipole
– As the name implies, this antenna is made up of two
elements and is half a wavelength long
λ/2
OUTPUT
You could probably guess that this antenna is
horizontally polarized
i.e. polarization is parallel to the axis of the
antenna
ANTENNAS
Types of Antennas – Half Wave Dipole
VOR/ILS (Navigation) Antenna on CL601
ANTENNAS
Types of Antennas
• Quarter Wave Monopole
– A conducting surface acts like a mirror for EM waves.
– This is used to make more compact antennas.
– E.g.
λ/4
OUTPUT
ANTENNAS
Quarter Wave Monopole
– This example is vertically polarized
λ/4 Monopoles (VHF Communications)
ANTENNAS
Loop Antenna
– The λ/2 and λ/4 antennas respond to the Electric Field
Component of the EM wave
– They are also tuned i.e. they respond to a narrow range
of frequencies defined by their linear dimension.
– The Loop Antenna responds to the magnetic component
of the EM wave
ANTENNAS
Loop Antenna
OUTPUT
CURRENT
ANTENNAS
Loop Antenna
Polarization?
Loop antennas are not tuned.
Uses:
VLF/LF/HF
Direction Finding
VHF Navigation (ILS/VOR)
ANTENNAS
Long Wire Antenna
– Not practical for high speed aircraft
– Used for HF communications
ANTENNAS
Horn Antenna
– Used for microwave frequencies (1 GHz +)
– e.g. Weather radar
– Highly directional
– Usually used with a parabolic reflector
ANTENNAS
Installation Considerations
– Antenna Patterns
• Usually as close to omnidirectional as possible
– Or directional in the vertical plane only
– Usually downwards but for GPS, upwards
• The other and of the communications/navigation
link is usually on the ground in an unknown
direction.
• Exceptions
– Weather radar
– Satellite communications (these need external pointing
information)
ANTENNAS
Installation Considerations
– Preferably Low Drag
– On bottom of aircraft, require protection from debris from
runway
– Siting Considerations
• Shadowing from wings or other structures
• Sufficient separation from other antennas to avoid
interference (severe problem on small aircraft)
• As close as possible to transmitter/receiver installation
– Reduce cable losses
– Reduce number of connectors
ANTENNAS
Installation Considerations
– Determining Location
• Analytical (mathematical) modelling
– Fairly accurate for How Frequencies (aircraft structure
modelled as a series of conducting rods) and for
High Frequencies (aircraft modeled as a series of conducting
plane surfaces)
– In between (VHF/UHF) this not very useful (wavelengths are
close to size of aircraft structures
ANTENNAS
Installation Considerations
– Determining Location
• Scale Modelling
– Requires a large anechoic chamber
» A room lined with EM absorbing structures
– Scale model of aircraft used (usually about 1/10)
– Difficult to scale some properties (e.g. resistivity of skin should
be 1/10 the resistivity of aluminum.
ANTENNAS
Determining Location
ANECHOIC CHAMBER
ANTENNAS
Problems with Antennas
•
Poor bonding between antenna and aircraft skin.
(especially for λ/4 monopoles)
• Cabling losses and faults
• Faulty or loose Connectors