REVIEW

Radio Theory
At the end of this lecture, the student should
be able to:
Describe about radio principles
Explain the applications of HF, VHF, UHF in
aviations
Radio is wireless transmission through space of
ELECTROMAGNETIC WAVES in the approximate
frequency range from 10kHz to 300’000MHz.
Radio theory is essential knowledge for the
understanding of the reasons why particular
frequencies are used for particular
navigational aids system (DME,VOR & etc)
 Communication is a process of transmitting
INFORMATION from one location to another
MEDIUM is required for the delivery of the
information to be exchanged.
For example,
Transmission medium for television or
telephone is cable or fiber optics
Radio transmission starts in US in 1920 with the
invention of RADIO TRANSMITTING
EQUIPMENT that capable of transmitting voice
and music.
TRANSMITTER : a device used to generate and
transmit radio signals [ELECTROMAGNETIC WAVES].
RECEIVER : a device that receives incoming radio
signals and converts them to sound or light.
Example: receiver on radio or television converting
broadcast signals into sound or images.
In communication link, the TRANSMITTER is
connected through a cable to one ANTENNA.
ANTENNA is a device which provides means for
radiating or receiving radio waves.
The signal is radiated to ANOTHER ANTENNA,
and then passes through another cable to the
RECEIVER.
Broadcast systems such as TV or radio can used
one transmitter to serve many receivers via a
free space link.
Types of antenna
Reflector
Array Antenna antenna

Aperture antennas
Types of antenna
Wire Antennas

Printed Circuit
Antenna
Electromagnetic Wave = Electric wave + Magnetic Wave
Electromagnetic wave are used to transmit information
by wave motion.
Both waves oscillate at the same frequency

ti o n Electric Field
ag a z
o p
f Pr
y
tio no
i r e c
D Magnetic Field

x
The types of electromagnetic wave including:
RADIO WAVES (for TV, radio, aircraft
communication)
Microwaves (radar, microwave oven)
Infrared radiation (Body screening, remotes)
Visible Light (Bulb)
Ultraviolet (Medical Equipments)
X-rays (Medical X-rays, Baggage Screening)
Gamma rays (Space Observations)
Everyday technologies that depend on radio waves:
Standard broadcast radio and television
Wireless networks
Cell phones
GPS receivers
Aircraft Communications
ATC communications
Satellite communications
Police radios, Wireless clocks, Cordless phones ,
Garage door openers
 Radio waves are
radiated energy.

Subject to Consist of
attenuation Properties
of Radio oscillating
& electric and
interference Waves
magnetic fields

Can be reflected, In free space, they travel in

refracted and straight lines at the speed of
diffracted light c = 3108 m/s.
In free space, radio wave travel in straight lines at
the speed of light c = 3108 m/s.

Frequency is the number of wave cycles that occur

within 1 second. It is measured in Hertz.
f = c/c is speed of &  is wavelength

Wavelength is the distance a radio wave travels

during one cycle.
Amplitude is the strength of the signal.
THE HIGHER THE FREQUENCY, THE SHORTER THE
WAVELENGTH
What is the frequency of an Air Traffic Control
operating at a wavelength of 2.5m? [speed of
light c = 3108 m/s.]

f = c/
f= 3108 m/s / 2.5m
f= 120’ 000 000 Hz
f= 120 MHz
What is the frequency of Distance Measuring
Equipment (DME) operating at a wavelength of
0.3m? [speed of light c = 3108 m/s.]

f = c/
f= 3108 m/s / 0.3m
f= 1 x 109Hz @ 1000 x 106 Hz
f= 1 GHz @ 1000 MHz
What is the wavelength of ILS Localizer
operating at a frequency of 100MHz? [speed of
light c = 3108 m/s.]
What is the wavelength of an aircraft
communications operating at a frequency of
30MHz? [speed of light c = 3108 m/s.]
Reflection is the change in direction of travel of a wave,
due to hitting a reflective surface.
This is the same characteristic displayed by a radio wave as
it is reflected from the ionosphere.
When reflection of wave happen, the wavelength,
frequency and speed do not change.
Refraction occurs when the radio waves go from one
medium to another medium.
Refract means change the direction of radio propagation of
by causing them to travel at different speeds and at
different direction along the wave front.
Diffraction of waves is spreading out of waves when
they move through a gap or around an obstacle.
Frequency, wavelength and speed of waves do not
change.
The direction of propagation and the pattern of waves
are change.

Direction of wave propagation

Obstacle
Radio wave can pass through an opaque object, but
may suffer attenuation.
Attenuation is the loss of wave energy as it travels
through a medium .
Or in short word the strength of the radio wave is
reduced.
Interference is the prevention of reception of a clear
radio signal.
Interference also means the superposition of two or
more waves from same sources.
There are a large number of users of radio
communication
How can these users coexist without interfering with
each other?
Radio communicators can operate without
interfering by choosing different radio
frequency
To eliminate confusion and facilitate
international understanding the majority of the
countries of the world have joined in
establishing standard classification systems.
The use of the various Radio Frequencies all
over the World is allocated by ITU
(International Telecommunications Union).
• Why do you think airline attendants ask
passengers to turn off electronic devices
including phones, radios, TVs and computer
during take-off and landing?
 Table of Radio Frequencies
Description Abbreviation Frequency Wavelength

Very Low
Frequency VLF 3 KHz - 30 KHz 100,000m - 10,000m

Low Frequency LF 30 KHz - 300 KHz 10,000m - 1,000

Medium Frequency MF 300 KHz - 3 MHz 1,000m - 100m

High Frequency HF 3 MHz - 30 MHz 100m - 10m

Very High
Frequency VHF 30 MHz - 300 MHz 10m - 1m

Ultra High
Frequency UHF 300 MHz - 3 GHz 1m - 0.10m

Super High
Frequency SHF 3 GHz - 30 GHz 0.10m - 0.01m

Extremely High
Frequency EHF 30 GHz - 300 GHz 0.01m - 0.001m
Band Frequency
LF 30– 300kHz
MF 300kHz– 3MHz
HF 3 – 30MHz
VHF 30 – 300MHz
UHF 300MHz– 3GHz
There are three principle paths which radio
waves may follow over the earth between the
transmitter and the receiver:
Surface Wave
(Ground wave)

Wave Propagation Sky wave

Space waves
Follows the contour
of the Earth

Line of Sight: Clear path between

transmitting and receiving antennas
Surface Wave (Ground Wave)
A wave which follows the contours of the earth’s surface.
Propagate Low Frequencies (LF,MF)
Used for short distance radio transmissions
Sky Wave
A wave that is refracted by the Ionosphere and returned to
earth.
Propagate Middle Range Frequencies (HF).
Used for long distance transmissions
Space Wave
A wave which is line of sight (reception dependant on altitude).
Propagate Upper Range Frequencies.
Used for VHF/UHF Transmissions.
Antennas on Aircraft
Type and size of antenna varies with the different types of aircraft.
The location of antenna depends on the design of aircraft.
HF1 = Typically used for Long-Range ATC
communications.
HF2 = Back up to HF1. (frequently used to listen
to the BBC to overcome boredom on longer
flights).

VHF1 = Used for ATC communications

VHF2 = Emergency frequency monitoring.
VHF3 = Typically used for Data transmissions
HF-High Frequency (3– 30 MHz)

HF is the basic band for long-range communications,

mainly because its transmissions are reflected from
the ionosphere.

HF is widely used for domestic aircraft voice

communications.
HF undergoes the “Skywave” phenomena where
ionosphere reflects the HF radio waves and can be
utilized for medium and long range radio
communications,

HF transmissions are reflected from the ionosphere.

Disadvantage of HF
Efficiency of HF is affected by:
All kinds of electrical interference caused by
ionosphere disturbances such as
thunderstorms. This provides the typical radio
noise.
Other weather phenomena:
Sunlight
Season
VHF: Very High Frequency
VHF-Very High Frequency (30 – 300MHz)

Normally, VHF between 100 to 200MHz are used for

ATC communications, emergency and navigational aids
(VOR, DME, ILS).
The propagation characteristics of VHF are optimized
for short range communications.
The range varies depending the atmospheric
conditions but normally is about 180 nautical miles.
However it is more prone to blockage by Land
Features & buildings.
Specific Usage
VHF Voice communication Source Carrier
118-121.4 MHz Air Traffic Control

121.5 MHz Emergency

121.6 – 121.9 MHz Airport Ground control

123.1 MHz Search And Rescue

123.675-128.8 MHz Air traffic control

128.825-132.0 MHz En Route

132.05-135.975 MHz Air traffic control

UHF-Ultra High Frequency (300MHz– 3GHz)

UHF: similar to VHF, but is restricted mainly for

military aviation use.

In Malaysia, UHF is mostly used by Combat Aircraft of

the Royal Malaysian Air Force (RMAF).

Other applications includes Navigations/Landing Aids

such as the Glide path component of the
ILS(Instrument Landing Systems).
• At current count, there are also UHF signals receivable
by Television, example, TV8, TV3, NTV7 and TV9 and
ASTRO.