1. Radio Waves
AVIONICS
TECHNOLOGY
Radio is a method of transmitting intelligence from one location to
another by means of electromagnetic radiation.
The first radios installed in airplanes were used for communications
and it was only much later that navigational radio systems were
developed.
Radio systems for other purposes have also been developed,
especially in the last 20 to 30 years.
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Some of the uses for radio in modern aircraft can be categorized as
follows.
 Communications ‑ Both voice and data.
 Navigation ‑ Many different systems are in use today.
 ATC Radar ‑ The Air Traffic Control system relies on radar.
 Weather Avoidance ‑ Used to avoid areas of adverse weather.
 Approach Aids ‑ A specialized type of navigation to guide an aircraft

down to the runway in bad weather.
Altitude Measurement ‑ Gives precise alti­tude above ground level.

Airborne Collision Avoidance ‑ Warns the pilot of nearby aircraft.
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Radio Wave Spectrum
The RF portion of the electromagnetic wave spectrum extends from
approximately 3 kHz to 300 GHz.
Table radio frequency bands:
FREQUENCY RANGE
BAND
Very Low Frequency (VLF)
3 to 30 kHz
Low Frequency (LF)
30 to 300 kHz
Medium Frequency (MF)
300 to 3000 kHz
High Frequency (HF)
3 to 30 MHz
Very High Frequency (VHF)
30 to 300 MHz
Ultra High Frequency (UHF)
300 to 3000 MHz
Super High Frequency (SHF)
3 to 30 GHz (Giga Hertz)
Extremely High Frequency (EHF)
30 to 300 GHz
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Radio Wave Spectrum (cont’d)
The frequencies are divided into
bands which, starting at the low
end, are: very low frequency (VLF),
low frequency (LF), medium
frequency (MF), high frequency
(HF), very high frequency (VHF),
ultra high frequency (UHF), super
high frequency (SHF) and extremely
high frequency (EHF).
Radio frequency chart showing the
operating frequencies of common
aircraft systems:
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Radio Wave Spectrum (cont’d)
The range and diversity of
electromagnetic waves, or EM
waves, is very broad.
The entire spectrum of EM waves
includes not only radio waves but
visible light, gamma rays, infrared,
etc.
Common aircraft radio systems are
included on the right side in the
Figure.

Notice: there is an aviation
application for all of the bands
except EHF.
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Radio Wave Spectrum (cont’d)
Radio waves are produced by an
alternating current that is fed to an
antenna to produce radio‑frequency
waves.
Radio waves travel as an energy
field from the antenna at the speed
of light, approximately 300,000,000
meters per second.
An RF wave is composed of an
electromagnetic field and a 90o
displaced electrostatic field.
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Radio Wave Spectrum (cont’d)
The polarization of an RF wave is
determined by the direction of the
lines of force in the electromagnetic
field.
The direction of these lines of force
is dependent on the polarization of
the radiating element.

A vertical antenna produces a
vertically polarized wave

A horizontal antenna emits a
horizontally polarized wave.
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Simple vertical antennas have
omnidirectional characteristics in
that they transmit and receive in a
360o pattern,
 They are used for
communication.
Horizontally polarized antennas
are basically directional,
 They are primarily used for
navigation.
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Radio Wave Spectrum (cont’d)
All EM waves consist of two different invisible energy fields that travel through
space. The electric field and the magnetic field are at right angles to each other
and to the direction of propagation or travel.
Figure below shows the two fields and the direction of propagation.
A radio wave has two components,
an electric wave (E) and a magnetic wave (H)
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Radio Wave Spectrum (cont’d)
Radio waves are produced when a
radio frequency electrical signal is
sent down a conductor to an
antenna.
The antenna transforms the
electrical signal into EM waves
which propagate outward from the
antenna through space.
The EM waves travel through space
at the velocity of light, which is
186,284 miles per second or
300,000,000 meters per second.
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The basic operating frequency of a
radio is called the carrier frequency,

the signal carries the data or
information that needs to be
transmitted from one place to
another.
The wavelength of an EM wave is
the distance from peak to peak for
the invisible waves in the electric
and magnetic fields.
Wavelength is measured in meters
and it is inversely proportional to
the frequency.
The wavelength in meters can be
found by dividing the constant
300,000,000 by the frequency in
hertz.
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Frequency Band Use
The usable range of frequencies
has been divided and bands
assigned for various communication
and navigation purposes.
The frequencies used for aviation
communication and navigation are
shown in Figure on the right side.
Communication radios use highly
sensitive and selective transmitters
and receivers for two-way
communication between aircraft
and ground stations or between
aircraft in flight.
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VLF and LF Band Communication
The very low frequency (VLF) and
low frequency (LF) bands were
originally used for radio navigation.
Because the wavelengths were in
the kilometer range and higher (30
kHz has a wavelength of 10
kilometers, or about 6.2 miles),
enormous antennas had to been
used.
Until today, these frequency bands
are still used in aircraft for long
range navigation (LORAN-C, Omega
and Decca) and direction finder
navigation (ADF).
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MF and HF Band Communication
The medium-frequency (MF) and
high-frequency (HF) bands are not
only used by commercial AM
broadcasting stations, but portions
are also used by aircraft for air-toground communication.
Aircraft flying over the oceans
typically use HF communication
because it can travel great
distances.
HF equipment operates in the
frequency range of 2 to 25
megahertz and is normally singlesideband.
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MF and HF Band Communication
(cont’d)
Signal radiation in these frequency
ranges have the important property
of being reflected by the
ionosphere.
When a radio wave in the MF or HF
range hits ionosphere layer, it is
reflected back to earth.
Multiple reflections between this
layer and earth are possible,
allowing great distances to be
obtained in these ranges,
particularly the high-frequency
band.
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MF and HF Band Communication
(cont’d)
The disadvantage of this type of
propagation is that it depends on
the characteristics of the
ionosphere, which varies widely,
especially during daylight hours.
As a result of this varying, the
waves are reflected differently and
take different paths over a period of
time.

This causes the signal at the
receiver to vary in strength,
which causes the output to fade
in and out.
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VHF and UHF Band Communication
Signal radiation in these frequency
ranges gets very little ionospheric
reflection.
Communications in these ranges
tend to be line-of-sight and over a
short distance.
Line-of-sight means exactly what
the name says the transmitter and
receiver must be within a straight
visual sighting line from each other.
Buildings and uneven terrain may
affect the transmission.
For commercial use, the lower part
of the UHF band and the VHF band
is also used for mobile
communications and television.
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VHF and UHF Band Communication
(cont’d)
Very high frequency (VHF) radio
transmissions operate in the
118.000 to 135.975 megahertz
range.
This frequency range is used for air
traffic control (ATC) communication
and for communication between
civil aircraft operated domestically.
VHF communication use singlechannel simplex operation in which
a single frequency is used for both
transmitting and receiving (singlechannel), but only one person can
talk at a time (simplex).
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