Radio Communication
Presentation created by:
András Balogh
AM and FM
• The goal is to transmit a modulating signal S(t) via a
wave sin(ωt).
• In case of AM, the product of the modulation is
f(t)=(A+S(t))*sin(ωt);
• In case of FM, the product is f(t)=A*sin((ω+S(t))*t);
• AM easily picks up noise, any random absorption of
energy is a source
• Above a noise threshold, FM has a better signal to
noise ratio
AM and FM
Quadrature AM
• Uses sine and cosine carrier waves
• Transmits 2 digital or analog signals
• f(t)=(A+S1(t))sin(ωt)+(A+S2(t))cos(ωt)
Stereo FM
• Transmits 2 audio signals, L(t) and R(t)
• Compatible with mono FM
• Main channel: L+R, decoded by mono, 30 Hz –
15 kHz
• Second channel: L-R, codes the stereo signal,
23 kHz – 53 kHz
• Phase alignment: pilot wave, 19 kHz
Stereo FM
Radiowave propagation on Earth
• The medium of propagation is the air, not vacuum, it
has to be considered.
• The opacity of the atmosphere is highly dependant
on the radio frequency
• The Earth can be considered as a conductor
• The ionosphere is also a conductor in the form of
plasma
Atmospheric opacity
Direct mode propagation
• Direct mode or line-of-sight propagation is the free
propagation of radiowaves in the air.
• The range of direct mode is limited by the curvature
of the Earth.
• Small antennae:
𝑅ℎ𝑜𝑟𝑖𝑧𝑜𝑛 𝑘𝑚 = 3.57 ℎ𝑎𝑛𝑡𝑒𝑛𝑛𝑎 [𝑚]
• Very High Frequency (VHF)
• Tropospheric scatter and ducting can propagate
waves over the horizon.
Direct mode propagation
Surface Modes
• The conducting properties of the ground can attach
the radiowave to the surface
• A groundwave propagates along the curved surface
of the Earth, beyond the horizon.
• The ground dissipates the energy of the groundwave
• Only low frequency waves (30-3000 Hz) can travel via
groundwave modes.
Surface Modes
The Ionosphere
The Ionosphere
• The D layer: innermost layer from 60 km to 90 km. Solar
radiation, only in daylight.
• High absorption for high frequency (HF) radiowaves, long
distance AM communication impossible during daytime.
• The E layer: from 90 km to 150 km. It reflects radiowaves
up to 10 MHz. Sporadic events: additional plasma clouds
form, it can reflect up to 225 MHz.
• The F layer: 150 km to 500 km. Any radiowave that
penetrates the layer will go out to space.
Ionospheric propagation
• Ionospheric propagation: skywave or skip, reflection
and refraction from the Ionosphere’s E and F layers.
• The radiowaves also reflect from the ground, they
can reach the far side of the Earth.
• HF radiowaves can travel in this manner.
• The Earth and the Ionosphere also creates a
waveguide effect, allowing an other form of
propagation.
Skywave
Waveconduction
Signal weakening
• Absorption in the air
• Dissipation in the ground
• Scattering effects
• Scattering effects also generate noise
• Some examples of scattering sources:
Rain, airplanes, lightning, meteors, auroras,
tropospheric humidity, metal objects on the
surface
Band acronym
Band name
Frequency
range
Wavelength
range
Propagation
modes
VLF
Very Low
Frequency
3-30 kHz
100-10 km
Waveguide
LF
Low Frequency
30-300 kHz
10-1 km
Waveguide,
groundwave
MF
Medium
Frequency
300-3000 kHz
1000-100 m
Groundwave,
skywave
HF
High Frequency 3-30 MHz
100-10 m
Skywave
VHF
Very High
Frequency
30-300 MHz
10-1 m
Direct wave,
skywave, trop.
duct.
UHF
Ultra High
Frequency
300-3000 GHz
1000-100 mm
Direct wave,
trop. duct.
SHF
Super High
Frequency
3-30 GHz
100-10 mm
Direct wave
EHF
Extremely High
Frequency
30-300 GHz
10-1 mm
Direct wave
with
absorption
Communication satellites
• Geostationary orbit: 35786 km, more than 100 satellites, no
tracking required
• Low-Earth-orbit: 200 km, 90 min, visible in 1000 km, small
signal strength, cheap to launch, satellite constellations,
discontinuous data transmission
• Molniya orbit: highly elliptical orbit, 63.4° inclination, arctic
communication, 40000 km apogee
• Medium-Earth-orbit: 8000 km to 18000 km, wider coverage
than LEOs, more visible time, weaker signal, less satellites
required for network
• Polar orbit: sun syncronous, less used
Geostationary
orbits from
Earth
Molniya orbit
Molniya orbit projection
Orbital altitude classification
Comsat applications
• Telephone communication: islands, sparsly populated areas,
polar regions
• Television
• Digital Cinema: movies are digitalized, then distributed to
theatres via satellites among other ways
• Internet
• Military
Thank you for your attention