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+s 1 (t))sin(ωt)+(a+s 2 (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: R horizon km = 3.57 h antenna [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 VLF Very Low Frequency Wavelength range Propagation modes 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 UHF SHF EHF Very High Frequency Ultra High Frequency Super High Frequency Extremely High Frequency 30-300 MHz 10-1 m Direct wave, skywave, trop. duct. 300-3000 GHz 1000-100 mm Direct wave, trop. duct. 3-30 GHz 100-10 mm Direct wave 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
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