# Lecture 4: Propagation Models, Antennas and Link Budget Anders Västberg 08-790 44 55.

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Lecture 4: Propagation Models, Antennas and Link Budget Anders Västberg vastberg@kth.se 08-790 44 55

Digital Communication System Source of Information Source Encoder ModulatorRF-Stage Channel RF-Stage Information Sink Source Decoder Demodulator Channel Encoder Digital Modulator Channel Decoder Digital Demodulator [Slimane]

Propagation between two antennas (not to scale) No Ground Wave for Frequencies > ~2 MHz No Ionospheric Wave for Frequencies > ~30 Mhz

The Radio Link Design considerations –The distance over which the system meets the performance objectives –The capacity of the link. Performance determined by –Frequency –Transmitted Power –Antennas –Technology used [Black et. al]

Noise Thermal noise – White Noise Spectral density:(W/Hz) kBoltzmann’s constant (1.38 10 -23 J/K) TAbsolute temperature (in Kelvins) Noise power (in W) BBandwidth (Hz)

Signal to noise ratio (SNR)

Propagations Models Free Space Model Plane Earth Model Power Law Model

Dipole antenna Omnidirectional L=  II Half-wave dipole –Gain 1,64 = 2.15 dBi –Linear Polarisation Quarter-wave dipole –Conducting plane below a single quarter wave antenna. Acts like a half- wave dipole L= /4 I

Yagi-antenna Directional http://www.urel.feec.vutbr.cz/~raida/multimedia_en/chapter-4/4_3A.html 3-30 element and a gain of 8-20 dBi

Parabolic antenna Directional Effective area A e =  d 2 /4  [Stallings, 2005]

Corner Reflectors Multiple images results in increased gain Example: G=12 dBi  Driven Element Images

Loop-antenna - Directional http://www.ycars.org/EFRA/Module%20C/AntLoop.htm Linear Polarisation Gain 1,76 dBi

Helical antenna Directional Normal mode Axial mode http://hastingswireless.homeip.net/index.php?page=antennas&type=helical

Microwave Communication [Slimane]

Propagation in the Atmosphere The atmosphere around the earth contains a lot of gases (10 44 molecules) It is most dense at the earth surface (90% of molecules below a height of 20 km). It gets thinner as we reach higher and higher attitudes. The refractive index of the air in the atmosphere changes with the Height This affects the propagation of radio waves. The straight line propagation assumption may not be valid especially for long distances. [Slimane]

Line-of-Sight Range [Slimane]

Ray Paths and Wave fronts

Fresnel Zone [Slimane]

Ionospheric Communication [Davies, 1993]

Indoor models