Lecture 2: Antennas and Propagation Anders Västberg

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]

Maxwell's Equations Electrical field lines may either start and end on charges, or are continuous Magnetic field lines are continuous An electric field is produced by a time-varying magnetic field A magnetic field is produced by a time-varying electric field or by a current

Radiation Only accelerating charges produce radiation [Saunders, 1999]

Electromagnetic Fields Poyntings Vector: Power density:

Impedance of Free Space Both fields carry the same amount of energy Free space impedance is given by The power density can be expressed as [Slimane]

Free Space Propagation

Antenna Gain The antenna gain is defined by its relative power density

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

Diffraction [Saunders, 1999]

Diffraction For radio wave propagation over rough terrain, the propagation is dependent on the size of the object encountered. Waves with wavelengths much shorter than the size of the object will be reflected Waves with wavelengths much larger than the size of the obstacle will pass virtually unaffected. Waves with intermediate wavelengths curve around the edges of the obstacles in their propagation (diffraction). Diffraction allows radio signals to propagate around the curved surface and propagate behind obstacles. [Slimane]

Propagation in the Atmosphere The atmosphere around the earth contains a lot of gazes (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]

Effective Earth Radius [Slimane]

Microwave Communication [Slimane]

Line-of-Sight Range [Slimane]

Fresnel Zone [Slimane]

Ionospheric Communication [Davies, 1993]

Propagation Modelling [Slimane]

Indoor models

Dipole antenna 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

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

Yagi-antenna element and a gain of 8-20 dBi

Loop-antenna Linear Polarisation Gain 1,76 dBi

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

Helical antenna Normal mode Axial mode

Multipath propagation