Lecture 5: Antennas and Wave 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]

Isotropic Antenna An isotropic radiator is an hypothetical antenna that generates a uniform field, i.e., energy flows with equal strength in all directions. Let P t be the total power emitted by this antenna. This total power will be uniformly distributed over the surface of a sphere enclosing the antenna. The power density on a sphere of radius r is given by: [Slimane]

Effective Aperture of an Antenna A e is defined as the area of a perfect lossless antenna The received power can be written as follows: A e is dependent on the type of receiving antenna [Slimane]

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

Wave propagation The field at the receiver can be decomposed into three components –Direct wave, Line-of-Sight Path –Ground reflected wave –Ground Wave (less than 2 MHz, less than 10 MHz over water) [Slimane]

Plane Earth Model [Slimane]

Plane Earth Model [Slimane]

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]

Propagation Modelling [Slimane]