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Lecture 2: Antennas and Propagation

Lecture 2: Antennas and Propagation. Anders Västberg vastberg@kth.se 08-790 44 55. Digital Communication System. Source of Information. Source Encoder. Channel Encoder. Digital Modulator. Modulator. RF-Stage. Channel. Information Sink. Source Decoder. Demodulator. RF-Stage.

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Lecture 2: Antennas and Propagation

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  1. Lecture 2: Antennas and Propagation Anders Västberg vastberg@kth.se 08-790 44 55

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

  3. 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

  4. Radiation Only accelerating charges produce radiation [Saunders, 1999]

  5. Electromagnetic Fields Poyntings Vector: Power density:

  6. 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]

  7. Free Space Propagation

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

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

  10. Diffraction [Saunders, 1999]

  11. 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]

  12. Propagation in the Atmosphere • The atmosphere around the earth contains a lot of gazes (1044 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]

  13. Effective Earth Radius [Slimane]

  14. Microwave Communication [Slimane]

  15. Line-of-Sight Range [Slimane]

  16. Fresnel Zone [Slimane]

  17. Ionospheric Communication [Davies, 1993]

  18. Propagation Modelling [Slimane]

  19. Indoor models

  20. L=l/2 I I Dipole antenna • 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=l/4 I

  21. Corner Reflectors • Multiple images results in increased gain • Example:G=12 dBi Images l/2 Driven Element

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

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

  24. Parabolic antenna • Effective area Ae =hp d2/4 h=0.56 [Stallings, 2005]

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

  26. Multipath propagation

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