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Lecture 2: Radio Wave Propagation
Anders Västberg
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Maxwell's Equations
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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
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Radiation Uniform motion Reversing direction Direction change
Osscillating periodic motion Only accelerating charges produce radiation [Saunders, 1999]
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Propagation Mechanisms
The higher frequency The more radio waves resamble the properties of light At lower frequencies Electrical properties of obstacles are important (but we tend to express these in terms of optical properties) If the wave length is of the same order of magnitude (or larger), diffraction or surface effects dominate
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Propagation mechanisms
When the dimension of the object is: Very Large compared to the wavelength Reflection Larger compared to the wavelength Diffraction Small compared to the wavelength Scattering Very small compared to the wavelength Unaffected
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Diffraction [Saunders, 1999]
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Propagation between two antennas (not to scale)
No Ground Wave for Frequencies > ~2 MHz No Ionospheric Wave for Frequencies > ~30 Mhz
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Free Space Propagation
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Knife edge loss
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Effective Earth Radius
Re=8500 km=4/3R0 [Slimane]
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Fresnel Zones
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