lecture 2: radio wave propagation anders västberg 08-790 44 55 [email protected]
TRANSCRIPT
Lecture 2:Radio Wave Propagation
Anders Västberg
08-790 44 55
Maxwell's Equations
D f B 0
E B
t
H J f D
t
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]
Uniform motion
Reversing direction
Direction change
Osscillating periodic motion
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
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
Diffraction
[Saunders, 1999]
Propagation between two antennas (not to scale)
No Ground Wave for Frequencies > ~2 MHzNo Ionospheric Wave for Frequencies > ~30 Mhz
Direct Wave
Ground ReflectedWave
Ground Wave
Sky Wave
Free Space Propagation
Ptr
Ae
2
2
4
4
r
APASP
r
PS
eterr
tr
Knife edge loss
Out[12]=
4 2 2 4v
5
10
15
20
25
LdB
Effective Earth Radius
[Slimane]
Re=8500 km=4/3R0
Fresnel Zones