Transcript Lecture 2: Radio Wave Propagation

Lecture 2:
Radio Wave Propagation
Anders Västberg
08-790 44 55
[email protected]
Maxwell's Equations
D
B
f
0
B
E
t
D
H
Jf
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
Uniform motion
Reversing direction
Direction change
Osscillating periodic
motion
Only accelerating charges produce
radiation
[Saunders, 1999]
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)
Sky Wave
Direct Wave
Ground Wave
Ground Reflected
Wave
No Ground Wave for Frequencies > ~2 MHz
No Ionospheric Wave for Frequencies > ~30 Mhz
Free Space Propagation
Ae
Pt
r
Pt
Sr 
2
4r
Pt Ae
Pr  S r Ae 
2
4r
Knife edge loss
L dB
25
20
15
Out[12]=
10
5
v
4
2
2
4
Effective Earth Radius
Re=8500 km=4/3R0
[Slimane]
Fresnel Zones