Lecture 2: Radio Wave Propagation Anders Västberg 08-790 44 55 [email protected].

Lecture 2:Radio Wave Propagation

Anders Västberg

08-790 44 55

[email protected]

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