Lecture 2: Antennas and Propagation

Anders Västberg

vastberg@kth.se

08-790 44 55

Digital Communication System

Source of Information

SourceEncoder

Modulator RF-Stage

Channel

RF-StageInformation

SinkSource

DecoderDemodulator

ChannelEncoder

DigitalModulator

ChannelDecoder

DigitalDemodulator

[Slimane]

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]

Electromagnetic Fields

)cos(}{),( tetrE tj EE

(V/m),2

1ErmsE

HEP

H2

1rmsH

)(W/m,2

1

2

1 2HEP S

Poyntings Vector:

Power density:

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

H/m104

F/m10854185.87

0

120

22

0

HE

3770

00

Z

20

0

2

rmsrms HZ

Z

ES

[Slimane]

Free Space Propagation

Ptr

Ae

2

2

4

4

r

APASP

r

PS

eterr

tr

Antenna Gain

2

2

2

44

c

AfAG ee

• The antenna gain is defined by its relative power density

),(max SG

24

),(),,(

r

PS

SSrS

tr

rr

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

Diffraction

[Saunders, 1999]

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]

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]

Effective Earth Radius

[Slimane]

Microwave Communication

[Slimane]

Line-of-Sight Range

[Slimane]

Fresnel Zone

[Slimane]

Ionospheric Communication

[Davies, 1993]

Propagation Modelling

[Slimane]

Indoor models

Dipole antenna

L=

I I

• 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=/4

I

Corner Reflectors

• Multiple images results in increased gain

• Example:G=12 dBi

Driven Element

Images

Yagi-antenna

http://www.urel.feec.vutbr.cz/~raida/multimedia_en/chapter-4/4_3A.html

3-30 element and a gain of 8-20 dBi

Loop-antenna

http://www.ycars.org/EFRA/Module%20C/AntLoop.htm

• Linear Polarisation

• Gain 1,76 dBi

Parabolic antenna

• Effective area

Ae = d2/4

[Stallings, 2005]

Helical antenna

• Normal mode

• Axial mode

http://hastingswireless.homeip.net/index.php?page=antennas&type=helical

Multipath propagation