# Lecture 2: Antennas and

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16 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

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Lecture 2: Antennas and
Propagation

Anders Västberg

vastberg@kth.se

08
-
790 44 55

Digital Communication
System

Source of

Information

Source

Encoder

Modulator

RF
-
Stage

Channel

RF
-
Stage

Information

Sink

Source

Decoder

Demodulator

Channel

Encoder

Digital

Modulator

Channel

Decoder

Digital

Demodulator

[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(
}
{
)
,
(

t
e
t
r
E
t
j
E
E
(V/m)
,
2
1
E

rms
E
H
E
P

H
2
1

rms
H
)
(W/m
,
2
1
2
1
2
H
E
P

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/m
10
4
F/m
10
854185
.
8
7
0
12
0
2
2
0

H
E

377
0
0
0

Z
2
0
0
2
rms
rms
H
Z
Z
E
S

[Slimane]

Free Space Propagation

P
t
r
A
e
2
2
4
4
r
A
P
A
S
P
r
P
S
e
t
e
r
r
t
r

Antenna Gain

2
2
2
4
4
c
A
f
A
G
e
e

The antenna gain is defined by its relative power
density

)
,
(
max

S
G

2
4
)
,
(
)
,
,
(
r
P
S
S
S
r
S
t
r
r
r

Propagation between two
antennas (not to scale)

No Ground Wave for Frequencies > ~2 MHz

No Ionospheric Wave for Frequencies > ~30 Mhz

Direct Wave
Ground Reflected
Wave
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 (10
44

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=
/2

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

/2

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

A
e

=

h
d
2
/4

h0.56

[Stallings, 2005]

Helical antenna

Normal mode

Axial mode

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

Multipath propagation