Awp unit i a (1)

Antenna and Wave
Propagation
S.Mahendrakumar
Asst. Prof. (Sr. Gr.)
Department of ECE
Velalar College of Engineering and
Technology, Erode.

Basic Communication System
Transmitter Transmission Medium Receiver
Input
Transducer
Output
Transducer
Noise
wired / wireless

2. Wireless Communication
Tx Rx
Communication media
 Cellular
 WLAN
 Satellite Communication
 Radio, TV Broadcast

What is an antenna?
• An antenna is an electrical conductor or system of conductors
• Transmission - radiates electromagnetic energy into space
• Reception - collects electromagnetic energy from space
• Region of transition between guided and free space propagation
• Often part of a signal transmitting system over some distance
• Not limited to electromagnetic waves (e.g. acoustic waves)

Basic Antenna Parameters
• Radiation pattern
• Beam area and beam efficiency
• Effective aperture and aperture efficiency
• Directivity and gain
• Radiation resistance

Radiation pattern
• Far field patterns
• Field intensity decreases with increasing distance, as 1/r
• Radiated power density decreases as 1/r2
• Pattern (shape) independent on distance
• Usually shown only in principal planes

2
D
2
r
:
field
Far  D : largest dimension of the antenna
e.g. r > 220 km for APEX at 1.3 mm !

Radiation pattern (2)
)
,
( 


E )
,
( 


E
2
0
2
2
)
,
(
)
,
(
)
,
( r
Z
E
E
P





 
 

Field patterns
max
)
,
(
)
,
(
)
,
(






P
P
Pn 
+ phase patterns
)
,
( 

 )
,
( 


HPBW: half power beam width

0
30
60
90
120
150
180
210
240
270
300
330
Power pattern of 2 isotropic sources
Pn

d 1

2
  0 deg

0
30
60
90
120
150
180
210
240
270
300
330
Pn

d 1

2
  90
 deg

0
30
60
90
120
150
180
210
240
270
300
330
1.5
1
0.5
0
Field P attern of 2 isotropic sources
E i
 
i
0
30
60
90
120
150
180
210
240
270
300
330
Pn

d 1

2
  45
 deg

0
30
60
90
120
150
180
210
240
270
300
330
1.5
1
0.5
0
Field P attern of 2 isotropic sources
E i
 
i
0
30
60
90
120
150
180
210
240
270
300
330
Pn

d 1

2
  135
 deg

Radiation Pattern

Beam Area and Beam Efficiency

  





 







4
2
0 0
)
,
(
)
sin(
)
,
( d
P
d
d
P n
n
A
Main beam area
Minor lobes area


  d
P
beam
Main
n
M )
,
( 



  d
P
lobes
or
n
m
min
)
,
( 

m
M
A 




Beam area
A
M
M




Main beam efficiency

Effective Aperture and Aperture Efficiency
Receiving antenna extracts power from incident wave
e
in
rec A
S
P 

For some antennas, there is a clear physical aperture and an aperture efficiency can
be defined
p
e
ap
A
A


A
e
A


2

Aperture and beam area are linked:

Directivity and Gain
average
P
P
D
)
,
(
)
,
( m ax





A
n d
P
D










4
)
,
(
4
4
Isotropic antenna: 
4

A 1

D
2
4

 e
A
D 
From pattern
From aperture
only
losses
ohmic
to
due
lower than
is
)
1
(0
factor
efficiency
Gain
D
G
k
k
D
k
G
g
g
g




Directivity

Radiation Resistance
• Antenna presents an impedance at its terminals
A
A
A jX
R
Z 

• Resistive part is radiation resistance plus loss resistance
L
R
A R
R
R 

The radiation resistance does not correspond to a real resistor present in the antenna but to the resistance of space
coupled via the beam to the antenna terminals.

Radiation Resistance
• Antenna presents an impedance at its terminals
A
A
A jX
R
Z 

• Resistive part is radiation resistance plus loss resistance
L
R
A R
R
R 

The radiation resistance does not correspond to a real resistor present in the antenna
but to the resistance of space coupled via the beam to the antenna terminals.

Short dipole
)
1
1
(
2
)
cos(
3
2
0
)
(
0
r
j
cr
le
I
E
r
t
j
r








)
1
1
(
4
)
sin(
3
2
2
0
)
(
0
r
j
cr
r
c
j
le
I
E
r
t
j






 



)
1
(
4
)
sin(
2
)
(
0
r
cr
j
le
I
H
r
t
j









2
r
1
as
varies
P
r
1
as
vary
H
E
,
2




and
r
for 
•Length much shorter than wavelength
•Current constant along the length
•Near dipole power is mostly reactive
•As r increases Er vanishes, E and H gradually become in phase


 


l
r
e
I
j
E
r
t
j
)
sin(
60 )
(
0



RADIATED POWER AND RADIATED RESISTANCE

RADIATION FROM HALF-WAVE DIPOLE

(1)

(2)

(3)
(4)

(5)
(6)

(7)

(8)

dBi versus dBd
•dBi indicates gain vs. isotropic antenna
•Isotropic antenna radiates equally well in all directions,
spherical pattern
•dBd indicates gain vs. reference half-wavelength dipole
•Dipole has a doughnut shaped pattern with a gain of 2.15 dBi
dB
dBd
dBi 15
.
2



Received power
,
,
, 
t
t
et G
P
A
• Receiving antenna
• Transmitting antenna
r
r
er G
P
A ,
,
t
r
t
r
t
t
r P
G
G
r
G
r
P
G
P
2
2
2
4
4
4















S, power density Effective receiving area

Classification of Antenna
• Linearly polarised antennas Circularly polarised antennas
• Narrow-band Broad-band
• Wire
• Aperture
• Arrays

Polarisation of EM wave
Electrical field, E
vertical
horizontal
circular

Types of Antenna
• Wire
• Aperture
• Arrays

Wire Antenna
• Dipole
• Loop
• Folded dipoles
• Helical antenna
• Yagi (array of dipoles)
• Corner reflector
• Many more types
Horizontal dipole

Aperture antenna
• Collect power over a well defined aperture
• Large compared to wavelength
• Various types:
• Reflector antenna
• Horn antenna
• Lens

Antenna Arrays
 Motivations: to achieve desired high gain or radiation pattern, and the
ability to provide an electrically scanned beam.
 It consists of more than one antenna element and these radiating
elements are strategically placed in space to form an array with desired
characteristics which are achieved by varying the feed (amplitude and
phase) and relative position of each radiating element;
 The main drawbacks are the complexity of the feeding network required
and the bandwidth limitation (mainly due to the feeding network)

 The driven element (feeder) is the very heart of the antenna. It determines the
polarisation and centre frequency. For a dipole, the recommended length is about 0.47
to ensuring a good input impedance to a 50 Ω feed line.
 The reflector is longer than the feeder to force the radiated energy towards the front. The
optimum spacing between the reflector and the feeder is between 0.15 to 0.25
wavelengths.
 The directors are usually 10 to 20% shorter than the feeder and appear to direct the
radiation towards the front. The director to director spacing is typically 0.25 to 0.35
wavelengths,
 The number of directors determines the maximum achievable directivity and gain.

Awp unit i a (1)

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Awp unit i a (1)