CHAP3 dan CHAP4-Antena dan perambatan sinyal.ppt

Antennas and Propagation
Chapter 5

Introduction
 An antenna is an electrical conductor
or system of conductors
 Transmission - radiates electromagnetic
energy into space
 Reception - collects electromagnetic
energy from space
 In two-way communication, the same
antenna can be used for transmission
and reception

Radiation Patterns
 Radiation pattern
 Graphical representation of radiation
properties of an antenna
 Depicted as two-dimensional cross section
 Beam width (or half-power beam width)
 Measure of directivity of antenna
 Reception pattern
 Receiving antenna’s equivalent to radiation
pattern

Types of Antennas
 Isotropic antenna (idealized)
 Radiates power equally in all directions
 Dipole antennas
 Half-wave dipole antenna (or Hertz
antenna)
 Quarter-wave vertical antenna (or
Marconi antenna)
 Parabolic Reflective Antenna

Antenna Gain
 Antenna gain
 Power output, in a particular direction,
compared to that produced in any
direction by a perfect omnidirectional
antenna (isotropic antenna)
 Effective area
 Related to physical size and shape of
antenna

Antenna Gain
 Relationship between antenna gain and
effective area

G = antenna gain
 Ae = effective area

f = carrier frequency

c = speed of light 3 108
m/s)

 = carrier wavelength
2
2
2
4
4
c
A
f
A
G e
e 





Propagation Modes
 Ground-wave propagation
 Sky-wave propagation
 Line-of-sight propagation

Ground Wave Propagation
 Follows contour of the earth
 Can Propagate considerable
distances
 Frequencies up to 2 MHz
 Example
 AM radio

Sky Wave Propagation
 Signal reflected from ionized layer of
atmosphere back down to earth
 Signal can travel a number of hops, back
and forth between ionosphere and earth’s
surface
 Reflection effect caused by refraction
 Examples

Amateur radio
 CB radio

Line-of-Sight Propagation
 Transmitting and receiving antennas must be
within line of sight
 Satellite communication – signal above 30 MHz not
reflected by ionosphere
 Ground communication – antennas within effective line
of site due to refraction
 Refraction – bending of microwaves by the
atmosphere
 Velocity of electromagnetic wave is a function of the
density of the medium
 When wave changes medium, speed changes
 Wave bends at the boundary between mediums

Line-of-Sight Equations
 Optical line of sight
 Effective, or radio, line of sight

d = distance between antenna and
horizon (km)

h = antenna height (m)

K = adjustment factor to account for
refraction, rule of thumb K = 4/3
h
d 57
.
3

h
d 
 57
.
3

Line-of-Sight Equations
 Maximum distance between two
antennas for LOS propagation:
 h1 = height of antenna one
 h2 = height of antenna two
 
2
1
57
.
3 h
h 



LOS Wireless Transmission
Impairments
 Attenuation and attenuation distortion
 Free space loss
 Noise
 Atmospheric absorption
 Multipath
 Refraction
 Thermal noise

Attenuation
 Strength of signal falls off with distance
over transmission medium
 Attenuation factors for unguided media:
 Received signal must have sufficient strength
so that circuitry in the receiver can interpret the
signal
 Signal must maintain a level sufficiently higher
than noise to be received without error
 Attenuation is greater at higher frequencies,
causing distortion

Free Space Loss
 Free space loss, ideal isotropic antenna
 Pt = signal power at transmitting antenna
 Pr = signal power at receiving antenna

 = carrier wavelength

d = propagation distance between antennas

c = speed of light 3 10 8 m/s)
where d and  are in the same units (e.g., meters)
   
2
2
2
2
4
4
c
fd
d
P
P
r
t 





Free Space Loss
 Free space loss equation can be
recast:









d
P
P
L
r
t
dB
4
log
20
log
10
    dB
98
.
21
log
20
log
20 


 d

    dB
56
.
147
log
20
log
20
4
log
20 








 d
f
c
fd


Free Space Loss
 Free space loss accounting for gain of other
antennas
 Gt = gain of transmitting antenna
 Gr = gain of receiving antenna
 At = effective area of transmitting antenna
 Ar = effective area of receiving antenna
       
t
r
t
r
t
r
r
t
A
A
f
cd
A
A
d
G
G
d
P
P
2
2
2
2
2
2
4







Free Space Loss
 Free space loss accounting for gain of
other antennas can be recast as
     
r
t
dB A
A
d
L log
10
log
20
log
20 

 
      dB
54
.
169
log
10
log
20
log
20 



 r
t A
A
d
f

Categories of Noise
 Thermal Noise
 Intermodulation noise
 Crosstalk
 Impulse Noise

Thermal Noise
 Thermal noise due to agitation of
electrons
 Present in all electronic devices and
transmission media
 Cannot be eliminated
 Function of temperature
 Particularly significant for satellite
communication

Thermal Noise
 Amount of thermal noise to be found in a
bandwidth of 1Hz in any device or
conductor is:
 N0 = noise power density in watts per 1 Hz of
bandwidth

k = Boltzmann's constant = 1.3803 10-23
J/K

T = temperature, in kelvins (absolute temperature)
 
W/Hz
k
0 T
N 

Thermal Noise
 Noise is assumed to be independent of
frequency
 Thermal noise present in a bandwidth of B
Hertz (in watts):
or, in decibel-watts
TB
N k

B
T
N log
10
log
10
k
log
10 


B
T log
10
log
10
dBW
6
.
228 




Noise Terminology
 Intermodulation noise – occurs if signals with
different frequencies share the same medium
 Interference caused by a signal produced at a
frequency that is the sum or difference of original
frequencies
 Crosstalk – unwanted coupling between signal
paths
 Impulse noise – irregular pulses or noise spikes
 Short duration and of relatively high amplitude
 Caused by external electromagnetic disturbances,
or faults and flaws in the communications system

Expression Eb/N0
 Ratio of signal energy per bit to noise power
density per Hertz
 The bit error rate for digital data is a function
of Eb/N0

Given a value for Eb/N0 to achieve a desired error
rate, parameters of this formula can be selected
 As bit rate R increases, transmitted signal power
must increase to maintain required Eb/N0
TR
S
N
R
S
N
Eb
k
/
0
0



Other Impairments
 Atmospheric absorption – water vapor
and oxygen contribute to attenuation
 Multipath – obstacles reflect signals so
that multiple copies with varying delays
are received
 Refraction – bending of radio waves as
they propagate through the atmosphere

Multipath Propagation
 Reflection - occurs when signal encounters
a surface that is large relative to the
wavelength of the signal
 Diffraction - occurs at the edge of an
impenetrable body that is large compared
to wavelength of radio wave
 Scattering – occurs when incoming signal
hits an object whose size in the order of
the wavelength of the signal or less

The Effects of Multipath
Propagation
 Multiple copies of a signal may arrive
at different phases
 If phases add destructively, the signal
level relative to noise declines, making
detection more difficult
 Intersymbol interference (ISI)
 One or more delayed copies of a pulse
may arrive at the same time as the
primary pulse for a subsequent bit

Types of Fading
 Fast fading
 Slow fading
 Flat fading
 Selective fading
 Rayleigh fading
 Rician fading

Error Compensation
Mechanisms
 Forward error correction
 Adaptive equalization
 Diversity techniques

Forward Error Correction
 Transmitter adds error-correcting code to
data block
 Code is a function of the data bits
 Receiver calculates error-correcting code
from incoming data bits
 If calculated code matches incoming code, no
error occurred
 If error-correcting codes don’t match, receiver
attempts to determine bits in error and correct

Adaptive Equalization
 Can be applied to transmissions that carry
analog or digital information
 Analog voice or video
 Digital data, digitized voice or video
 Used to combat intersymbol interference
 Involves gathering dispersed symbol energy
back into its original time interval
 Techniques
 Lumped analog circuits
 Sophisticated digital signal processing algorithms

Diversity Techniques
 Diversity is based on the fact that individual
channels experience independent fading events
 Space diversity – techniques involving physical
transmission path
 Frequency diversity – techniques where the
signal is spread out over a larger frequency
bandwidth or carried on multiple frequency
carriers
 Time diversity – techniques aimed at spreading
the data out over time

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