Antennas, Propagation & Signal Encoding Techniques

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"Antennas, Propagation & Signal Encoding Techniques"

```					Antennas and
Propagation
Lecture Learning Outcomes
   Understand the radiation pattern of an
antenna and calculate parameters for
different antenna types.

   Understand the basis of signal
propagation.
Lecture Learning Outcomes
   Understand the concepts associated with
LoS transmissions.

   Been able to calculate noise parameters,
antenna gain and transmission losses for
different types of antennas in LoS
transmissions.
Class Contents
Antennas
• Antenna Types & Gains
Propagation Modes
• Ground Wave
• Sky Wave
• Line of Sight
Line of Sight Transmission
• Attenuation
• Free Space Loss
• Noise
• Atmospheric Absorption
• Multipath
• Refraction
• Multipath Propagation
Antennas
An antenna is an electrical conductor or system of
conductors used either for radiating electromagnetic
energy into space or for collecting electromagnetic
energy from space.

is a graphical representation of the radiation properties
of an antenna as a function of space coordinates.

Radiation patterns are almost always depicted as 2-dimensional
cross section of the three-dimensional pattern
The Isotropic Antenna

An Isotropic Antenna radiates power in all directions
equally. (Omnidirectional Antenna)
Beam Width (Half-Power Width)
Is the angle within which the power radiated by the antenna
is at least half of what is in the most preferred radiation
position.

Directional Antenna: Power radiated in the direction of B is
greater than that radiated in the direction of A
Antenna Types & Gains
Dipoles
• Half-Wave Dipole (Hertz Antenna)

• Quarter Wave Dipole (Marconi Antenna)

Half-Wave      Marconi
Dipole        Antenna
Parabolic Reflective Antenna

(a) Parabola Properties

(b) Parabolic Antenna: principle of operation

Typical beam width for parabolic antennas at
12 GHz

Antenna Diameter (m)    Beam Width (degrees)

0.5                     3.5
0.75                     2.33
1.0                     1.75
1.5                    1.166
2.0                    0.875
2.5                     0.7
5.0                     0.35
Antenna Gain
Is a measure of directionality of an antenna

It is defined the power output in a particular direction
compared to that produced in any direction by a perfect
omnidirectional antenna (isotropic antenna).

G  antenna gain
4   Ae     4   Ae  f 2
G               
A e  effectivearea                  2               c2
f  carrier frequency
c  speed of ligth (3x108 m/s)
  carrier wa velength
Effective Area of typical antennas

Type of Antenna Effective Area Ae      Power Gain
(m2)            (Relative to
Isotropic)
Isotropic                               1
 / 4 
2

1.5  2 / 4  
Infinitesimal                            1.5
Dipole or loop
Half-Wave Dipole
1.64  2 / 4        1.64

Parabolic (face
area A)
0.56  A           7  A / 2
Propagation Modes
 Ground Wave Propagation
 Sky Wave Propagation
 Line of Sight
Ground Wave

• Frequency Below 2 MHz

• Slowed down wave front due to EM current induced into
the earth. (downwards tilt)

• Suffer from difraction and scattering from the atmosphere

Sky Wave

• Frequency between 2 and 30 MHz

• Transmitted signal is refracted by the ionosphere and reflected
By the earth.

• Bouncing allows signal to be picked up thousands of kilometres
from the transmitter.

broadcast (BBC & Voice of America)
Line of Sight

• Above 30 MHz, ground wave and sky wave do not operate

• There is no reflection from the ionosphere (allowing satellite
communications not beyond the horizon and back).

• For Ground Based communications, the antennas need to be
in LOS with each other.

Optical LOS with no intervening obstacles

d  3.57  h
d  3.57  K  h            used to compensate for the
refraction
Maximum distance between two antennas (radio LOS) with K=4/3

                       
d  3.57  K  h1  K  h2  4.12  h1  h2                        
• h is measured in metres
• d is measured in kilometres
•K depends on weather conditions
Standard Atmosphere           Without mist          Sub-standard    Surface Ducts,   Wet Mist
Light Mist         ground         over
mist         water
Typical   Mild Climate (Non            Dry, Mountainous        Plains, some    Tropical Coast    Coast
tropical), air mix            without mist             mist
day and night

K                  1,33                    1,33  1              1  0,66       0,66  0,5      0,5  0,4
Line of Sight Transmission
Sources of Impairment

 Attenuation & Attenuation Distortion
 Noise
 Atmospheric Absorption
 Multipath
 Refraction
Attenuation & Attenuation Distortion
Attenuation

Defined as the loss of strength of the signal over the communications
channel. It is a complex function of the distance and the make of the
atmosphere.

Attenuation Distortion

Occurs when the frequency components of the received signal
have different relative strengths than the frequency components
of the transmitted signal.
Factors encountered when dealing with
attenuation

   Strength on the received signal (solved using amplifiers or repeaters
in the communications path).

   SNR considerations (must be high enough to avoid errors in the
transmission) – solved using amplifiers of repeaters.

   Attenuation increase with frequency (known as attenuation
distortion) – solved using equalizing techniques across a band of
frequencies.
Free Space Loss

Is the ratio of power radiated by the transmitter antenna

PT 4    d              PT=transmitted power (W)
2
Isotropic
:        L                            PR=received power (W)
Antenna               PR      2                  d = distance
 = wavelength (same
units as distance
It is usually expressed in dB

PT(dB)  PR(dB)  L dB
f is expressed in Hz
L dB  20  log(d )  20  log( f )  147.56 dB           d is expressed in m

PT(dB)  PR(dB)  20  log( d )  20  log( f )  147 .56 dB
Free Space Loss – Other Antennas
For non-isotropic antennas, the gain of the antenna, with respect
to isotropic, should be taken into consideration:

L
PT   4    d 
 2
2

PR   GT  GR
Expressed in dB:

PT(dB)  PR(dB)  20  log( d )  20  log( f )  10  log(G T  G R )  147 .56 dB

PT(dB) and PR(dB) must be expressed in the same dB unit: dBW or dBm
The gains inside the logarithm should be expressed in adimensional
Quantities. If expressed in dB, they should be in dBi

PT(dB)  PR(dB)  20  log( d )  20  log( f )  G T(dBi )  G R(dBi)  147 .56 dB
Free Space Loss – Other Antennas

Free space loss can also be expressed in terms of
effective area:

L dB  20  log( d )  20  log( f )  20  log( A eT  A eR )  169 .54 dB
Noise
Noise are unwanted signals that combine and distort
the signal intended for transmission and reception in
a communications system.

   Thermal Noise

   Intermodulation Noise

   Crosstalk

   Impulsive Noise
Thermal Noise

 Due to thermal agitation of electrons

 It is present in all electronic devices and transmission
media.

 It is a function of the temperature

The amount of thermal noise is defined as noise power density
in watts per 1 Hz of bandwidth.

N 0  k  T ( W/Hz)

K is the Boltzmann’s constant: 1.3803x10-23 J/K
T is the absolute temperature in Kelvins
Thermal Noise

At room temperature (250 C), the noise power density is:

N 0 dB  10  log(1.38 10 23 (J/K )  (298 .15 K))  203 dBW/Hz

For any given bandwidth B, the noise present in the band is:

N0  k T  B
in dBW

N 0  228 .6 dBW  10  log(T )  10  log( B)
Intermodulation Noise

   Produced when there is nonlinearities in the transmitter,
receiver or transmission system, when 2 or more
different frequencies share the medium.

   The effect is the production of new signals at frequencies
that are the sum or difference of the original frequency
and multiples of those frequencies.
Cross Talk

   Defined as unwanted coupling between signal paths.

   Can occur when unwanted signals are picked up by
microwave antennas or by electrical coupling between
twisted pair (in guided media transmissions)

   Can be identified when in the telephone line, another
conversation can be heard.

   Typically is in the same order of magnitude or less than
the Thermal Noise
Impulsive Noise

   Non-continuous noise consisting of irregular pulse or
noise spikes of short duration and relatively high
amplitude.
   Causes include external electromagnetic disturbances
(lightning) and faults and flaws in the communication
system.
   It is a minor concern in analogue signals, but is a major
concern when dealing with digital data transmissions
Impulsive Noise
Example

In a voice communication, impulsive noise will generate
clicks and crackles of short duration, however, the
conversation will still be intelligible.

In a digital transmission, a small spark of energy
(10 ms in duration) would wash out 560 bits of data
being transmitted at 56 kbps.
Ratio of Signal Energy per bit to Noise Power
Density

   The short name for this equivalent is the Eb/N0
expression

   The advantage of Eb/N0 over SNR is that the latter
depends on the bandwidth
Ratio of Signal Energy per bit to Noise Power
Density
A signal containing a binary data transmitted at a data rate of R, is
subjected to thermal noise N0

The Energy per bit in such a signal is:
S = signal power
E b  S  Tb       Tb = time needed to
transmitt 1 bit:
The expression Eb/No can be written:
Tb = 1/R
S       S             k = Boltzman Constant
Eb N0                               (1.3803x1023 J/K)
R  N0 R  k  T        T = Temp in Kelvin

 Eb 

 N   SdBW  10  log( R )  228.6 dBW - 10  log(T)

 0  dB
Ratio of Signal Energy per bit to Noise Power
Density

Example:
Suppose a signal encoding technique requires that Eb/N0 = 8.4 dB
for a bit error rate of 10-4. If the effective noise temperature is 290K
(room temperature) and the data rate is 2.4 Kbps, what received
signal level is required to overcome thermal noise

Solution:
 Eb 

 N   SdBW  10  log( R )  228.6 dBW - 10  log(T)

 0  dB
8.4 dB  SdBW  10  (3.38)  228.6 - 10  (2.46)
SdBW  161.8 dBW
Achievable Spectral Density

The parameter N0 is the noise power density in watts/hertz.
The noise in a signal with a bandwidth B is:

N  N0  B

Substituting in the Eb/N0 expression

S     S B
Eb N0          
R  N0 N  R

Considering that the Shannon’s capacity formula (in bps)

C  B  log 2 (1  S N )
S    C
 2 B 1
N
Achievable Spectral Density

Equating the channel capacity C with the data rate R, and using
the Eb/N0 expression:

B  CB 
Eb   N 0    2  1.
C        

This expression is a formula that relates the achievable
spectral efficiency C/B to Eb/No
Atmospheric Absorption

   Additional loss between the transmitting and receiving antenna.

   The main contributors are the water vapour and oxygen present in
the atmosphere.

   Water Vapour generates attenuation peaks at frequencies close to
22 GHz

   Absorption due to oxygen has a peak in the vicinity of 60 GHz

   Rain and Fog cause scattering of radio waves that results in
attenuation
Multipath

   Occurs in environments where
there is no direct LOS between
the transmitting and receiving
antenna due to the presence of
intervening obstacles.

   Obstacles can reflect the signal
creating multiple copies that
arrive at delayed times to the
Refraction

   Is the bend that suffer radio waves when propagating
through the atmosphere

   It is caused by changes of speed of the signal with
altitude or by other spatial changes in atmospheric
conditions.

   Normally the speed of the signal increases with altitude,
causing the radio waves to bend downwards.

power caused by changes in the transmission medium or
path(s).

   The most important fading mechanism is multipath
propagation.
Multipath propagation

 Reflection
(surface > wavelength)

  Diffraction
(edge of body > wavelength)

  Scattering
(obstacle = wavelength)
Effects of multipath propagation

   Copies of the signal arriving at different phases.

   If copies add destructively, SNR declines

   Signal interpretation then becomes difficult.

   Intersymbol interference (ISI)

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