Download Introduction to telecommunication_part 2

Channels of transmission
are the transmission lines
Can be either
i.
Hard medium
On Electrical conductor
On Optical fibers
or
ii. Soft medium
Such as open space.
Modes of transmission
modes of propagation of energy.
• Sound waves:
Longitudinal propagation
• Electro-magnetic waves:
Transverse Propagation
Electromagnetic propagation
Optical waves are electromagnetic waves.
Radio waves are electromagnetic waves.
When current flows in a conductor, it
follows electromagnetic propagation.
Properties of EM waves and
Optical waves are same.
Reflection
Refraction
Diffraction
Polarization
Interference
Absorption
Doppler effect.
(to be revised by the students)
Frequency-wavelength relationship
• The wavelength-frequency product of a
transmitted wave is constant for a given media.
f =c/r
c = velocity of light = 3 x108 meter/s
f = frequency of wave in Hz.
 = wavelength of the wave in meters.
r = relative permittivity of the media (for air it is unity).
Generally constant, can vary with temperature,
moisture content, oxygen, ionization, impurity in
insulating material etc.
For air: as media
• The spectra of frequencies with air as channel
extends from 3x10 Hz to 3x 1012 Hz. Lowest is the
voice frequency while highest is optical.
• For convenience, we split this frequency spectra in
several ranges in terms of powers (n) in 3x10n Hz
The wavelength is 1011-n mm
• When n=1, 3 Hz wavelength is 1010 mm
• When n=6, 3 MHz wavelength is 105 mm
• When n=9, 3 GHz wavelength is 102 mm
Spectra: for every n the range is n:10n
n
01
02
03
04
05
06
07
08
09
10
11....
abb
ELF
VF
VLF
LF
MF
HF
VHF
UHF
SHF
EHF
optical
application
Power frequencies & their Harmonics
Voice frequencies, Audio Signals
Sonar: Marine navigation, ultrasound
Sonar: Marine navigation, ultrasound
Medium wave AM Broadcasting
Short wave AM broadcasting
FM, TV, public service
TV, Cellular, WLAN
Satellite, TV, radar, LMDS
mm waves, radar, LMDs
sub-mm, visible light, medical instrumentation
Optical spectra
Range of
application
n
11 14
Infra red
14 15
Partial: visible light
followed by Ultraviolet
15 16
Ultra violet
16 18
Soft x-rays
18 19
Hard x-rays
19 21
Gamma Rays.
Characteristic Impedance
• Input impedance of an infinite length
transmission line (TL) is termed as
Characteristic impedance Zo.
• It is the ratio of electric field intensity (E) volt per
meter to magnetic field intensity (H) ampere per
meter of TL.
• In infinite length TL, power inputted is fully
absorbed in it, load connected to the sinking
end is irrelevant.
Mullett, ”Basic Telecommunication: physical layer”, p.295; Thomson Learning.
Balanced Transmission line
• Here the signal current circulates is one
pair of wires running simultaneously. One
wire carries forward current while the
other, return current.
• The properties of transmission line
depends whether or not these pair of wires
twisted and /or
shielded.
Unbalanced transmission line
• Only one wire conducts the signal.
The return path is through shield or, ground.
• Coaxial cable, instrument probes are the
examples.
The equivalent electric circuit of
a section
no. of sections per meter is frequency dependent.
unbalanced section of transmission line
R
Zo
balanced section of transmission line
L
C
Zo
G
values of inductance and capacitance per uni t length.
R/2
L/2
R/2
C
L/2
G
values of inductance and capaci tance per uni t length.
When several such sections are connected in cascade,
Zo in both case is as below. Should R and G 0, Zo = L/C
Z0 
R
sL
G
sC
sections/length increases with frequency.
ABCD parameters are preferred for
Calculations of the sections.
The characteristic impedance is
resistive at radio frequencies.
Kennedy and Davis, ”Electronic Communication Systems”, 4/e,McGraw Hills, pp185-193
More on Zo…
• We take two sections of TL for our review.
1’
1
S1
2
Zo
2’
Zo
Infinite sections
S2
Zo
• For infinite sections, the impedance seen at 1-2
would be the same as seen at 1’-2’ etc.
• It is equivalent to taking one section and
loading it with Zo..
More on Zo…
• This implies that: Impedance measured at the
input of a TL now of finite length with the output
terminated in Zo will be Zo itself.
• For maximum power to be transferred, connect
conjugate terminating impedance Zo*. conjugate
• Note that Zo = [ZocZsc] also.
For one section or equivalent one section,
zo = [zseries arm / Yshunt arm].
Free space impedance
•The free space impedance, also known as
characteristic impedance of vacuum/air, depends
on ratio of permeability of air and permittivity of air.
Zo = [o/o]
= 120 = 377 ohm
It varies depending on the values of o & o at any
instance.
?? Can the impedance in any other media also be
written as Zx= [x /x] ?
Components of
Characteristic impedance
decides
the characteristic of the channel
Characteristic impedance for the loss less media is
Zo = [L/C] . It is resistive at radio frequencies.
L and C are defined per unit length of the media.
L and C decide the characteristic of the channel.
It is band-limited: can be base band or pass band.
Recall that frequency of resonance of a loss less LC filter is
fo = 1/2LC and its critical resistance: Rc = [L/C].
• As you will learn in electro-magnetics and antenna,
Any system of conductors will
RF energy if the conductor separation
nears half the wavelength of the
operating frequency.
Such transmission line acts as antenna.
Essentials for propagation
A time varying electric signal is applied to a
conductor. This makes a current to flow in the
conductor. It creates an electric field.
This current is parallel to the electric field.
This current carrying conductor surrounds a
magnetic field.
This magnetic field is perpendicular to electric field.
Thus the time varying electric field and time varying
magnetic field are mutually perpendicular.
Polarization
• Polarization refers to physical orientation
of the radiated waves in space.
• A vertical antenna will have vertical
Electric field.
• The propagation of em wave takes place
in the direction perpendicular to the
electric field helically.
Helix propagation of E.M.Waves
vertical Antenna, propagation
erpendicular to the electric field.
1
electric field
0.5
0
-0.5
-1
1
0.5
40
30
0
20
-0.5
magnetic field
10
-1
0
time
Speed of signaling is media dependant
Velocity ‘c’ of EM wave in a media is c = 1/( )
velocity of EM wave in free space = 3x108 M/s,
o =1.257x10-6 H/M, o= 8.854x10-12 F/M
cvac= 1/ (1.257x10-6 H/M x 8.854x10-12 F/M)
= (0.08985) 109 M/  (FM)
 3 x 108 M/s
as  FM =1/ f = s
The velocity of signal in open space is
at the rate of the velocity of light in
vacuum or, air.
Velocity of signal in a media…
Thus velocity of an EM wave
i.e. electrical and optical signal,
in a loss-less media,
is decided by
the permeability and permittivity
of the media.
Velocity factor
• The velocity factor VF = c/cvac
• Since c = 1/( ) and cvac=1/(oo)
Denoting r as relative permeability and r as the relative
permittivity
VF = {(o/)(o/)} = 1/ rr
• As the relative permeability of the vacuum/air is the same as
that of any dielectric material, velocity factor depends only
on relative permittivity and thus can be written as
VF =1/r
• You may relate the above velocity factor with coefficient of
refraction in optics.
The Zo can also be calculated by
physical dimensions of the channel.
s

d
D
d
Parallel wire
d diameter
s distance
Coaxial Cable
d diameter of core
D diameter of insulation
• Zo = {120/(ek)} log (D/d) 
: coaxial cable
= (138/k) log (D/d) :
: coaxial cable
• Zo = (2x138/k) log (2s/d) 
: parallel wires
120/e = 377/e = 138 as e is natural base.
k is the relative dielectric constant of the insulation.
Example:
A piece of RG-59B/U coaxial cable has a 75 ohm
characteristic impedance and nominal capacitance of 69 pF/m.
What is its inductance pr meter? If the diameter of the inner
conductor is 0.584 mm and the dielectric constant of the insulation
is 2.23, what is the outer diameter.
•
Soln:
Given Zo = 75 , C = 69 pF/M, k = 2.23 (relative)
(a) Since Zo= L/C, Hence L = 0.388 H/m.
(b) Zo = [138/k] log (D/d)
log (D/d)=Zo/ [138/k] = 0.81
D=3.77 mm.
Propagation of EM waves near the
earth surface
Properties of EM Waves.
• Reflection
• Refraction
• Absorption
• Diffraction
• Interference,
• Doppler effect.
Terrestrial Propagation
a.
b.
c.
d.
e.
Terrestrial Propagation is not hurdle free.
The hurdling factors can be
curvature of earth,
Hills & high buildings,
Changes in atmospheric conditions,
Certain layers that are formed in the sky
above the earth as a result of pollution,
ionization, solar radiations etc and are
ambience dependent.
Rivers and water ponds conduct em waves.
Reflection, absorption and
refraction
When an EM Wave hits a surface having
change in physical properties, the wave can
take a series of recourse in different
proportions.
 It can refract through the media due to change
in the velocity.
 It can be absorbed by the media.
 It is reflected back.
Absorption
Part energy is always absorbed by the media. The
absorption factor depends on
frequency dependent behavior of
the media
temperature,
Whether day ?
humidity, contents of oxygen etc.
See next slides
Average Atmospheric attenuation due to water
vapors and oxygen vrs frequency in GHz range.
Oxygen
water
Absorption
A. Sea Level at 20C at 760 mm atmospheric pressure and Humidity 7.5 g/M3
B. 4 kM Elevation: at 0 C and humidity 1g/M3.
Attenuation Charactristics
Absorption characteristics:
• The experimental investigations showed
that in sub millimeter and millimeter
wavelength range, the attenuation
characteristic of transmission is highly
dependant on the presence of oxygen and
water vapors.
• In the following slide, we see the effect of
objects in routine partitions such as wood
and concrete.
Absorption in wood and concrete
In wood, absorption is instant
In concrete absorption depends on thickness
of the concrete block.
Skin depth is the distance where the wave intensity reduces to 37 %
Reflection
a) The two mirrors in a barber shop, one in the front and
other at the back of the dressing chair,
b) An object kept at an angle results in multiple
reflections in them. They get blur/echoed after each
reflection.
• Why??
a. The mirrors are uneven surfaced and polished,
b. They absorb part of the optical energy
c. Multiple reflections
Refraction here is negligible.
Reflection..
This we treat as property of the mirror.
We can “see” this property of EM waves
in mirror at ‘vision’ frequencies.
Similar property is held by other media in
other frequency range that we can not
“see”.
• Earth is a good reflecting media for e.m. waves
including light. It also partly absorbs the em
wave energy.
• It does not refract them.
Multiple hop sky wave propagation:
frequency range 3-30 MHz.
Ionospheric layers
Earth
multiple reflections in the forward direction, called hopping, are due
to reflection of em waves between earth and F ionospheric layers.
Reflections…
Reflected waves add to reception.
• In a given frequency range, the ionospheric
layers reflect the em waves.
• Depending on the angle () of transmitting
antenna of the polarized wave and extending
maximum useable reflecting frequency (MUF),
this media refracts the e.m.wave through
ionospheric layers. The wave thus “escapes”
through the layers. (Next slide)
• Critical frequency fc = (MUF) cos ()
• The range of operating reflecting frequency
(<f c ), is 3 MHz to 30 MHz.
Sky wave Propagation
> fc
Antenna at
different angles
Line of Sight (LOS) propagation
LOS communication
• When frequency range is above 300 MHz, e
m Waves do not get absorbed and are not
sufficiently refracted to be reflected by
ionosperic layers as shown in next slide.
• At 10’s of GHz and above; such as in upper
W and optical range, get absorbed by
water vapors and presence of oxygen.
• Fiber optic cables are must for unattenuated
“surface communication” while W links work
in ambience.
• There is no optical link for satellites yet.
Ziemer+Tranter,”Principles of communication” 5/e,Wiley p 10-11.
Overall effect of skywave reflection
Total internal reflection
Refraction
Attenuation Charactristics
Interference
• Multipath reception are due to multiple
reflections of the same signal.
• It creates echo effect in the receiver.
• It is called interference.
• The multipath reception can be from
objects on earth, reflection from
ionosphere, LOS reception, surface wave
transmission etc.
• Signal from unknown source, called noise
may also be found at the receiver.
Realistic link behavior
Multiple reception
multiple reflection create
interference
Satellite communication
• The technique to refract the EM wave
though the ionospheric layers is called
trans-ionospheric propagation, basic for
satellite communications.
• It depends on angle of antenna and
frequency of operation.
• Beyond 300 MHz, the ionospheric layers
refracts the em waves but does not reflect.
The ionospheric layers
• At about 70 KM to 350 KM above the earth
there exists several D, E, F1 & F2 layers
created by ionization of ultraviolet, , 
and  rays emitted by the solar system.
• These layers have varied properties that
depend on position of the Solar planets
with respect to earth, presence of clouds
and industrial wastes, and whether it is
day or, night.
Effect of sudden atmospheric
changes
D layer
• Lowest layer is D. It is 10 kM thick at
about 70 kM above the earth.
• It disappears at night.
• It reflects VLF and LF waves (3-300 kHz)
and
partially absorbs the MF and HF waves
(0.3-30 MHz)
E-Layer and sporadic E-layer
• It exists at 100 kM above earth and has
thickness of about 25 kM.
• During the day sun creates ionization in this
layer that disappears at night.
• It reflects HF waves (3-30 MHz) during day.
• It partially helps surface wave propagation in
MF range (0.3-3 MHz) .
• The sporadic E-layer exists during the night
also and its cause is still not known. It is found
to contribute to long distance propagation.
F- layers: F1 and F2
• F1 layer exists at about 180 kM in day time. Its
thickness is 20 kM.
• F2 layer exists at about 250 to 400 kM. Its
height rises with atmospheric temperature. It’s
thickness at times can be about 200 kM.
• At night, F1 & F2 layers merge.
• It is the topmost layer and with reduced degree,
remains ionized during the night.
• It reflects HF (3-30 MHz) waves which in turn
yield better reception.
Ionospheric Layers
Frequency range for different
propagation layers
D-Layer E- Layer
VLF:
3-30kHz

LF :
30-300kHz

MF:
0.3-3 MHz

HF:
3-30 MHz

VHF:
30-300MHz
F-Layer
Transionosphere


Surface wave propagation
• This mode of propagation of the EM wave is over the
surface of the earth.
• The polarization of antenna is vertical.
Else, earth being a good conductor, the electrical
component will be short circuited.
• As the wave propagates over the surface, due to
induction and absorption of induced em Wave by
earth, jungle, hills, buildings etc. the wave “lies down
and dies”. It reduces amplitude.
• Diffraction tilts the wave.
• Increasing the transmitting and receiving antenna
heights, the process can be slowed.
Propagation through surface waves
frequency range 0.3 to 3 MHz
Surface wave propagation
The electric field strength and voltage developed are
•
•
 = 120ht I/ d volt/meter
V= 120ht hr I/ d volt
Where:
ht and hr : effective heights of transmitting and receiving
antennas
d: distance from transmitting antenna
I: Antenna current,
: wavelength.
Diffraction
• In optics, we saw that when a parallel
wave incident on any sharp object, it
creates diffraction and causes change in
the shape of the wave-front.
• Due to reflection of this diffracted light
from an object, we see objects in a room.
• Such effect are eminent in em waves.
Differaction of EM wave
Doppler Effect
• If there is a relative motion between the
source (or, reflected source) and the
receiver, there is a change seen in the
frequency of reception.
• If it moves object moves towards the
receiver, it increases and vice versa.
• Fdopple= f[1 + vr/c]
velocity of object toward receiver/velocity of light
Ex.
A radar emitting 10.5 GHz, finds that the
reception frequency is increased by 1172.5 Hz.
Calculate the speed and the direction of the reflecting
surface, which in this case is an automobile.
•
•
•
•
Soln:
Let the speed of the automobile be vr mtrs /sec.
Since the frequency is found increased, the
automobile is coming towards the radar.
Doppler frequency is 1172.5 Hz.
Hence vr = 33.5 mtr/sec.