Embed Size (px)
Citation preview
Transmission lines
Outline
Types of transmission linesparallel conductorscoaxial cablestransmission line wave propagationLossescharacteristics impedanceincident and reflected wave and
impedance matching
transmission media
Guided some form of conductor that provide conduit in
which signals are contained the conductor directs the signal examples: copper wire, optical fiber
Unguided wireless systems – without physical conductor signals are radiated through air or vacuum direction – depends on which direction the
signal is emitted examples: air, free space
transmission media
Cable transmission media guided transmission medium and can be any
physical facility used to propagate EM signals between two locations
e.g.: metallic cables (open wire, twisted pair), optical cables (plastic, glass core)
incident and reflected wave
Incident voltage voltage that propagates from sources toward the load
Reflected wave Voltage that propagates from the load toward the
sources
classifications of transmission lines
Balanced Transmission line 2 wire balanced line. both conductors carry current. But only one
conductor carry signals.
classifications of transmission lines
classifications of transmission lines
Unbalanced Transmission line One wire is at ground potential the other wire is at signal potential advantages – only one wire for each signal disadvantages –reduced immunity to noises
classifications of transmission lines
classifications of transmission lines
Baluns Balanced transmission lines connected to
unbalanced transmission lines e.g.: coaxial cable to be connected to antenna
Metallic Transmission Lines types
Parallel conductors Coaxial cable
parallel conductors
consists of two or more metallic conductors (copper)
separated by insulator – air, rubber etc. Most common
Open Wire Twin lead Twisted Pair (UTP & STP)
parallel conductors
Open Wire two-wire parallel conductors Closely spaces by air Non conductive spaces
support constant distance between conductors (2-6 inches)
Pro – simple construction Contra – no shielding, high radiation loss, crosstalk application – standard voice grade telephone
parallel conductors
Twin lead spacers between the two conductor are replaced
with continuous dielectric – uniform spacing application – to connect TV to rooftop antennas material used for dielectric – Teflon, polyethylene
parallel conductors
Twisted pair formed by twisting two insulated conductors
around each other Neighboring pairs is twisted each other to
reduce EMI and RFI from external sources reduce crosstalk between cable pairs
parallel conductors
Unshielded Twisted Pair two copper wire encapsulated in PVC twisted to reduce crosstalk and interference improve the bandwidth significantly Used for telephone systems and local area
network
parallel conductors
UTP – Cable Type Level 1 (Category 1)
ordinary thin cables for voice grade telephone and low speed data
Level 2 (Category 2) Better than cat. 1 For token ring LAN at tx. rate of 4 Mbps
Category 3 more stringent requirement than level 1 and 2 more immunity than crosstalk for token ring (16Mbps), 10Base T Ethernet (10Mbps)
parallel conductors
UTP – Cable Type Category 4
upgrade version of cat. 3 tighter constraints for attenuation and crosstalk up to 100 Mbps
Category 5 better attenuation and crosstalk characteristics used in modern LAN. Data up to 100Mbps
Category 5e enhanced category 5 data speed up to 350 Mbps
parallel conductors
UTP – Cable Type Category 6
data speed up to 550 Mbps fabricated with closer tolerances and use more
advance connectors
parallel conductors
Shielded Twisted Pair (STP) wires and dielectric are enclosed in a conductive
metal sleeve called foil or mesh called braid the sleeve connected to ground acts as shield –
prevent the signal radiating beyond the boundaries
parallel conductors STP – Category
Category 5e Feature individually shielded pairs of twisted wire Category 7
4 pairs surrounded by common metallic foil shield and shielded foil
twisted pair 1Gbps
Foil twisted pair Four pairs of 24-AWG copper wires encapsulated in a common
metallic-foil shield with a PVC outer sheath to minimize EMI susceptibility while maximizing EMI immunity > 1Gbps
shielded-foil twisted pair Four pairs of 24-AWG copper wires surrounded by a common
metallic-foil shield encapsulated in a braided metallic shield offer superior EMI protection > 1Gbps
Coaxial cable
used for high data transmission coaxial – reduce losses and isolate
transmission path basics
center conductor surrounded by insulation shielded by foil or braid
Metallic transmission lines
Coaxial cable
Rigid air filled solid flexible
BNC Connectors To connect coaxial cable to devices, it is necessary to use coaxial connectors. The most common type of connector is the Bayone-Neill-Concelman,
or BNC, connectors. Types: BNC connector, BNC barrel, BNC T, Type-N, Type-N barrel. Applications include cable TV networks, and some traditional
Ethernet LANs like 10Base-2, or 10-Base5.
Guided Media – Coaxial Cable
Two-wire parallel transmission lineelectrical equivalent circuit
CjGLjRZ ω
ω0
Characteristic Impedance of a Line A terminated transmission line that is matched in its
characteristic impedance is called a matched line The characteristic impedance depends upon the electrical
properties of the line, according to the formula: The characteristic impedance can be calculated by using Ohm’s
Law:
Zo = Eo / Io
where Eo is source voltage and Io is transmission line current
Characteristic Impedance The characteristic impedance for any type of transmission line
can be calculated by calculating the inductance and impedance per unit length For a parallel line with an air the dielectric impedance is:
Zo = the characteristic impedance (ohms) D = the distance between the centers r = the radius of the conductor
r
DZ log2760
Coaxial cable
0
138log
r
DZ
d
Z0 = the characteristic impedance (ohms)D = the diameter of the outer conductord = the diameter of the inner conductor = the permittivity of the materialr = the relative permittivity or dielectric constant of the medium0 = the permeability of free space
For extremely high frequencies, characteristic impedance can be given by
Zo =
0r
0
1
o
c
CL /
Wave propagation on Metallic transmission lines
Velocity factor The ratio of the actual velocity of propagation of EM wave
through a given medium to the velocity of propagation through vacuum
Vf = velocity factor
Vp = actual velocity of propagation c = velocity of propagation in vacuum
pf
VV
c
transmission line wave propagation
rearranged equation the velocity via tx. line depends on the dielectric
constant of insulating material
ϵr = dielectric constant
The velocity along tx. line varies with inductance and capacitance of the cable
f pV c V
1p
r
V
transmission line wave propagation
as
velocity x time = distance
therefore
normalized distance to 1 meter
Vp = velocity of propagation √LC = seconds L = inductance C = capacitance
T LCdistance
timep
DV
T
p
DV
LC
1 meters
secondpVLC
transmission line wave propagation
Question A coaxial cable with
distributed capacitance C = 96.6 pf/H Distributed inductance L = 241.56 nH/m Relative dielectric constant. ϵr = 2.3
Determine the velocity of propagation and the velocity factor
Losses
Conductor Losses conductor heating loss - I2R power loss the loss varies depends on the length of the tx. line
Dielectric Heating Losses difference of potential between two conductors of a metallic tx lines Negligible for air dielectric increase with frequency for solid core tx line
Radiation Losses the energy of electrostatic and EM field radiated from the wire and transfer to the nearby conductive material Reduced by shielding the cable
Losses
Coupling Losses whenever connection is made between two tx line discontinuities due to mechanical connection where
dissimilar material meets tend to heat up, radiate energy and dissipate power
Corona luminous discharge that occurs between two conductors
of transmission line when the difference of potential between lines exceeds
the breakdown voltage of dielectric insulator
