Transmission lines

Outline Types of transmission lines parallel conductors coaxial cables transmission line wave propagation Losses characteristics impedance incident and reflected wave and impedance matching

transmission media Guided Unguided 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 Category 5 Category 5e 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

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

Guided Media – Coaxial Cable 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.

Two-wire parallel transmission line electrical equivalent circuit

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

Coaxial cable Z0 = the characteristic impedance (ohms) D = the diameter of the outer conductor d = the diameter of the inner conductor = the permittivity of the material r = the relative permittivity or dielectric constant of the medium 0 = the permeability of free space For extremely high frequencies, characteristic impedance can be given by Zo =

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

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

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

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 Corona 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