communication theory 3

7/29/2019 Communication Theory 3

1/4

Communication Theory 3

Propagationmovement through a medium

Wavedisturbance that moves through a medium

Wave Motionrecurring disturbance advancing through space with or without a medium

Wave Propagationthe travel of electromagnetic waves or sound waves through a medium, orthe travel of a sudden electric disturbance along a transmission line

Types of Wave Motion

Transverse Wavestravels at right angles to the direction of propagationLongitudinal Waves (Compression Waves)travels back and forth in the same direction asthe wave motion

Two Categories of Transmission Media

Guided- Wired system- Uses conductor that provides conduit in which electromagnetic signals are contained

Ex: copper cables, pair of wire, coaxial cable, fiber optic

Unguided

- Wireless system- Emitted then radiated through air or vacuum

Ex: Earths atmosphere, free space (vacuum)

Metallic Transmission Linesmetallic conductor system used to transfer electrical energyfrom one point to another using electrical current flow

Transverse Electromagnetic Wavesa TEM propagates primarily in non conductor(dielectric) that separates the two conductors. Electromagnetic waves are produced through

acceleration of electric charge.

Three Primary Characteristics of Electromagnetic Waves

- Wave Velocity- Frequency- Wavelength

Wave Velocityspeed of wave that travelsFrequencythe rate at which the wave repeat itselfWavelengthdistance of one cycle

=c/Where:

= wavelength (meter per cycle)c = velocity of light (3x10

8m/s)

= frequency (Hertz)

Types of Transmission Line

Balanced- With two wired, both conductors carry current. One conductor carries signal while the

other is a return path.- Signal that travels in opposite direction (metallic circuit current)- Signal that travels in the same direction (longitudinal current)

Unbalanced

- One wire is at ground potential, other wire at signal potential


2/4

ADV: requires only one wire for each signal

D-ADV: less immunity noise

BALUN (Balanced-UNBALANCED)

- Circuit that connects balanced and unbalanced transmission line.Ex: narrowband balun or sleeve

Parallel Conductor Linescomprised of two or more metallic conductors separated by nonconductive insulating material called dielectricEx: air, rubber, propylene

Most Common Parallel Conductor Transmission Lines

Open Wire Transmission Line

- Two parallel wires, closely spaced, separated by air- The distance between tow conductors is generally between 2 to 6 inches, the dielectric is

air- Radiation losses are high- Crosstalk is present- Used for standard voice grade telephone

Twin Lead Transmission Line

- Same as open wire transmission line, except the insulating material is a solid dielectric- Commonly used to connect TV to rooftop antenna

Twisted Pair Transmission Line- Is formed by twisting two insulated conductors around each other, mostly stranded in

units, the units are cabled into cores containing up to 3000 pairs of wire and is covered by

sheaths.- Neighboring pairs are sometimes twisted with different pitches (twist length) to reduce

the effects of EMI (Electromagnetic Interference) and RFI (Radio Frequency

Interference) from external sources (man-made)- Twisting the wires also reduces crosstalk between cable pairs- The size varies from 16-26 gauge (AWG-American Wire Gauge)- The higher the gauge the smaller the diameter and the higher the resistance- Used in telephone networks- Simple to install and relatively inexpensive

Two Types of Twisted Pairs

Unshielded Twisted Pair (UTP)

- Consists of two copper wires where each wire is separated by PVC (Poly Vinyl Chloride)insulation

- The maximum number of twists for UTP cable is two/foot- Inexpensive, flexible, easy to install- Least expensive transmission medium and most susceptible to noise

Shielded Twisted Pair

- Parallel two wire transmission line separated by dielectric material- The wires are enclosed in a conductive metal sleeve called Foil- Sleeve is a woven into a mesh called Braid (SLEEVEconnected to ground and acts as a

shield, preventing signals from radiating beyond their boundaries)- More expensive- Greater immunity noise

Metallic Transmission Line Equivalent Circuit

Primary Electrical Constant (Distributed Parameters)uniformly distributed throughout the

length of line

A. Series DC Resistance (R)B. Series Inductance (L)


3/4

C. Shunt Capacitance (C)D. Shunt Conductance (G)

Lumped Parameters Combined parameters

Transmission Characteristics

for a maximum power transfer from the source to load;transmission line must be terminated in a purely resistive load equal to the characteristic

impedance, Zo, of a transmission line

Characteristic Impedance, Zodefine as the impedance seen looking into an infinitely long

line or the impedance seen looking into a finite line that is terminated in a purely resistive load

equal to Zo of the line

Zo = (R + jL)/(G + jC)

For extremely low frequency, use

Zo = R/G

For extremely high frequency, use

Zo = L/C

Characteristic Impedance of a Two Wire Parallel Line

Zo = 276log D/r

Characteristic Impedance of a Concentric Coaxial Cable

Zo = (138/r) log D/d

r = Relative dielectric constant of the insulating material

Summary of a Transmission Line

1. The input impedance of an infinite long line radio frequency is resistive and equal to Zo 2. Electromagnetic waves travel down without reflections, such a line is called non resonant3. The ratio of the voltage and current at any point along the line is equal to zero4. The incident voltage and current at any point along the line is in phase5. Line losses on a resonant line are minimum per unit length6. Any transmission line that is terminated in purely resistive load equal to Zo acts as if it

were an infinite linea. Zi = Zob. No reflected wavesc. V and I are in phased. There is maximum power transfer

Propagation Constant (Propagation Coefficient) used to express the attenuation (signal loss)and the phase shift per unit length

= + j

propagation constant (unitless) phase shift coefficient (radians/unit length)attenuation coefficient (meters/unit length)

expressing with complex quantity

= (R + jL)(G + jC)

because a phase shift of 2 rad occurs over a distance of 1


4/4

= 2/

an intermediate and radio frequency,

L > R and C > G:

= LC

Velocity Factor (Velocity Constant), Vfdefined by the ratio of the actual velocity ofpropagation of an electromagnetic wave through a given medium to the velocity of propagation

through a vacuum

Vf = Vp/c; Vf = 1/r; r = /o

rdielectric constant of a given material - permittivity of dielectric

opeermitivity of air

Dielectric Constantrelative permittivity of a material

T = LC, secVp = D/T = D/LC, m/sec

If distance is normalized to 1 meter, the velocity of propagation for a LOSSLESS Transmission

Line is

Vp = 1/LC, m/sec

Electrical Length of a Transmission Line- At low freq (long wavelength), the voltage along the line remains constant- At high freq (short wavelength), several wavelengths of the signal maybe present on the

line at the same time. Therefore, the voltage along the lines varies- A transmission line is defined as long if its length exceeds 1/16th of a wavelength,

otherwise it is considered short

Transmission Line Losses

1. Conductor Losses (I2R Power Loss) inherent and unavoidable power loss2. Dielectric Heating Losses increases with freq caused by difference of potential

between 2 conductors

3. Radiation Loss- reduced by properly shielding the cable, directly proportional to freq- depends on dielectric material, conductor spacing and length4. Coupling Lossesdiscontinuities tend to heat up, radiate energy, dissipate power5. Corona- luminous discharge that occurs between two conductors of the transmission line when the

potential exceeds the breakdown voltage- destroy transmission line (most destructive)

communication theory 3

Documents