esat: transline

Transmission Line

A metallic material or structure that forms all or part of a path from one point to another directing the transmission of energy.

1. Electromagnetic radio wave

2. Electromagnetic acoustic wave

energy, it could be in a form of (2):

Electric power transmission

it delivers:

1. Parallel wire lines

2. Coaxial Cables

types of transmission lines

Parallel wire lines

Currents in two conductors are equal in magnitude, flow in opposite directions and exactly opposite in phase.

Coaxial cables

concentric conductors

1. To guide electrical energy from one point to another.

2. To connect transmitter to antenna and vice versa.

two major applications of Parallel wire lines

It should maintain the integrity of signals in both directions
it must be delivered with a very much low of loss, following the Maximum power transfer theorem.

the most important part of transmission line (antenna application)

1. Balanced

2. Unbalanced

Transmission lines based on measure of electrical symmetry.

Balanced

Neither conductor at ground potential.

Balanced-to-Ground

A measure of electrical symmetry wrt ground potential

Unbalanced

One of its two conductors at ground potential.

1. Balanced Transmission Lines (Parallel lines)
1.1. Two-Wire open line
1.2. Two wire ribbon (twin lead)
1.3. Shielded pair
1.4. Unshielded twisted pair.

2. Unbalanced Transmission Lines (Coaxial Lines)
2.1. Rigid coaxial line
2.2. Flexible Coaxial line
2.3. Semi Rigid Coaxial line
2.4. Formable Cable

3. Balanced to unbalanced (BALUN)

different kinds of transmission lines

Two-wire open line

This line consist of two wires that are generally spaced from 2 to 6 inches apart.

Most often used for power lines, rural telephone lines, and telegraph lines.

Advantage:
Simple Construction

Disadvantage:
High loss and noise pickup because of the lack of shielding.

Two-wire ribbon (twin lead)

Uniform spacing is assured by embedding the two wires in a low-loss dielectric.

Commonly used to connect a television receiving antenna to a home television set.

Advantage:
Lowest signal losses than coaxial cable

Disadvantage:
Vulnerable and prone to get damaged by the weather and by its established environment.

Shielded Pair

Parallel conductors separated from each other and surrounded by a solid dielectric.

Looks like the power cord of washing machine or refrigerator.

Advantage:
Cancels out the electromagnetic interference thoroughly to maintain faster data transfer speeds.

Disadvantage:
More expensive in cost.

Unshielded Twisted Pair (UTP)

Line consist of two insulated wires twisted together to form a flexible line without the use of spacers.

Use for Local Area Network, Fast Ethernet connections and telephone lines.

Advantage: Its flexibility and reliability in high speed data rate.
Disadvantage: Prone to noise and very sensitive in Electromagnetic Interference.

Rigid Coaxial line

Insulating spacers or beads at regular intervals insulates the inner conductor.

Mostly used for radio frequency (AM - FM) transmission.

Advantage: Minimize radiation losses
Disadvantage: Expensive and must be kept dry.

Flexible coaxial line

Outer conductor is made of metal braid, which gives the line flexibility and uniform insulator (Polythylene).

Ideal for high speed data and video applications that require flexibility

Advantage: Support high bandwidth transmission compare to twisted pair, high flexibility and high transfer rates.

Disadvantage: Bulky, Expensive, must be grounded to prevent interference.

Semi rigid coaxial line

This type of coax offers superior screening at higher frequencies. The major disadvantage is not very flexible, thus also called "hard line" cable.

Formable cable

Alternative to semi-rigid coaxial cable used where flexibility is required. Formable cable can be stripped and formed by hand without the need for special tools. Ideal for laying out and designing pre-bent semi-rigid assemblies.

Balun

A type of transformer to convert an unbalanced one or vice versa.

Balun

isolate a transmission line and provide a balanced output.

I^2R Loss

Whenever current flows the conductors, some energy is dissipated as heat.

note:
Ideal conductor, Resistance = Zero
real conductor, Resistance not equal to zero

1. Copper Losses
1.1. I^2R Loss
1.2. skin effect
2. Dielectric Losses
3. Induction Losses
4. Radiation Losses

Transmission line losses

Skin effect

A tendency for alternating current to flow mostly near the outside surface of the conductor.

note:
Eddy Current
Lenz's Law

Eddy current

Current induced by changing magnetic fields and flow in closed loops, perpendicular to the plane of the magnetic field.

Lenz's Law

The direction of magnetically induced current, like an eddy current, will be such that the magnetic field produced will oppose the change of magnetic field which created it.

Effective radius of conductivity

Eddy current is concentrated at the center of the transmission line, thus cause cancellation of the current at the center.

Dielectric loss

Heating effect on the dielectric material between the conductors.

Polyethylene is often used as a dielectric.

Dielectric Loss

Change in the path of electrons requires more energy, introducing a power loss.

Induction loss

When the electromagnetic field about a conductor cuts through any nearby metallic object.

Radiation loss

Some magnetic lines of force about a conductor do not return to the conductor when the cycle alternates.

Inductance
Resistance
Capacitance
Conductance

Transmission Line Constants

Inductance (Transmission Line Constants)

When current flows through a wire, magnetic lines of force are set up around the wire.

inductance

change of current, change of magnetic field, then there will be value of _____

unit: microhenrys per unit length

Resistance (Transmission Line Constants)

Because there is no perfect conductor, _______ also exist through the wires.

unit: Ohms per unit length

Capacitance (Transmission Line Constants)

Both parallel wires act as plates of a capacitor and that the air between them acts as a dielectric.

unit: picoFarads per unit length

Conductance (Transmission Line Constants)

Because any dielectric is not a perfect insulator, a small current known as LEAKAGE CURRENT flows between the two wires.

unit: Micromhos per unit length

distributed constants

Electrical characteristic that are spread along the entire length of the transmission line and cannot be distinguished separately.

1. Size of wires
2. Spacing
3. Dielectric

distributed constants depends on (3)

Propagation Constant

The measure of change in its amplitude and phase per unit distance as it propagates to a certain medium (example: space or physical bounded medium)

Propagation constant

also called a transmission function or transmission constant.

refraction of light

the bending of a wave when it enters a medium where its speed is different

note: light is a form of electromagnetic wave

refraction index

velocity of light in vacuum over
velocity of light in the medium

velocity factor

ratio of a transmission line's true propagation velocity and the speed of light in a vacuum.

used instead of refraction index (optics).

Related to the values of inductance and capacitance.

Electrically short (L<λ/4) [preferred]

Electrically long (L> λ/4)

Length of a transmission line

Maximum Power Transfer Theorem

"To obtain maximum external power from a source with a finite internal resistance, the resistance of the load must equal the resistance of the source as viewed from its output terminals." -Moritz Von Jacobi

Maximum Power Transfer Theorem

sometimes called Jacobi Theorem

Incident wave

signal propagating from the source to the load end.

Reflected wave

Signal is propagating from the load end to the source.

Considered as loss for it is an energy not delivered to the end device.

Standing wave

Stationary waveforms due of reflected waves mixed with more of the incoming incident waveform

reflected wave = incident wave

mismatch impedance worst case scenario

1. open ended line

2. close ended line

mismatch impedance worst case scenarios (2)

open ended line

A transmission line that has an infinitely large terminating impedance.

Maximum voltage
Minimum current

Voltage, reflected wave is in phase.
Current, reflected wave is out of phase.

close ended line

Maximum current

Minimum voltage

Current, reflected wave is in phase
Voltage, reflected wave is out of phase.

reflection coefficient

percentage of the reflected wave to incident.

relationship between the line's characteristic impedance and the actual load at the end of the list.

impedance transformer

used to match other unmatched impedance.

stub matching

a short section of transmission line either open or short ended to tune out the reactance component of the load before quarter wave transformer matching.