Transmission Lines

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  • Topic: RF connector, Coaxial cable, Coaxial connectors
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  • Published : December 4, 2012
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TRANSMISSION LINES
Introductory Lecture Engr. Duwi E. Iscala SoE/APC

What are transmission lines?
• Transmission lines are pieces of wire or cable used to carry telephone signals, computer data in LANs, TV signals in cable TV systems, and signals from a transmitter to an antenna, or vice versa.

Transmission Line
• Definition:
– Any means of conveying signal or power from one point to another

What are the different types of transmission lines?
• Parallel-wire lines

What are the different types of transmission lines?
• Coaxial cable

What are the different types of transmission lines?
• Twisted-pair cable

What are the different types of transmission lines?
• Fiber optic cable

Why are transmission lines important?
• Because they are MORE than pieces of wire or cables. They are CIRCUITS. • They act as RESONANT CIRCUITS and REACTIVE COMPONENTS at very high frequencies

REACTIVE COMPONENTS

TUNED CIRCUITS

Electrical Characteristics
• They act as tuned circuits and filters. Their ELECTRICAL CHARACTERISTICS are critical in any communication system

Filters

MATCHING CIRCUIT
• Equipment and transmission lines must be MATCHED to each other for successful communication to take place. Transmission lines perform such matching.

Matching Circuit

General Classifications of Transmission Lines
• Balanced lines – used when balanced properties are considered • Unbalanced lines – used when unbalanced properties are considered

Balanced Line
• Transmission line connection in which neither wire is connected to ground. Signal in each wire is referenced to ground.

Balanced Lines

Unbalanced Lines
• Transmission line connection in which one conductor is connected to ground.

Unbalanced Lines

Conversion
• To convert from balanced to unbalanced operation, or vice versa, uses BALUN (balanced-unbalanced)

Balun

Common-mode Rejection
• Balanced lines protection from noise pick up or cross talk from inductive and capacitive coupling due to external signals.

Historical Glimpse
• The distributed circuit analysis of uniform transmission lines was started by William Thomson (Lord Kelvin) in 1855 and completed by Oliver Heaviside about 1885

Electrical Characteristics
Part 2

Electrical Characteristics
• At low frequencies, the transmission line acts as a carrier of the ac voltage. For these applications, the only characteristic of the cable of interest is resistive loss. • Examples: AC power lines, audio wires for sound system, telephone lines (local loop)

Electrical Characteristics
• Cables used to carry RF energy are not simply resistive conductors but are complex equivalents of inductors, capacitors, and resistors

Electrical Characteristics
• Whenever the length of a transmission line is the same order of magnitude as or greater than the wavelength of the transmission signal, the line takes on a special characteristics and requires a more complex analysis

Wavelength
• Descriptive Definition:
– Wavelength is the length or distance of one cycle of an ac wave – Wavelength is the distance that an ac wave travels in the time it required for one cycle of that signal

Wavelength
• Definition by Formula:
– Wavelength is the ratio of the speed of light to the frequency of the signal λ = 300,000,000 (m/s) f (Hz)

Wavelength
• Definition by Formula:
– Wavelength is the ratio of the speed of light to the frequency of the signal λ = 186,400 (mi/s) f (Hz)

Wavelength
For a 60-Hz power line signal:
λ = 300,000,000(m/s) 60 (Hz) = 5 x 106 m

• Wavelength >> transmission line length

Wavelength
At radio frequencies, say 3 Mhz or more:
λ = 300,000,000(m/s) 3,000,000(Hz) = 100 m

• Wavelength = or < transmission line length

Wavelength
Simplified formula:
λ = 300 f (frequency is in Mhz)
NOTE: wavelength is in meter

λ = 984 f (frequency is in Mhz)
NOTE: wavelength is in feet

Electrical Characteristics
• A pair of...
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