Transmission Line Fundamentals & Hardware

Comms 4 Transma Fundamentals

Transmission Line

A device designed to guide electrical energy from one point to another; a metallic conductor system used to transfer RF energy.

Guided Transmission Media

Media with some form of conductor that provides a conduit for electromagnetic signals (e.g., Copper Wire, Optical Fiber).

Unguided Transmission Media

Wireless systems where propagation depends on direction and obstacles (e.g., Air, Free Space).

Input Impedance (Zin)

The impedance presented to the transmitter by the transmission line and its load (antenna).

Output Impedance (Zout)

The impedance presented to the load by the transmission line and its source.

Characteristic Impedance (Zo)

The ratio of voltage to current at any point on a transmission line; the impedance seen looking into an infinitely long transmission line.

Parallel-Line (Two-Wire Open Line)

Consists of two wires spaced 2 to 6 inches apart by insulating spacers; simple construction but high radiation losses and noise pickup.

Two-Wire Ribbon

A parallel line where two wires are embedded in a low-loss dielectric, usually polyethylene.

Twisted Pair

Consists of two insulated wires twisted together to form a flexible line; not used for high frequencies.

Shielded Pair

Consists of two parallel conductors separated from a twisted braid shield by a dielectric.

Rigid Coaxial Line

Consists of a central, insulated inner conductor mounted inside a tubular outer conductor; minimizes radiation losses but is expensive and must be kept dry.

Flexible Coaxial Line

Consists of a flexible wire inner conductor insulated from a copper braid outer conductor by a solid dielectric (usually polyethylene).

Waveguide

A hollow metal tube (cylindrical or rectangular) used to efficiently interconnect high-frequency electromagnetic waves.

Fiber Optics

A dielectric waveguide (thin glass or plastic cable) that transmits radiant power or light energy.

Velocity of Propagation (Vp)

The speed at which a signal travels through the transmission line, often expressed as a percentage of the speed of light.

Attenuation

The amount of loss in signal strength as it propagates down a wire or cable.

Wavelength (lambda)

The distance occupied by one cycle of a wave.

Balanced Transmission Line

A line where neither wire is connected to the ground (e.g., Twin lead).

Unbalanced Transmission Line

A line where one conductor is connected to the ground (e.g., Coaxial cable).

Common-Mode Rejection (CMR)

The ability of a balanced line to cancel external signals that appear on both wires simultaneously.

Balun

A device used to convert a transmission line from balanced to unbalanced operation (or vice versa).

Crosstalk

Unwanted coupling caused by overlapping electric and magnetic fields between adjacent lines.

Near End Crosstalk (NEXT)

A measure of the level of crosstalk or signal coupling within the cable at the source end.

ACR (Attenuation-to-Crosstalk Ratio)

A combined measurement of attenuation and crosstalk.

Delay Skew

The difference in propagation time between the fastest and slowest wire pairs in a UTP cable.

Power Sum NEXT (PSNEXT)

Measures the total crosstalk from all other cable pairs into a specific pair.

Return Loss

A measure of the ratio of power reflected or returned into a cable to the amount of power transmitted.

Connector

A device that connects a cable to equipment or another cable.

Coaxial Connector

Designed to maintain the physical integrity and electrical properties of the coaxial cable while connecting.

PL-259 Connector (UHF)

A male connector that fits on the end of a coaxial cable (often RG-8/U) and attaches the shield braid and inner conductor.

BNC Connector

A bayonet-style connector commonly used to attach test instruments and in LANs.

SMA Connector

A sub-miniature connector characterized by a hexagonal body, used for high-frequency applications.

Copper Losses (I^2R Losses)

Energy dissipated as heat whenever current flows through the conductors.

Dielectric Heating Losses

Energy dissipated as heat in the dielectric material due to the potential difference between conductors; negligible in air dielectrics.

Radiation Losses

Occur when magnetic lines of force do not return to the conductor; effectively energy radiated into space like an antenna.

Induction Losses

Occur when the electromagnetic field around the conductor cuts through a nearby metallic object, inducing a current in it.

Coupling Losses

Occur at connections between transmission lines or equipment.

Corona

A luminous discharge between conductors when the potential difference exceeds the breakdown voltage of the dielectric insulator.

RG-58

A common coaxial cable type with approx 50-53 ohms impedance, used in Ethernet and radio.

RG-59

A common coaxial cable type with approx 73-75 ohms impedance, used for video and cable TV.

RG-8

A thicker coaxial cable with approx 50-52 ohms impedance, lower attenuation than RG-58.

RG-11

A thick coaxial cable with 75 ohms impedance.

Twin Lead

A common balanced transmission line with 300 ohms impedance, used for TV lead-ins.

CAT-3 Cable

Unshielded Twisted Pair (UTP) used for telephone installations (up to 16 Mbps).

CAT-5 Cable

UTP used for computer networks (up to 100 Mbps).

CAT-5e Cable

Enhanced CAT-5 with improved noise performance (100 Mbps+).

CAT-6 Cable

UTP for higher speed computer networks (up to 250 Mbps).

CAT-7 Cable

Shielded Twisted Pair (STP) for ultra-high-speed networks (up to 600 Mbps/1 Gbps).