12 Transmission Lines

Types of Transmission Lines Two-Wire Open Line Transmission line between antenna and transmitter or antenna and receiver. Simple construction. Twisted Pair Two insulated wires twisted to form flexible line without use of spacers.

Types of Transmission Lines Unshielded Twisted Pair (UTP) Used for computer networking; CAT6 and CAT5e. See Table 12-1: T568A/T568B Wiring

Table 12-1 T568A/T568B Wiring

Types of Transmission Lines Unshielded Twisted Pair (UTP) Used for computer networking; CAT6 and CAT5e. See Table 12-2: Different Categories for Twisted-Pair Cable

Table 12-2 Different Categories for Twisted-Pair Cabl3

Types of Transmission Lines Shielded Pair Parallel conductors separated from each other, and surrounded by solid dielectric.

Types of Transmission Lines Coaxial Lines Rigid or air; flexible or solid. Balanced/Unbalanced Lines Unbalanced Amplitude of electrical signal by center conductor in coaxial line measured with respect to grounded outer conductor. Balanced Same current flows in each wire but 180° out of phase.

Electrical Characteristics of Transmission Lines Two-Wire Transmission Line Electrical characteristics Construction of line. Characteristic Impedance Impedance measured at any point on line would be the same.

Electrical Characteristics of Transmission Lines Transmission Line Losses Copper Dielectric Radiation or induction

Propagation of DC Voltage Down a Line Physical Explanation of Propagation To understand characteristics of transmission line with ac voltage applied, infinitely long transmission line analyzed with dc voltage applied. Velocity of Propagation Current moving down the line, its electric and magnetic fields are propagated down the line.

Propagation of DC Voltage Down a Line Delay Line Delay signal by some specific amount of time. Wavelength Distance traveled by wave during a period of one cycle.

Nonresonant Line Traveling DC Waves Nonresonant line Traveling waves Line of infinite length or as one terminated with resistive load equal in ohmic value to characteristic impedance of line. Traveling waves Voltage and current waves; move in phase with one another from source to load.

Nonresonant Line Traveling AC Waves Little difference between charging of line when ac voltage applied to it and when dc voltage applied.

Resonant Transmission DC Applied to an Open-Circuited Line Resonant line Transmission line terminated with impedance not equal to characteristic impedance. Because impedances are equal, applied voltage divided equally between source and line.

Resonant Transmission Incident and Reflected Waves Voltage on open-circuited wire equal to source voltage, and current is zero.

Resonant Transmission DC Applied to a Short-Circuited Line Voltage reflection from open circuit is in phase, while from short circuit it is out of phase. Current reflection from open circuit is out of phase, while from short circuit it is in phase.

Resonant Transmission Standing Waves Open Line Mismatch Incident and reflected waves interact. Standing wave Remains in one position, varying only in amplitude.

Resonant Transmission Standing Waves Shorted Line Out-of-phase reflection that occurs for current on open line and voltage on shorted line. Impedance Voltage divided by current.

Resonant Transmission Quarter-Wavelength Sections Phase inversion of voltage and current every quarter wavelength. Cavity filter or selective cavity extends idea of quarter-wavelength coaxial stub filter to high-power operation.

Standing Wave Ratio Standing Wave Reflections Instantaneous vector addition of incident-wave amplitude with that of reflected wave as result of impedance mismatch between transmission line and load. Reflections Occur when impedance mismatch.

Standing Wave Ratio Degree of Mismatch Reflection coefficient, voltage standing-wave ratio, return loss.

Standing Wave Ratio Voltage standing wave ratio (VSWR) Ratio of maximum voltage to minimum of standing wave on a line. Standing wave ratio (SWR) Equal to ratio of maximum current to minimum current.

Standing Wave Ratio Effect of Mismatch Flat line Perfect condition of no reflection when load purely resistive and equal to Z0. Higher the VSWR, the greater is mismatch on line.

Standing Wave Ratio Quarter-Wavelength Transformer Electrical Length Not physically a transformer; offers property of impedance transformation. Electrical Length Line can be miles long physically and electrically short at low frequencies.

The Smith Chart Transmission Line Impedance Impedance Constantly changing along line and equal to ratio of voltage to current at given point. Smith chart presents solution to impedance-matching problems posed by complex sources and complex loads.

The Smith Chart Transmission Line Impedance Smith chart impedance-matching tool for transmission lines. Graphical Smith chart used to represent network analyzer solutions.

The Smith Chart Smith Chart Normalizing Dividing all impedances by characteristic impedance of the line. Reciprocal of impedance defined as admittance and reciprocal of reactance is called susceptance.

The Smith Chart Smith Chart Greatest utility as impedance-matching calculator. Allows for simple conversion of impedance to admittance, and vice versa.

The Smith Chart Smith Chart Spiral True standing wave representation on Smith chart. Many calculations with transmission lines pertain to matching load to line and keeping VSWR as low as possible. Use of short-circuited stubs prevalent in matching problems.

Transmission Line Applications Discrete Circuit Simulation Transmission line sections: used to simulate inductance, capacitance, LC resonance. Baluns Unbalanced-to-balanced transformer.

Transmission Line Applications Transmission Lines as Filters Quarter-wave section of transmission line used as efficient filter or suppressor of even harmonics.

Transmission Line Applications Slotted Lines Section of coaxial line with lengthwise slot cut in outer conductor; pickup probe inserted into slot. VSWR, generator frequency, unknown load impedance.

Transmission Line Applications Time-Domain Reflectometry Short-duration pulse transmitted into a line. Monitored with oscilloscope.

Impedance Matching and Network Analysis Network analyzer Test equipment to characterize linear impedance characteristics of devices under test (DUTs). Scalar network analyzer Measures magnitude of DUT impedance.

Impedance Matching and Network Analysis Vector network analyzer (VNA) Determines impedance magnitude and phase characteristics of DUTs. Scattering or S-parameters Linear transmission and reflection characteristics of signals applied to DUTs.

Impedance Matching and Network Analysis Scattering matrix Describes all voltages incident to and reflected from all ports. Impedance analysis not confined to microwave systems.

Impedance Matching and Network Analysis Vector network analyzer System of three receivers and two-port “test set” that separates forward and reflected components of applied signals through directional couplers.