Transmission Lines and Impedance Matching

Section 1: Introduction to Transmission Lines

A transmission line is a specialized cable designed to carry alternating current of radio frequency.
Transmission lines are used in applications such as connecting radio transmitters and receivers with their antennas, and in the design of high-frequency circuits.
Two types of straightforward transmission lines exist: parallel line (or ladder line) and coaxial line.
The primary parameters for a transmission line include: resistance (R), inductance (L), capacitance (C) and conductance (G).
The secondary parameters include: characteristic impedance (Zo) and propagation constant (γ). These are derived from the primary parameters.

Section 2: Understanding Impedance

Impedance, denoted as Z, is a measure of the opposition offered to a varying electric current by a circuit element.
Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase.
In ideal transmission lines, impedance is independent of the line length.
Complex impedance includes resistance (real part) and reactance (imaginary part).

Section 3: Impedance Matching

Impedance matching is an essential concept in electronics that allows maximum power transfer between source and load.
Impedance matching is achieved when the load impedance equals the source impedance or the characteristic impedance of the transmission line.
Mismatched impedance can lead to signal reflections or echo signals.
Common impedance matching techniques include L-section match, transformer match, and stub match.

Section 4: Standing wave ratio (SWR)

The standing wave ratio (SWR) on a transmission line is a measure of impedance matching.
A SWR of 1:1 indicates an ideal match between the line and its load, meaning there’s no reflection.
SWR directly affects power transfer. A high SWR can lead to poor performance in communication systems.

Section 5: Losses in Transmission Lines

Various factors can contribute to losses in transmission lines, including dielectric losses, radiation losses, and copper losses.
Poor impedance matching can also lead to losses by causing reflections.
Minimising losses in transmission lines is important in maintaining the overall efficiency of a communication system.