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Keysight Technologies
Transmission Lines and Reflected Signals
University Engineering Lab Series - Lab 3
Application Note
Introduction
Transmission lines are a very important circuit element in electrical engineering. Transmission lines
are distributed elements, in contrast to lumped elements such as resistors (R), capacitors (C), and
inductors (L). The electrical characteristics of a distributed element depend upon its physical geom-
etry, and the electromagnetic fields which dictate its circuit behavior are distributed over a range of
space. Unlike in a purely lumped element circuit, geometry matters and is a primary factor for how the
circuit behaves. A transmission line cannot be described by a simple current-voltage (I-V) relation-
ship like RCL elements can. Distributed elements, such as transmission lines, are more fundamentally
described by the electromagnetic field distributions, which can be either static (DC), time harmonic
(AC), or transient waves or pulses. Transmission lines, the subject of this laboratory, are conduits for
the propagation of electromagnetic waves which carry both information and power. Travelling waves
interact with inhomogeneities in their path, and these produce reflections. These reflected waves are
an integral aspect of transmission line behavior which can be both useful and a challenge to manage.
03 | Keysight | Transmission Lines and Reflected Signals, University Engineering Lab Series - Lab 3 - Application Note
A quick overview
The characteristic impedance of a transmission line Z0 is the ratio of the voltage and
current of a wave travelling along the line; that is, a wave travelling in one direction in the
absence of reflections in the other direction. The characteristic impedance is determined
by the geometry and materials of the transmission line. In a lumped-element model,
Z0 can be expressed in terms of RLGC parameters and the angular frequency of the
electromagnetic wave:
R + j L
Z0 = .
G + jC
The phase velocity, also called the propagation velocity, is determined by the material
itself:
1
v= ,