• Electromagnetics I
  • Ch 3: Transmission Lines
  • Loc 3.18
  • Electromagnetics I
  • Ch 3
  • Loc 3.18

Measurement of Transmission Line Characteristics

This section presents a simple technique for measuring the characteristic impedance

, electrical length

, and phase velocity

of a lossless transmission line. This technique requires two measurements: the input impedance

when the transmission line is short-circuited and

when the transmission line is open-circuited.

In Section 3.16, it is shown that the input impedance

of a short-circuited transmission line is

and when a transmission line is terminated in an open circuit, the input impedance is

Observe what happens when we multiply these results together:

that is, the product of the measurements


is simply the square of the characteristic impedance. Therefore


If we instead divide these measurements, we find




is known in advance to be less than

, then the electrical length

can be determined by taking the inverse tangent. If

is of unknown length and longer than

, one must take care to account for the periodicity of tangent function; in this case, it may not be possible to unambiguously determine

. Although we shall not present the method here, it is possible to resolve this ambiguity by making multiple measurements over a range of frequencies.


is determined, it is simple to determine




, and then

. For example, the phase velocity may be determined by first finding

for a known length using the above procedure, calculating

, and then



Ellingson, Steven W. (2018) Electromagnetics, Vol. 1. Blacksburg, VA: VT Publishing. https://doi.org/10.21061/electromagnetics-vol-1 CC BY-SA 4.0

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