EMC Question of the Week: April 21, 2025

illustration of a transmission line with a voltage standing wave pattern

The voltage standing wave ratio (VSWR) in a transmission line is a function of the line's characteristic impedance and the 

  1. source impedance
  2. termination impedance
  3. both (a) and (b)
  4. neither (a) nor (b)

Answer

The best answer is “b.” The VSWR depends on the magnitude of the reflection coefficient, Γ, at the load end of the transmission line. Specifically, it is 

𝑉𝑆𝑊𝑅 = (1+|Γ|)/(1−|Γ|), where

|Γ| = |(ZL-Z0)/(ZL+Z0)|. 

The VSWR is not impacted by the source impedance. In fact, there is no inherent advantage to matching the transmission line at the source end when all signals flow from the source end to the load end. On the other hand, transmission lines that carry signals in both directions should normally be matched at both ends.

Note that all the power is reflected in a lossless transmission line terminated with a capacitance (as shown in the figure). In this case, the magnitude of the reflection coefficient is equal to one, and the VSWR is infinite. This is true for any purely reactive load impedance.

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