
What is VSWR?
March 22, 2023VSWR, or Voltage Standing Wave Ratio, is found from the reflection coefficient:
If a transmission line is feeding a DUT and there is an impedance mismatch between the DUT input impedance and the characteristic impedance of the line, there will be a nonzero reflection coefficient Γ.
The existence of a reflection on the transmission line gives rise to constructive and destructive interference along its length and stationary peaks and valleys in the RF voltage will occur. The ratio between the maximum and minimum RF voltage along the length is equal to the VSWR. If there is no variation, then the VSWR = 1 (or often written 1:1), and there is a perfect match with no reflection.
The magnitude of the reflection coefficient may be found from:
See this and more important radio frequency charts and formulas in this comprehensive application note.
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What is a Reflection Coefficient?
March 22, 2023
Just as a beam of light might bounce off a sheet of glass, some or all of an RF signal may reflect from an impedance other than one equal to the characteristic impedance of the transmission line. If the characteristic impedance is Z0 and the encountered impedance is Z, then:If the encountered impedance is complex, then the reflection coefficient is also complex. If Z = Z0, then Γ = 0 and there is no reflection. If Z is 0, a short, then Γ = -1 and the reflection is total but inverted. If Z is infinite, an open, then Γ = 1 and again the reflection is total but in phase.For complex impedances, the reflection coefficient will also be complex, and may be expressed as a vector with magnitude less than one and some angle such as 0.5∠53°. It can be expressed in Real/Imaginary Cartesian coordinates as well, such as 0.3+j0.4 which is the same vector. Figure 1 depicts a reflection coefficient as a vector on a polar chart.Figure 11 - Vector on a Polar ChartSee this and more important radio frequency charts and formulas in this comprehensive application note.

Useful Radio Frequency Engineering Formulas and Charts
February 9, 2023
This application note is a collection of essential formulas and charts for Radio Frequency Engineering.

Voltage and Current Measurements with a VNA and DMM
April 20, 2018
Vector Network Analyzers (VNAs) are used to measure the reflection and transmission coefficients, or S-parameters, of a Device Under Test (DUT). When the DUT is passive, the VNA may be the only tool necessary. But some devices are active and require an external power source for operation. In such active applications, it may be useful to perform voltage or current measurements of the DUT as the VNA generator sweeps over frequency or power.