Visualizing VSWR

Updated March 10, 2008

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New for March 2008! We now offer an Excel spreadsheet that will help you visualize how voltage standing waves are created. It's located in our download area. Note: you have to enable macros in order for the spreadsheet to work!

The hypothetical phenomenon we are trying to illustrate is as follows. A TEM transmission line of impedance Z1 is connected to a second transmission line of impedance Z2 (think of this as two coax lines in series, we'll add a figure one of these days). The discontinuity occurs at the value X=25 (whatever units you like). The Z2 transmission line is terminated in a matched load (perfect resistor of value R=Z2) so that there is no additional reflection down the line. The waveforms represent intensity (and polarity) of the electric field, which is entirely perpendicular to the direction of propagation (that's what TEM, or transverse electromagnetic, means)

The two charts below are snapshots in time. On the first chart, the composite wave on the left (forward plus reflected, color red) has peaks around 2, 8, 14 and 20. These are the maxima along the standing wave (and later there are other maximum between these points at 5, 11 and 17).

You'll also not that the composite waveform on the left is contiguous with the transmitted waveform on the right of the mismatch (blue line). This has to be the case, it is not possible to have a discontinuity in the electric field.

The chart below the composite wave is at the minima located at 0, 6, 12 and 18. Note there are other minima between these at 3, 9 and 15.

These images are nothing compared to seeing the waveform move before your eyes, with your mouse controlling the speed. Go get the download called Standingwave_101.xls!