Voltage and current on a transmission line with standing waves

Folk often ask how to calculate the maximum voltage on an antenna feed line with standing waves, often to get a feel for the necessary voltage withstand of baluns, feed line, switches and relays, and ATUs.

Feeding at a current maximum outlines the method described in detail at (Duffy 2011), but the approach is more complex than a lot of hams want.

A simpler method is to treat the transmission line as lossless, and to simply find the worst case voltage and current that can occur… and design for that, or perhaps do the more detailed analysis depending on the outcome.

A new calculator, Calculate Vmax, Vmin, Imax, Imin for lossless line from Zload (or Yload) and Zo, does just that.

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Above is the built-in example of a G5RV with tuned feeder on 80m with feed point impedance derived from a modelling package. The voltage and currents calculated are those for a long lossless feed line.

If you were using a low-end ATU rated for 1500Vpk, the above analysis would warn that even at 100W, that might well be exceeded. Note that some of these ATUs that incorporate a voltage balun might lose so much power in the balun core losses that the power to the antenna is only half the transmitter output, so reducing voltage stress on the feed system (including the balun… though it will get hot due to core loss).

If you don’t know the feed point impedance but know the VSWR, select VSWR from the drop down box and enter the VSWR in the Real column.

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Above is an example for a 50Ω feed line at 1kW with VSWR=3.

A conservative position is to ensure that all components on the feed line can withstand the maximum peak voltage and maximum RMS current.

Again, using a low-end ATU with 1500Vpk rating, it should comfortably be able to withstand the peak voltage in this scenario.