Standing Waves and Reflection in Transmission Lines Page 3

3.4) Current in the Single Yellow Wire

  Next a single sample wire (yellow in figure 12) is placed through the toroidal transformer and the current in the transformer secondary is now shown on the second trace down on the oscilloscope and is producing a waveform (of 21.9 mV) into 50 Ω. The voltage and current in the sample wires as shown on the two lower traces remain unchanged

Figure 12 Current in the Yellow Wire

3.5) Current with Both Wires in the Same Direction

  The two sample wires (yellow and violet) are now passed through the transformer in the same direction. It can be seen from figure 13 and the second trace down on the oscilloscope that the displayed voltage has practically doubled (in this case from 21.9 mV to 43.1 mV), thus showing that the current has doubled and therefore the two currents passing through the transformer in the same direction have been added.

Figure 13 Two Wires in the Same Direction

3.6) Current With Wires in the Opposite Directions

  With the sample wires (yellow and violet) now placed in opposite directions through the toroidal transformer, it can be seen from the second trace down in figure 14 that the two currents have been magnetically cancelled in the transformer and there is no output from the current transformer. The two currents are still present in the actual sample wires, as can be seen by the voltages displayed on the two lower traces of the oscilloscope but the transformer has effectively subtracted the currents and it looks as though there is no current at all from the second trace down!

Figure 14 Two Wires in Opposite Directions

  We can now see quite clearly the behaviour of the current transformer and there should be no confusion as to what is really happening. When two identical currents of the same phase in two wires flow in opposite directions through the toroidal transformer their magnetic effects cancel, the resulting measured net current from these two sample wires is zero. It can be logically deduced from this that when there are two identical currents of the same phase going in the Forward and Reflected directions in a single wire, the measured net Current is also zero because the transformer responds to the net enclosed Current.

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Technical Articles Index First Published on 21/10/2026 by William J Highton