# transmission line standing wave puzzle

Discussion in 'Feedback and Suggestions' started by Unregistered, Nov 18, 2008.

1. ### Unregistered Thread Starter Guest

In the following example on the above page, the current at node 2 is 13.33 mA,
a fixed number.

But in the next diagram, the current at node 2 is changing between MAX and -MAX
in red (a standing wave, current antinode).

How can you explain this?

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At f=0: input: V=1, I=0; end: V=1, I=0.

At 250 kHz, we see zero voltage and maximum current at the source-end of the transmission line, yet still full voltage at the load-end: (Figure below)

(figure not copied)

At f=250 KHz: input: V=0, I=13.33 mA; end: V=1 I=0.

You might be wondering, how can this be? How can we get full source voltage at the line's open end while there is zero voltage at its entrance? The answer is found in the paradox of the standing wave. With a source frequency of 250 kHz, the line's length is precisely right for 1/4 wavelength to fit from end to end. With the line's load end open-circuited, there can be no current, but there will be voltage. Therefore, the load-end of an open-circuited transmission line is a current node (zero point) and a voltage antinode (maximum amplitude): (Figure )

(figure not copied)

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,182
415
I think you're just confusing the average and the instantaneous voltages. The 13.33A as read with a meter will be the RMS. If you were looking with a scope, you'd see the voltage (and currents) oscillate between their maximum (absolute) values.

eric

Eric,