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I am currently learning about transmission lines form an online book hosted by a website called “All About Circuits”. If anyone is interested, the particular chapter I am currently reading can be found here: http://www.allaboutcircuits.com/vol_2/chpt_14/6.html.

Basically, the book talks about having a circuit with a transmission line that looks like this:

enter image description here

The transmission line is supposed to be long enough so that:

  • At 250 kHz, the line's length is precisely right for 1/4 wavelength.
  • At 500 kHz, the line's length is precisely right for 1/2 wavelength.
  • At 750 kHz, the line's length is precisely right for 3/4 wavelength.
  • At 1000 kHz, the line's length is precisely right for 1 wavelength.

The images below show that standing wave graphically depicted with the frequencies mentioned above.

At 250 kHz, 1/4 wavelength:

enter image description here

At 500 kHz, 1/2 wavelength:

enter image description here

At 750 kHz, 3/4 wavelength:

enter image description here

At 1000 kHz, 1 wavelength:

enter image description here

If you look at the standing wave for the mentioned frequencies, you will notice that the load end of the transmission line always has a standing wave antinode. I can understand this because the line is open so in my mind this makes sense.

What I don’t understand is why (depending on the frequency), we sometimes get a node and sometime an antinode on the source side of the transmission line. I don’t understand this because both the source and load sides are supposed to be endpoints which should mean that they should always be nodes or always be antinodes but not vary (at least that is my understanding).

Nevertheless, I am obviously missing something here so I was hoping someone could help me out and clarify what is going on here.

Thank you.

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The devil's in the details, even if they are printed.

The trick is in the fact that the transmission-line is of characteristic 75 Ohm, and the excitation side is also 75 Ohm. Were that end very low source impedance it would tend much more to wanting a node (and as such forcing the need for an adjusted frequency = half the allowable frequencies), were it a very high impedance it would tend more to wanting an anti-node and again half the allowable frequencies.

The fact it is the exact characteristic makes that end reflection-less and in effect not a defining factor in the orientation of the standing wave. As such the open end gets to "dictate" the orientation on its own and the number of allowable frequencies for standing waves on the same line gets doubled.

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