Visualizing RF Standing Waves

14 Comments

Standing waves are one of those topics that lots of people have a working knowledge of, but few seem to really grasp. A Ham radio operator will tell you all about the standing wave ratio (SWR) of his antenna, and he may even have a meter in the shack to measure it. He’ll know that a 1.1 to 1 SWR is a good thing, but 2 to 1 is not so good. Ask him to explain exactly what a standing wave is, though, and chances are good that hands will be waved. But [Allen], a Ham also known as [W2AEW], has just released an excellent video explaining standing waves by measuring signals along an open transmission line.

[Source: Wikipedia]
[Source: Wikipedia]
To really understand standing waves, you’ve got to remember two things. First, waves of any kind will tend to be at least partially reflected when they experience a change in the impedance of the transmission medium. The classic example is an open circuit or short at the end of an RF transmission line, which will perfectly reflect an incoming RF signal back to its source. Second, waves that travel in the same medium overlap each other and their peaks and troughs can be summed. If two waves peak together, they reinforce each other; if a peak and a trough line up, they cancel each other out.

All well and good, but what do standing waves actually look like in a transmission line? [Allen] obliges with a demo using a strip line, which is simply an open section of transmission line. With a special RF detector probe, [Allen] is able to sweep along the strip line and show a few cycles of the 2.4GHz standing wave generated by removing the line terminator. The nodes and anti-nodes are clearly separated on the strip line by a distance proportional to the wavelength.

For me personally, this was a real eye-opener. Now I not only understand where the standing waves come from, but that the nodes and anti-nodes map to physical locations on a line that are characteristic of the frequency of the standing wave. Simple concept, but without that visualization, it just never clicked. For even more eye-opening, be sure to check out [Eliot Williams’] recent “Say It With Me” column on input impedance.

14 thoughts on “Visualizing RF Standing Waves”

  4. Actually 2:1 is not so bad, many even call it reasonable on some antennas it’s when you get to 8:1 that things get really sticky. Some Antennas by design are considered really well tuned at 2:1 In fact the G5RV is designed to have a bad SWR as it uses the feedline as part of the radiation pattern. A choke and an antenna tuner is required for that antenna.

    Report comment
    Reply

  8. Whwn it really gets interesting is with a signal that swings above and below zero.
    You use various taps and a curved transmission line and create a compleatly solid state rectifier.

    Report comment
    Reply

  9. Whwn it really gets interesting is with a signal that swings above and below zero.
    You use various taps and a curved transmission line and create a compleatly solid state rectifier. Of course the hard part is keeping a stable in phase signal.

    Report comment
    Reply

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.