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.
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.