by Pete, NEØT
We are familiar with waves. We see them in water. Drop a stone in and a wave travels outward from the spot. This is called a “traveling wave”. If another wave in the water travels back toward the spot, the two waves will add to set up what’s called a Standing Wave. You can generate standing waves in a styrene coffee cup by dragging it across a surface which causes it to vibrate. The vibrating wave travels out to the cup and reflects back on itself causing a perfect standing wave, one which stands still as concentric circles in the coffee cup. I can show you this at a meeting using the coffee Padre brews there.
The radio waves on our coax or twin-lead feed lines begin as traveling waves going from the transmitter up the line to the antenna. If the antenna has a perfect 50 ohm radiation resistance there will be no reflection from the antenna back to the transmitter and the wave will remain traveling and only in the forward direction. The antenna’s radiation resistance is exactly equal to the feed line’s “characteristic impedance” and the system looks exactly as if the feed line is infinitely long, there’s nothing to reflect from. However, if the antenna has a radiation resistance different from the 50 ohm radiation resistance there will be a fraction of the forward wave that reflects back toward the transmitter. The result is the sum of the two waves which is a standing wave. Any wave which is a mix of traveling and standing or is pure traveling or pure standing can be broken down into forward and reflect components. The standing wave stands still along the feedline and has nodes and loops alternating each 1/2 wave length.
In the example above where the wave travels out to the antenna and some reflects; the standing wave builds up gradually (although the buildup is over in microseconds). Eventually, the transients are over and the wave takes on its permanent characteristic. This is the wave we measure in our antenna tuner. The transients occur each time we key the transmitter but are quickly over, long before we take our measurements of the steady-state forward and reflect waves.