standing waves.
Just to clarify. A standing wave is just that i.e. it does not move, the positions of it's maxima and minima, (often called nodes) are fixed. The position of the nodes and the amplitude of the wave depend on the degree and the nature of the mismatch between the feeder and the coupler/antenna.
When the forward wave from the transmitter reaches the line termination, (in this case the antenna coupler), if they have the same impedance all of the energy is absorbed and none is reflected back down the line. If a mismatch occurs some energy is reflected whose amplitude and phase depends on the type and size of mismatch. It is the interaction between the forward and reverse waves that form the fixed standing wave.
As said before the tuning is carried out at low power until the VSWR is within limits. Only then will the coupler allow the transmitter to operate at full power.
At one time aircraft like the Shackleton used trailing wire antennas which could be wound out to get a good match. Things got a bit sad if the operator forgot to wind it in before landing.
The earlier couplers used a series connected variable inductor and a parallel conected variable capacitor. The capacitor used to be made up of two blocks of copper machined into concentric intermeshing rings. The two blocks were more or less enmeshed to vary the capacitance. This capacitor was enclosed in a glass bottle and was a thing of beauty in itself. Much more fun than the new digital tuners.
Last edited by greatwhitehunter; 25th Aug 2011 at 19:32.
Reason: missed a word