In high frequency circuits, the wavelength of the signals in the circuit becomes as small as or smaller than the size of the physical components that make up the circuit. This can cause interesting effects that are completely non-intuitive.

In particular, two electrical components that are connected and pass a signal between them are required to have the same impedance to prevent reflections of the signal from occurring. Reflections due to un-matched impedances can carry significant power; enough to damage sensitive components that may be part of the circuit. The reflections can also seriously degrade a signal.

To prevent reflections, a number of impedance matching techniques can be used. Some the simplest techniques are reactive matching, the single stub tuner, the double stub tuner, and the quarter wavelength transformer.

Each of these techniques works in a relatively narrow band. They are generally designed around a single frequency that you want to carry, as the wavelength determines the length of the quarter wavelength transformer, and the length of the stubs for the stub tuners.