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.