Incident Wave

An incident wave is a current or voltage wave that travels through a transmission line from the generating source towards the load. It becomes incident when it arrives at a discontinuity or another medium with different propagation characteristics. At this point, some or all of the wave will be reflected back in the opposite direction to the original. Reflections will also occur at the end of the transmission line unless it is correctly terminated with its characteristic impedance. This situation occurs in practice when, for example, two lengths of dissimilar transmission lines are joined together.

These characteristics apply to electrically conducting transmission lines, often called copper lines as generally found in telecommunications circuits. However, the same wave behaviour occurs in other metals, particularly aluminium as used in power lines. Additionally, waves in fibre-optic lines and microwave reflections in waveguides perform in essentially the same way.

Reflections cause several undesirable effects, including modifying frequency responses, causing overload power in transmitters and overvoltages on power lines. However they can also be used in devices like stubs and impedance transformers. The reflections cause standing waves to be set up on the line, which are used by stubs. Their reactive properties are determined by their physical length in relation to the wavelength of the propagated waves. Therefore, stubs are most commonly used in UHF or microwave circuits whose short wavelengths allow them to be conveniently small. They are often used to replace discrete capacitors and inductors, because at UHF and microwave frequencies lumped components perform poorly due to parasitic reactance.