Unbalanced line
Examples of unbalanced lines are coaxial cable or the historic earth return system invented for the telegraph, but rarely used today.
Another benefit of unbalanced lines is that they do not require more expensive, balanced driver and receiver circuits to operate correctly.
These cables took the form of multiple insulated conductors enclosed by a metal screen and overall protective jacket.
It was originally constructed from rigid copper pipes, but the usual form today is a flexible cable with a braided screen.
The advantages of coax are a theoretically perfect electrostatic screen and highly predictable transmission parameters.
Coaxial lines are the norm for connections between radio transmitters and their antennae, for interconnection of electronic equipment where high frequency or above is involved, and were formerly widely used for forming local area networks before twisted pair became popular for this purpose.
Triaxial cable (triax) is a variant of coax with a second shield conductor surrounding the first with a layer of insulation in between.
Planar format transmission lines are flat conductors manufactured by a number of techniques on to a substrate.
Similarly, the audio frequencies used by telephones are relatively low and transmission line theory only becomes significant for distances of at least between buildings.
At the very high data rates handled by modern computer processors, transmission line considerations can even be important inside an individual integrated circuit.
The variant of stripline where the space between the two ground planes is completely filled with a dielectric material is sometimes known as triplate.
A sheet of copper foil may be wrapped around the two boards to electrically bond the two ground planes firmly together.
Strictly speaking, standard transmission line analysis does not apply because other modes are present, but it can be a usable approximation.
Typical circuit blocks implemented by transmission lines include filters, directions couplers and power splitters, and impedance matching.
At microwave frequencies discrete components need to be impractically small and a transmission line solution is the only viable one.
On the other hand, at low frequencies such as audio applications, transmission line devices need to be impractically large.
