Planar transmission line

Planar transmission lines are transmission lines with conductors, or in some cases dielectric strips, that are flat, ribbon-shaped lines. They are used to interconnect components on printed circuits and integrated circuits working at microwave frequencies because the planar format fits in well with the manufacturing methods for these components. Transmission lines are more than simple interconnections. With normal interconnections the propagation of the electromagnetic wave along the wire is fast enough to be considered instantaneous, and the voltage at each end of the wire can be considered identical. If the wire is longer than a large fraction of a wavelength (one tenth is often used as a rule of thumb) these assumptions are no longer true and transmission line theory must be used instead. At lower frequencies, these considerations are only necessary for the cables connecting different pieces of equipment, but at microwave frequencies the distance at which transmission line theory becomes necessary is measured in millimetres. Hence the need for transmission lines within a circuit.

There are several different forms of planar transmission line. The main ones are stripline, microstrip, suspended stripline, and coplanar waveguide*. Stripline was the earliest form, conceived during World War II by Robert M. Barrett. All four of these forms consist of a pair of conductors (although in three of them, the return conductor is through the ground plane). Consequently, they have a dominant mode of transmission (the field pattern of the electromagnetic wave) that is identical, or near-identical, to the mode found in a pair of wires. Other planar formats, such as slotline, finline, and imageline transmit along a strip of dielectric and substrate integrated waveguide forms a dielectric waveguide within the substrate with rows of posts. These formats cannot support the same mode as a pair of wires, and consequently they have different transmission properties. Many forms have a narrower bandwidth and in general they produce more signal distortion. However, advantages include (depending on exact formats being compared) low loss and a better range of characteristic impedance.

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