Stripline

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Cross-section diagram of stripline geometry. Central conductor (A) is sandwiched between ground planes (B and D). Structure is supported by dielectric (C).

Stripline was invented by R. Barret in the 1950's. Stripline is a TEM (transverse electromagnetic) transmission line media, like coaxial cable.

A stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes, The insulating material of the substrate forms a dielectric. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the strip which is a transmission line. As shown in the diagram, the central conductor need not be equally spaced between the ground planes. In the general case, the dielectric material may be different above and below the central conductor.

A stripline transmission line is similar to a microstrip, except that the microstrip is not sandwiched, it is on a surface layer, above a ground plane.

Like coaxial cable, stripline is non-dispersive, and has no cutoff frequency. Good isolation between adjacent traces can be achieved more easily than with microstrip.

Stripline has two disadvantages:

- it is much harder (and more expensive) to fabricate than microstrip;

- because of the second groundplane, the strip widths are much narrower for a given impedance and board thickness than for microstrip.