Stripline
In electronics, stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s.
Stripline is the earliest form of planar transmission line.
A stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes.
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.
To prevent the propagation of unwanted modes, the two ground planes must be shorted together.
This is commonly achieved by a row of vias running parallel to the strip on each side.
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 provides for enhanced noise immunity against the propagation of radiated RF emissions, at the expense of slower propagation speeds when compared to microstrip lines.
Hence striplines have higher effective permittivity in comparison to microstrip lines, which in turn reduces wave propagation speed (see also velocity factor) according to Stripline, now used as a generic term, was originally a proprietary brand of Airborne Instruments Laboratory Inc. (AIL).
The version as produced by AIL was essentially air insulated (air stripline) with just a thin layer of dielectric material - just enough to support the conducting strip.
The more familiar version with the space between the two plates completely filled with dielectric was originally produced by Sanders Associates who marketed it under the brand name of triplate.
[1] Stripline was initially preferred to its rival, microstrip, made by ITT.
Also, discontinuity elements on the line (gaps, stubs, posts etc) present a purely reactive impedance.
This is not the case with microstrip; the differing dielectrics above and below the strip result in longitudinal non-TEM components to the wave.
This results in dispersion and discontinuity elements have a resistive component causing them to radiate.
In the 1950s Eugene Fubini, at the time working for AIL, jokingly suggested that a microstrip dipole would make a good antenna.
[2] Stripline remained in the ascendent for its performance advantages through the 1950s and 1960s but eventually microstrip won out, especially in mass produced items, because it was easier to assemble and the lack of an upper dielectric meant that components were easier to access and adjust.
As the complexity of printed circuits increased, this convenience issue became more important until today microstrip is the dominant planar technology.
However, stripline is still chosen where operation over a wide band is required.
An accurate closed form equation for the characteristic impedance of a stripline with a thin centered conductor has been reported as[4]
{\displaystyle {\begin{aligned}W_{eff}&={\begin{cases}W-{\frac {(0.35-W)^{2}}{1+12T}},&W<0.35\\W,&W\geq 0.35\end{cases}}\\T&={\frac {t}{h}}\\W&={\frac {w}{h}}\\w&={\text{width of the stripline conductor}}\\t&={\text{thickness of the stripline conductor}}\\h&={\text{thickness of the substrate from the top ground plate to the bottom ground plate}}\\E_{r}&={\text{dielectric constant of the substrate dielectric material}}\end{aligned}}}
Note that when the conductor thickness is small, T<<1 or t< For thick conductors, Wheeler provides the following more accurate equations[5] For stripline conductors that are not centered, that is, the distance to the upper ground plane is not the same as to the lower ground plane, strategies exist to estimate the characteristic impedance in at least one of two ways. If the asymmetry of the conductor placement is not large, the lower and upper capacitance per unit length may be estimated for the upper ground plane and the lower ground plane using centered stripline equations and standard transmission line equations for homogeneous lines, are measured from center of the conductor to the lower and upper ground plane, respectively. Co and Lo are the capacitance and inductance per unit length of the associated transmission line. estimation is quantified and listed in the microstrip metallic enclosure equations.