What Is Stripline?

Stripline is a planar transmission line used in RF and microwave engineering to guide electromagnetic signals between circuit elements at controlled impedance and minimal loss. The structure consists of a flat conducting strip, centered between two parallel ground planes, with a homogeneous dielectric material filling the space between them. Because the conductor is fully enclosed within the dielectric, stripline supports pure transverse electromagnetic (TEM) mode propagation, making it non-dispersive and free of cutoff frequency effects over its operating range.

The technology traces to the early 1950s, when Robert M. Barrett of the Air Force Cambridge Research Center developed the triplate structure as a compact circuit-based alternative to waveguide and coaxial cable for radar applications. This work gave printed-circuit fabrication techniques access to the microwave frequency range for the first time. As Microwaves101 documents in its history of stripline, Barrett's innovation established planar transmission-line design as a mainstream engineering discipline.

Physical Structure and Characteristic Impedance

The standard stripline geometry places a conducting strip of width w and thickness t at the midpoint between two ground planes separated by a total distance b. The characteristic impedance of a zero-thickness strip is given approximately by the formula Z0 = (94.25 / sqrt(Er)) / (w_eff/b + 0.441), where Er is the relative permittivity of the dielectric and w_eff incorporates fringing capacitance at the strip edges. Because the dielectric is uniform throughout the cross-section, the effective permittivity equals the actual substrate permittivity, and the phase velocity of a signal does not change with frequency. This makes impedance design straightforward and predictable across a wide frequency range. Ground planes are often periodically shorted together at intervals to suppress parasitic cavity modes.

Comparison with Microstrip

Microstrip, derived from stripline when one ground plane is removed, places the conductor on an exposed substrate surface. The result is a simpler structure that is easier to solder and test, but the asymmetric field distribution between the dielectric substrate and the surrounding air introduces dispersion: effective permittivity varies with frequency. Stripline avoids this problem entirely because the electromagnetic fields are confined entirely within the dielectric. As detailed in the LibreTexts microwave engineering reference on planar transmission lines, this confinement also provides inherent shielding, reducing electromagnetic interference between adjacent traces. The tradeoff is increased fabrication complexity: accessing the strip for component attachment requires drilling or routing through the dielectric stack.

Suspended and Shielded Variants

Engineers have developed modified versions of the basic geometry to reduce dielectric loss or improve selectivity. Suspended stripline partially removes the dielectric so that the conducting strip is supported by a thin membrane, substantially lowering the effective permittivity and reducing dielectric loss. Resonators built on suspended substrates exhibit high quality factors, making this variant well suited to narrowband filter design. Air-filled stripline carries this concept to its limit, achieving the lowest dielectric loss of any planar structure while retaining TEM propagation. Miniaturized and multilayer stripline assemblies are a practical component of modern printed circuit board design, where Analog Devices' introduction to printed circuit transmission lines outlines the impedance control practices used in high-speed digital and RF boards.

Applications

Stripline has applications in a wide range of disciplines, including:

• Radar system front ends, where shielding and TEM propagation reduce spurious coupling
• Bandpass and notch filter construction using high-Q resonators on suspended substrates
• High-speed digital PCB routing where impedance control and crosstalk suppression are required
• Phased-array antenna feed networks demanding low dispersion at microwave frequencies
• Satellite and aerospace communications hardware requiring compact, lightweight RF interconnects