What Is Finline?

Finline is a planar transmission line structure designed for use in the millimeter-wave frequency range, consisting of a partially metalized dielectric substrate enclosed within a standard rectangular metal waveguide. The metallization on the substrate forms one or more narrow conducting fins with a slot at the center or offset from center, and the electromagnetic field is concentrated in the slot region, which guides the propagating wave. Because the structure is housed inside a waveguide, finline circuits retain the shielding and low-radiation-loss characteristics of conventional rectangular waveguide while offering the manufacturing economy and circuit integration advantages of planar printed-circuit technology.

The finline structure was proposed by Paul Meier in 1972 and takes its name from the fish-fin appearance of the metallization pattern viewed in cross section. It emerged as an alternative to microstrip and waveguide at frequencies above roughly 30 GHz, where conventional microstrip suffers from radiation loss and substrate mode excitation, and where machined waveguide components become difficult and expensive to fabricate. Finline occupies a middle ground in the quasi-planar transmission line family, which also includes slot line and coplanar waveguide, but with the advantage of the surrounding rectangular waveguide providing mechanical support and electromagnetic shielding.

Structure and Configurations

Finline appears in four principal geometric variants. In the unilateral configuration, fins on one side of the substrate form a single slot, with the other side left unmetalized. In the bilateral configuration, fins on both sides of the substrate are aligned to form a symmetric slot, which reduces the effective dielectric constant. The insular variant places the substrate fins away from the broad wall of the waveguide, and the antipodal design uses fins on opposite sides of the substrate arranged to form an asymmetric transmission line. The characteristic impedance of finline is tunable over a wide range (approximately 10 to 400 ohms) by varying the slot width relative to the waveguide dimension, a range substantially broader than that achievable with microstrip. Substrates typically use low-loss materials such as RT/Duroid, Cuflon, or quartz, and the dominant propagation mode is hybrid rather than pure TEM, TE, or TM. A detailed parameter study of these configurations appears in the Finlines chapter in Wiley's Encyclopedia of Electrical and Electronics Engineering.

Propagation and Circuit Integration

The propagation constant and characteristic impedance of a finline depend on the substrate permittivity, thickness, slot width, and the dimensions of the enclosing waveguide. Because the field is largely confined to the low-permittivity slot region rather than the substrate, the effective dielectric constant is close to unity, which reduces the wavelength shortening that would otherwise require smaller circuit dimensions at millimeter-wave frequencies and improves dimensional tolerances in fabrication. Finline circuits are compatible with standard rectangular waveguide flanges at the input and output ports, simplifying integration with test equipment and waveguide subsystems. Passive finline components including filters, directional couplers, power dividers, phase shifters, and transitions between finline and microstrip have been demonstrated across the Ka, V, and W bands (roughly 26 to 110 GHz), as reviewed in Menzel's Finline Components reference. Active circuits such as balanced mixers and frequency multipliers have also been realized by mounting diodes and transistor chips in the slot region, connecting to the fins by beam leads or bond wires. The RF Wireless World finline overview provides a concise comparison of the four geometric variants and their impedance ranges.

Applications

Finline has applications in a range of fields, including:

• Millimeter-wave filter and multiplexer design for satellite and radar systems
• Balanced mixer circuits at 60 GHz and above for point-to-point communication
• Transitions between planar circuits and standard WR-28, WR-22, or WR-15 waveguide flanges
• Wideband power dividers and hybrid couplers for phased-array antenna feed networks
• Automotive radar subsystems operating at 77 GHz