page_title: Planar transmission line
section_title: Finline variants
hierarchical_section_title: Planar transmission line / Types / Finline / Finline variants
context_page_description: 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 type fits in well with the manufacturing methods for these components. Transmission lines are more than simply interconnections. With simple interconnections, the propagation of the electromagnetic wave along the wire is fast enough to be considered instantaneous, and the voltages at each end of the wire can be considered identical. If the wire is longer than a large fraction of a wavelength, these assumptions are no longer true and transmission line theory must be used instead. With transmission lines, the geometry of the line is precisely controlled so that its electrical behaviour is highly predictable. 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, transmission lines are needed within circuits.
context_section_description: Unilateral finline is the simplest design and easiest to manufacture but bilateral finline has lower loss, as with bilateral suspended stripline, and for similar reasons.  The high Q of bilateral finline often makes it the choice for filter applications.  Antipodal finline is used where very low characteristic impedance is required.  The stronger the coupling between the two planes, the lower the impedance.  Insulated finline is used in circuits that contain active components needing bias lines.  The Q of insulated finline is lower than other finline types so it is otherwise not usually used.
caption: Finline variants: A, standard (unilateral),[57] B, bilateral,[58] C, antipodal,[58] D, strongly coupled antipodal[29] E, insulated[59]