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{{main|Helical resonator}}
A helical resonator is a [[helix]] of wire in a cavity; one end is unconnected, and the other is bonded to the cavity wall. Although they are superficially similar to lumped inductors, helical resonators are distributed element components and are used in the [[VHF]] and lower [[UHF]] bands.<ref>{{multiref|Whitaker, p. 227|Doumanis ''et al.'', pp. 12–14}}</ref>
=== Taper ===▼
A taper is a transmission line with a gradual change in cross-section. It can be considered the limiting case of the stepped impedance structure with an infinite number of steps.<ref>Zhurbenko, p. 310</ref> Tapers are a simple way of joining two transmission lines of different characteristic impedances. Using tapers greatly reduces the mismatch effects that a direct join would cause. If the change in cross-section is not too great, no other matching circuitry may be needed.<ref>Garg ''et al.'', pp. 180–181</ref> Tapers can provide [[Planar transmission line#Transitions|transitions]] between lines in different media, especially different forms of planar media.<ref>{{multiref|Garg ''et al.'', pp. 404–406, 540|Edwards & Steer, p. 493}}</ref> Tapers commonly change shape linearly, but a variety of other shapes may be used. The shape that produces a taper of the shortest length is known as a Klopfenstein taper and is based on the [[Chebychev filter]] design.<ref>{{multiref|Zhurbenko, p. 311|Misra, p. 276|Lee, p. 100}}</ref>▼
Tapers can be used to match a transmission line to an antenna. In some designs, such as the [[horn antenna]] and [[Vivaldi antenna]], the taper is itself the antenna. Horn antennae, like other tapers, are often linear, but the best match is obtained with an exponential curve. The Vivaldi antenna is a flat (slot) version of the exponential taper.<ref>{{multiref|Bakshi & Bakshi|pp. 3-68–3-70|Milligan, p. 513}}</ref>▼
=== Fractals ===
[[file:Hilbert resonator.svg|thumb|upright|Three-iteration Hilbert fractal resonator in microstrip<ref>Janković ''et al.'', p. 197</ref>]]▼
{{see also|Fractal antenna}}
▲[[file:Hilbert resonator.svg|thumb|upright|Three-iteration Hilbert fractal resonator in microstrip<ref>Janković ''et al.'', p. 197</ref>]]
The use of [[fractal]] curves as a circuit component is an emerging field in distributed element circuits.<ref>Ramadan ''et al.'', p. 237</ref> Fractals have been used to make resonators for filters and antennae. One of the benefits of using fractals is their space-filling property, making them smaller than other designs.<ref>Janković ''et al.'', p. 191</ref> Other advantages include the ability to produce [[wide-band]] and [[Multi-band device|multi-band]] designs, good in-band performance, and good [[out-of-band]] rejection.<ref>Janković ''et al.'', p. 191–192</ref> In practice, a true fractal cannot be made because at each [[Iterated function system|fractal iteration]] the manufacturing tolerances become tighter and are eventually greater than the construction method can achieve. However, after a small number of iterations, the performance is close to that of a true fractal. These may be called ''pre-fractals'' or ''finite-order fractals'' where it is necessary to distinguish from a true fractal.<ref>Janković ''et al.'', p. 196</ref>
Fractals that have been used as a circuit component include the [[Koch snowflake]], [[Minkowski island]], [[Sierpiński curve]], [[Hilbert curve]], and [[Peano curve]].<ref>Janković ''et al.'', p. 196</ref>. The first three are closed curves, suitable for patch antennae. The latter two are open curves with terminations on opposite sides of the fractal. This makes them suitable for use where a connection in [[cascade connection|cascade]] is required.<ref>Janković ''et al.'', p. 196</ref>
▲=== Taper ===
▲A taper is a transmission line with a gradual change in cross-section. It can be considered the limiting case of the stepped impedance structure with an infinite number of steps.<ref>Zhurbenko, p. 310</ref> Tapers are a simple way of joining two transmission lines of different characteristic impedances. Using tapers greatly reduces the mismatch effects that a direct join would cause. If the change in cross-section is not too great, no other matching circuitry may be needed.<ref>Garg ''et al.'', pp. 180–181</ref> Tapers can provide [[Planar transmission line#Transitions|transitions]] between lines in different media, especially different forms of planar media.<ref>{{multiref|Garg ''et al.'', pp. 404–406, 540|Edwards & Steer, p. 493}}</ref> Tapers commonly change shape linearly, but a variety of other shapes may be used. The shape that produces a taper of the shortest length is known as a Klopfenstein taper and is based on the [[Chebychev filter]] design.<ref>{{multiref|Zhurbenko, p. 311|Misra, p. 276|Lee, p. 100}}</ref>
▲Tapers can be used to match a transmission line to an antenna. In some designs, such as the [[horn antenna]] and [[Vivaldi antenna]], the taper is itself the antenna. Horn antennae, like other tapers, are often linear, but the best match is obtained with an exponential curve. The Vivaldi antenna is a flat (slot) version of the exponential taper.<ref>{{multiref|Bakshi & Bakshi|pp. 3-68–3-70|Milligan, p. 513}}</ref>
== Circuit blocks ==
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