Helical resonator
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A helical resonator is a passive electrical component that can be used as a filter resonator.
Physically, a helical resonator is a wire helix surrounded by a square or cylindrical conductive shield. One end of the helix is connected to the shield and the other end is left open (Weston, 2001, p. 660). The device works like a coaxial resonator, but it is much shorter because the helical inner conductor reduces the velocity of wave propagation (Lancaster, 2006, p. 99).