Helical resonator

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A helical resonator is a passive electrical component that can be used as a filter. Physically, a helical resonator is a wire helix surrounded by a square or cylindrical conductive shield. Like cavity resonators, helical resonators can achieve Q factors in the 1000s. This is because at high frequencies, the skin effect results in most of the current flowing on the surface of the helix and shield. Plating the shield walls and helix with high conductivity materials increases the Q beyond that of bare copper.

The length of wire is one quarter of the wavelength of interest. The helix is space wound, the gap between turns is equal to diameter of the wire. Helical resonators are well suited to UHF frequencies ranging from 600 MHz to 1500 MHz.

Design equations

$Q=35.9\cdot d\cdot {\sqrt {f}}$

$Z_{o}={\frac {136190}{d\cdot f}}$

$h=1.5\cdot d$

Q - quality factor (dimensionless)
$Z_{o}$ - resonator characteristic impedance (Ohms)
d - mean helix diameter (cm)
h - height of helix (cm)
f - frequency (MHz)

References

Kirt Blattenberger, "Helical resonator design", RF Cafe, 1989.
Anatol I. Zverev, Handbook of filter synthesis, pp.499-519, Wiley, 1967.
Practical filter design information

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