![Helical Resonator (3116177343)](https://upload.wikimedia.org/wikipedia/commons/f/fa/Helical_Resonator_%283116177343%29.jpg)
A helical resonator is a
passive
Passive may refer to:
* Passive voice, a grammatical voice common in many languages, see also Pseudopassive
* Passive language, a language from which an interpreter works
* Passivity (behavior), the condition of submitting to the influence of o ...
electrical component
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not ...
that can be used as a
filter
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component tha ...
resonator.
Physically, a helical resonator is a wire
helix
A helix () is a shape like a corkscrew or spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helic ...
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).
Like
cavity resonator
A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator ...
s, helical resonators can achieve
Q factors in the 1000s. This is because at high frequencies, the
skin effect results in most of the
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
flowing on the surface of the helix and shield. Plating the shield walls and helix with high
conductivity
Conductivity may refer to:
*Electrical conductivity, a measure of a material's ability to conduct an electric current
**Conductivity (electrolytic), the electrical conductivity of an electrolyte in solution
** Ionic conductivity (solid state), ele ...
materials increases the Q beyond that of bare copper (Blattenberger, 1989).
The length of wire is one quarter of the wavelength of interest. The helix is space wound, the gap between turns is equal to the diameter of the wire (Blattenberger, 1989). If the open end of the helix is close to the end cap of the metal shield the length is somewhat reduced due to the capacitance between the conductor and the shield (Whittaker, 2000, p. 227).
Coupling to the resonator can be achieved with a tap wire soldered to the helix at some distance from the shorted end. Input impedance varies with distance from the shorted end by
impedance transformer action. The tap point is chosen to achieve an impedance match with the connected circuit. Tuning of the resonator may be achieved by inserting a screw into the central axis of the helix (Weston, 2001, p. 660). Other means of input and output coupling used are a wire loop coupling to the magnetic field near the shorted end, or a probe capacitively coupling near the open end. Coupling between resonators in a multi-resonator filter is often simply achieved with apertures in the shielding between them (Whittaker, 2000, p. 227).
Helical resonators are well suited to
UHF frequencies ranging from 600 MHz to 1500 MHz (Blattenberger, 1989).
Design equations
* Q - quality factor (dimensionless)
*
- resonator characteristic impedance (Ohms)
* d - mean helix diameter (cm)
* h - height of helix (cm)
* f - frequency (MHz)
(Blattenberger, 1989)
References
*Kirt Blattenberger
"Helical resonator design", ''RF Cafe'' 1989.
*M. J. Lancaster, ''Passive Microwave Device Applications of High-Temperature Superconductors'', Cambridge University Press, 2006 .
*David Weston, ''Electromagnetic Compatibility: Principles and Applications'', Second Edition, CRC Press, 2001 .
*Jerry C. Whitaker, ''The Resource Handbook of Electronics'', CRC Press, 2000 .
*Anatol I. Zverev, ''Handbook of filter synthesis'', pp.499-519, Wiley, 1967 {{OCLC, 972252.
Resonators
Distributed element circuits