A helical resonator is a
passive electrical component that can be used as a
filter resonator.
Physically, a helical resonator is a wire
helix
A helix (; ) is a shape like a cylindrical coil spring or the thread of a machine screw. 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 for ...
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 microwave cavity or radio frequency cavity (RF cavity) is a special type of resonator, consisting of a closed (or largely closed) metal structure that confines electromagnetic fields in the microwave or radio frequency, RF region of the spect ...
s, helical resonators can achieve
Q factor
In physics and engineering, the quality factor or factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the resonator to the energy lost ...
s in the 1000s. This is because at high frequencies, the
skin effect
In electromagnetism, skin effect is the tendency of an alternating current, alternating electric current (AC) to become distributed within a Conductor (material), conductor such that the current density is largest near the surface of the conduc ...
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 (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
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter ...
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