A helical resonator is a passive electrical component

Electronic component

An electronic component is a basic electronic element and may be available in a discrete form having two or more electrical terminals . These are intended to be connected together, usually by soldering to a printed circuit board, in order to create an electronic circuit with a particular function...

 that can be used as a filter

Electronic filter

Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...

. Physically, a helical resonator is a wire helix

Helix

A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...

 surrounded by a square or cylindrical conductive shield. Like cavity resonators, helical resonators can achieve Q factor

Q factor

In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....

s in the 1000s. This is because at high frequencies, the skin effect

Skin effect

Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...

 results in most of the current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 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 - quality factor (dimensionless)
• - resonator characteristic impedance (Ohms)
• d - mean helix diameter (cm)
• h - height of helix (cm)
• f - frequency (MHz)