Helical resonator

Physically, a helical resonator is a wire helix surrounded by a square or cylindrical conductive shield.

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).

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 (Blattenberger, 1989).

Coupling to the resonator can be achieved with a tap wire soldered to the helix at some distance from the shorted end.

A helical resonator filter for the local oscillator injection of a VHF receiver. The view is from the bottom (shorted end) of the resonators. The local oscillator and RF amplifier boards attach to the bottom of the assembly and have loops that couple to the resonators forming respectively the input and output ports of the filter.