A helical bandpass filter consists of a series of cavities that are magnetically coupled together by an iris in each wall. Each cavity consists of a coil soldered to ground on one side and a tuning capacitor on the other. The length of the unfurled coil is equal to one-quarter wavelength of light at the center frequency of the structure. Cavity-based filter performance is determined entirely by geometry. Even though the devices are tuned, it can be challenging to size all the components such that the tuning elements are effective to meet desired synthesized response.
Helical resonator filters are widely used in ground based UHF mobile communication systems. They exhibit reasonable Q and excellent in-band performance over a wide temperature range with less volume and mass compared to conventional coaxial cavity filters operating in this band. Helical resonator filters are best suited in applications where conventional lumped-element filters are very small but are too lossy (lower Q) and coaxial resonator filters (higher Q) are too big and unpractical. Compared to SAW filters, although much bigger, the helical approach is much simpler and requires no costly fabrication setup. This type of filter becomes particularly attractive for satellite applications where low volume and mass coupled with high reliability and electrical performance is a must. Below is basic working principal of helical resonator.
Below example is a helix resonator combline filter. The 3D helix structure can be modeled in EM simulation tools and FEM solver then carries out the calculation. First a single cavity unit is designed and optimized then multi-section filter using these unit blocks are realized. Design start with Helical Filter parameter calculation using standard formulas.