Helical Filter

A Helical filter is a special case of a bandpass lumped element RF filter. Helical filters are realizable at frequencies above 100 MHz and up to several GHz. 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.

The size of a helical filter is greatly influenced by the frequency and, to a lesser extent, the bandwidth. At narrow bandwidths, larger structures are needed in order to get a higher resonator Q. Similarly, as frequency increases the size of the filter decreases due to the smaller wavelength. The principal advantage of a helical filter over a LC filter is bandwidth verses insertion loss. Helical resonators have a much higher Q than that of a similar lumped inductor. Moreover, a helical filter confines it’s magnetic fields in cavities that results in less stray coupling as compared to traditional LC filters. However, helical filters are limited to bandpass structures using monotonic responses.

Filtronetics, Inc. manufactures nearly every type of electronic RF filter available. For low frequency and power applications we offer EMI and LC filters. At higher frequencies we supply crystal, monolithic, LC, Helical, SAW and ceramic. And at high frequency we provide custom microwave filters. We can build from one prototype helical filter to over 1 million helical filters in production. We offer Helical filters in surface mount, through-hole, and connectorized packages.