A review of the design of helical resonator filters

The design procedure for helical resonator filters, the attempts to model the resonator and the derivation of the design equations are briefly reviewed. Also presented are a series of measurements that were made to assess the accuracy of the design equations, with a view to developing a computer-based design facility. The work has shown that the design equations do not allow a resonator to be designed in a single step, i.e. the dimensions require adjusting by measurement. A resonator made to the dimensions predicted by the equations will always require shortening to obtain the correct resonance. In addition to predicting the resonator dimensions, it is necessary to relate the inter-resonator aperture to the required coupling coefficient. The variation of coupling coefficient with aperture size has been measured and a series of curves are presented. Using the measured data in the form of look-up tables, design software has been developed that enables a one-step filter design to be performed.