New design techniques have been applied to microwave bandpass filters to demonstrate compact low cost filters using transmission lines, helical resonators and capacitors. Stripline filter structures which demonstrate very low radiation loss have been developed as shown. These filters do not require screening boxes and have the inherent high Qs of the materials used. Unloaded Qs of 520 have been obtained at 5 GHz in unscreened filters and unloaded Qs of 80 have been achieved at 21GHz on microwave resonators. New resonators (both hybrid and MMIC) which can tune over a near octave frequency range have been developed for use in tunable microwave oscillators and filters.
New high Q (235 at 2GHz) very compact resonators and filters base on printed helical resonators have been developed. These have been designed to ensure that the standing wave current is near zero through the via hole. Resistive loading has been used to suppress even order responses.
Ultra high Q resonators operating at room temperature are currently under investigation. Room temperature Qs of 24,000 and 60,000 are readily achieved with simple structures using BaTiO3 and Sapphire respectively. New structures with a 10 fold improvement in Q (at room temperature) are currently being modelled and investigated.