Folded Ring Microstrip Filter

A conventional microstrip ring resonator when folded up into a more compact structure is shown to exhibit similar characteristics to the annular-ring resonator. The folded ring resonator can resonate at identical resonant modes as the annular-ring resonator, which is designed to operate at the same fundamental mode frequency when fabricated on an identical dielectric substrate. The physical dimensions of this structure are much smaller than that of a conventional microstrip ring operating at the same frequency. Moreover, it can be easily integrated with other microwave devices, and its two flat sides enable effective coupling with adjacent resonators to form filters. Bandpass filters designed using the folded ring structure are significantly more compact than a traditional planar filter such as parallel-coupled bandpass filter designed at the same frequency even when fabricated on high dielectric constant substrates. The folded ring filters implemented provide relatively low insertion loss and good return loss at its passband.