Novel High-Quality Compact Microstrip Resonator and its Application to Bandpass Filter

A novel compact microstrip resonator based on interdigital structure is proposed. The resonator has several times higher quality factor than similar resonators presented earlier. It is shown that the size of the resonator can be significantly reduced with substantial increase in quality factor by incrementing the number of pins in the interdigital structure. A 4-pole bandpass filter with central frequency 900 MHz and an overall size 4.6 mm $\times$ 10.0 mm was designed and fabricated on the base of the proposed structure. The measured upper stopband of the filter was found to be up to $7.4f_{\rm o}$ at the level $-$ 40 dB.