An Investigation of Quarter-Wavelength Square-Spiral Resonator and Its Applications to Miniaturized Bandpass Filters

Abstract

In this paper, we investigate both quarter-wavelength square-spiral resonator and its application to miniaturized bandpass filters. Using the commercial simulation software, we examine the coupling coefficients and external quality factor of the proposed structure. With different combinations of the proposed resonators, four miniaturized bandpass filters are designed. The sizes of the designed filters are reduced by 96%, 96.7%, 96.6% and 94.8%, compared to the traditional square loop bandpass filter. It is also found that the first spurious passband of the filters is increased up to near three times the fundamental resonant frequency, and the rejection level around twice the fundamental resonant frequency is kept below 20 dB. To validate the proposed idea, four experimental filters are designed, simulated and fabricated. Measured results are in good agreement with the theoretical predictions.