A novel microstrip loop-type resonator with four resonant modes is proposed in this letter. The resonator is formed by a loop-type microstrip line loaded with four shorted stubs. It has a symmetrical structure, thus the odd-even-mode method is adopted to implement the resonant analysis. The novelty of the proposed resonator lies in two aspects. One is that its resonant frequencies can be adjusted in a more flexible way. The other is that its resonant modes have a uniform electromagnetic field distribution, which is beneficial for the excitation of resonant modes. For the purpose of demonstration, based on the novel resonator, a single-band bandpass filter with four transmission poles and a dual-band bandpass filter with two transmission poles in each passband are constructed. Additionally, source-load cross coupling is introduced, and several transmission zeros are generated in the stopband, which improves the out-of-band performance greatly. The designed single-band filter has the central frequency of 2.4 GHz and fractional bandwidth (FBW) of 4.5%, and the dual-band filter has the central frequency of 1.8/2.4 GHz and fractional bandwidth of 2.0%/2.5%. The two bandpass filters are designed, fabricated, and measured. Agreement between the simulated and measured results verifies the effectiveness of the proposed resonator and filters.
2. Hong, J.-S. and M. Lancaster, "Bandpass characteristics of new dual-mode microstrip square loop resonators," Electron Lett, Vol. 31, No. 11, 891-892, 1995.
3. Hong, J.-S. and M. J. Lancaster, "Microstrip bandpass filter using degenerate modes of a novel meander loop resonator," Microwave and Guided Wave Letters, IEEE, Vol. 5, No. 11, 371-372, 1995.
4. Gorur, A., "Description of coupling between degenerate modes of a dual-mode microstrip loop resonator using a novel perturbation arrangement and its dual-mode bandpass filter applications," IEEE Trans Microwave Theory Technol, Vol. 52, No. 2, 671-677, 2004.
5. Gorur, A., "Realization of a dual-mode bandpass filter exhibiting either a Chebyshev or an elliptic characteristic by changing perturbation’s size," IEEE Microwave Wireless Compon. Lett., Vol. 14, No. 3, 118-120, 2004.
6. Luo, S. and L. Zhu, "A novel dual-mode dual-band bandpass filter based on a single ring resonator," IEEE Microwave Wireless Compon. Lett., Vol. 19, No. 8, 497-499, 2009.
7. Luo, S., L. Zhu, and S. Sun, "A dual-band ring-resonator bandpass filter based on two pairs of degenerate modes," IEEE Trans. Microwave Theory Technol., Vol. 58, No. 12, 3427-3432, 2010.
8. Luo, S., L. Zhu, and S. Sun, "Compact dual-mode triple-band bandpass filters using three pairs of degenerate modes in a ring resonator," IEEE Trans. Microwave Theory Technol., Vol. 59, No. 5, 1222-1229, 2011.
9. Liu, L., X. Liang, R. Jin, X. Bai, H. Fan, and J. Geng, "A compact and high-selectivity tri-band bandpass filter based on symmetrical stub-loaded square ring resonator," Microwave and Optical Technology Letters, Vol. 62, No. 2, 630-636, 2020.
10. Mirzaei, M. and A. Sheikhi, "Design and implementation of microstrip dual-band filtering power divider using square-loop resonator," Electron Lett., Vol. 56, No. 1, 19-21, 2020.
11. Yan, J.-M., Z.-P. Xiao, and L. Cao, "A simple balanced bandpass filter using loop-type microstrip resonator loaded with shorted/opened stubs," Progress In Electromagnetics Research Letters, Vol. 107, 141-149, 2022.
12. Hong, J. S. and M. J. Lancaster, Microstrip Filter for RF/Microwave Applications, John Wiley & Sons, 2001.