New perturbation technique to generate split modes in electromagnetically coupled notch filters using different ring resonators.

A new method for generating split modes in ring resonators, used as notch filters with three geometries (square, triangular, and circular), is introduced by varying the feed position of electromagnetically coupled feed line. The resonators are fabricated (all at 2.4 GHz nominally) on TMM4 (ℇ_r = 4.5, h = 0.762 mm) substrates and coupled electromagnetically to a 50 Ω microstrip feed line on separate TMM4 substrate. Only the latter has ground plane under it. The offset distance between the center of feed line and center of resonator is varied and changes in the resonant (notch) frequency, rejection (S₂₁) of single and split modes are recorded at each offset position for all resonators. Experimental results are found to agree with simulated results to a greater degree for square and lesser for other two geometries. Frequency variations have much more disagreements. The angle of crossing between feed line and ring edges appears to be a critical factor. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1747–1752, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23483 [ABSTRACT FROM AUTHOR]