Side-Coupled Microstrip Open-Loop Resonator for Harmonic-Suppressed Bandpass Filters

Hang WANG
Lei ZHU

Publication
IEICE TRANSACTIONS on Electronics   Vol.E88-C    No.9    pp.1893-1895
Publication Date: 2005/09/01
Online ISSN: 
DOI: 10.1093/ietele/e88-c.9.1893
Print ISSN: 0916-8516
Type of Manuscript: LETTER
Category: Microwaves, Millimeter-Waves
Keyword: 
bandpass filter,  harmonic suppression,  open-loop resonator,  side-coupled resonator,  

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Summary: 
A side-coupled microstrip open-loop resonator is presented for design of harmonic-suppressed bandpass filters with compact size. In geometry, the open-ended microstrip feed line is put in close proximity to the loop resonator at the opposite side of an opened-gap. In design, its length is properly lengthened to establish the orthogonal even- and odd-symmetrical current distributions along the two coupled strip conductors. It thus results in cancellation the 1st parasitic resonance. The two-stage open-loop filter is first constructed and its performance is studied under varied feed line lengths. Furthermore, a four-stage filter block is optimally designed at 2.52 GHz and its circuit sample is fabricated with the overall length less than 60% of one guided wavelength. The measured insertion loss at the 1st harmonic is higher than 30 dB, the stopband covers the range from 2.8 GHz to 7.0 GHz, and the dominant pass bandwidth is about 9.0%.