Analysis of Bandpass Filters with Shielded Inverted Microstrip Lines

Ushio SANGAWA  

Publication
IEICE TRANSACTIONS on Electronics   Vol.E87-C   No.10   pp.1715-1723
Publication Date: 2004/10/01
Online ISSN: 
DOI: 
Print ISSN: 0916-8516
Type of Manuscript: PAPER
Category: Microwaves, Millimeter-Waves
Keyword: 
inverted microstrip line,  mode coupling,  dispersion,  bandpass filter (BPF),  parallel-coupled line,  attenuation pole,  

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Summary: 
A bandpass filter (BPF) with shielded inverted microstrip lines (SIMSL), previously demonstrated by the author, has shown the nontrivial asymmetry of filter responses in spite of adopting a conventional filter synthesis procedure. This paper will reveal the mechanism of the asymmetry and propose prescriptions for recovering the defect, in addition to observing the wave propagation property of SIMSL. Firstly, the behavior of phase constants or effective dielectric constants for various modes propagating on single SIMSL are indicated in terms of the line configuration, and the dispersion characteristics of the quasi-TEM mode are interpreted from the point of mode coupling between the pure TEM mode and dielectric slab modes. Then it is shown that the asymmetry is dependent only on the transmission characteristics of SIMSL parallel-coupled lines involved in the filter circuits. Theoretical considerations reveal that the asymmetry is due to the fact that SIMSL has quite different phase constants for the even- and odd-mode. On the basis of these results, the optimized BPF is designed and it is experimentally demonstrated that the symmetry of its responses is notably recovered. Furthermore, this optimization is still quite efficient for achieving high attenuation properties at its harmonics.