Microstrip Coupled Line Filters with Spurious Band Suppression

Abstract

Passband microstrip coupled line fllters are widely used in daily microwave en- gineering practice. These fllters present an undesired pass band at twice the design central frequency of the fllter. Two simple methods to suppress this drawback are reported in this com- munication. The proposed methods, apart from removing the spurious band, provide an extra coupling mechanism that relaxes tolerances of strip width and spacing in those cases where tightly coupled sections are required. DOI: 10.2529/PIERS060822161838 Parallel coupled lines microstrip fllters (PCMF) are well known devices in microwave practice. When implemented in their traditional form, i.e.,using a single layer substrate in microstrip tech- nology, these fllters present two relatively serious drawbacks. First, a spurious pass band appears at 2f0 (f0 is the central frequency of the fllter). The second problem comes from the di-culty of fabricating wide band fllters because of the weak coupling between the lines in the conventional structure. Strong coupling required by some fllter speciflcations leads to very small values of strip width, w, and strips spacing, s, which can not be sometimes accurately achieved in practice.