Concerning the influence of housing dimensions on the response and design of microstrip filters with parallel-line couplings

In this paper, measured results obtained from a narrow-band microstrip filt er are compared with computed responses obtained using two different classe s of software for various assumed housing conditions. (Some results from fi lters with wider bandwidths are also cited.) Housing modes are found to hav e a potentially significant affect on the bandwidth of microwave filers tha t involve microstrip coupled lines, even though the housing resonant freque ncies are much above the passband of the biter When analyzing such filters using full-wave three-dimensional (3-D) or 3-D planar field solver programs , it is found to be necessary to accurately model the housing if high accur acy in the computed response is needed. For carrying out filter design by o ptimization, it is usually convenient to use faster programs that utilize a full-wave two-dimensional (2-D) field solver to obtain line parameters and then use transmission-line analysis for the third dimension. Such programs can introduce housing-mode errors since 2-D full-wave analysis implies an infinitely long housing. Ways for getting around this problem are suggested . Physical explanations for the various effects observed are presented and are supported by computer studies of the natural frequencies of a coupled-l ine microstrip filter structure in the presence of various housing perturba tions.