Improved Design of Single-Section and Cascaded Planar Directional Filters

An improved analysis and design of single-section and cascaded planar directional filters is addressed, based on approximate and exact microwave circuit analyses, and the resulting characteristics of matching, coupling, band elimination, and isolation are investigated. The effects of finite even-mode reactance and the unavoidable loss of capacitively coupled parallel branch are also studied. It is known that better performance of single-section directional filters, especially in matching, can be gained by compensating the line length between coupling resonators. It is confirmed that cascading identical directional filters introduces transmission zeros in the stopband, allowing sharper cutoff characteristics. Furthermore, a flatter passband, as well as improved band elimination, are observed. It is also shown that the introduced transmission zeros can be controlled properly by adjusting the connecting-line lengths between directional filters or the parameters related to the coupling resonator. For an efficient design with better performance, accurate design equations are provided with graphs. As a validation check, a microstrip directional filter was designed with a fractional bandwidth of 2.5% at 2.5 GHz and transmission zeros of 2.40 and 2.61 GHz. Good agreement between the measurement and design specifications shows the validity of the proposed configuration, related analysis, and design theory.