CAD Modeling of a Microwave Rectangular Coaxial Filtering Device

In this paper we discuss the adequate filter parameter selection to obtain a low loss coaxial cable, out of which a narrowband filtering device is designed. The design parameters under interest are the attenuation constant (a) obtained, which will depend on the size of the rectangular coaxial cross section used. And the decision beyond which frequency higher order modes should start to propagate through the cable resulting in a dispersive coaxial cable. The tradeoff between these two parameters is discussed in detail. After the coaxial design parameters are chosen to meet a design specification, (in this paper a 4 pole, 4% fractional bandwidth filter, centered at 9 GHz), the design of an air filled coaxial feed line is discussed, followed by the design of the microwave filter, and finally the integration of the complete device is presented. The coaxial cable discussed in this paper uses air as the propagation medium, thus resulting in a conductor loss limited structure, where no dielectric or radiation losses are present. All calculations were done using Ansoft HFSS a commercial full wave 3D EM simulator